Evidence of evolution
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of living things has been discovered by scientists working in a variety of fields over many years. This evidence has demonstrated and verified the occurrence of evolution
and provided a wealth of information on the natural processes by which the variety and diversity of life on Earth developed. This evidence supports the modern evolutionary synthesis
, the current scientific theory
that explains how and why life changes over time. Evolutionary biologists document the fact of common descent
: making testable predictions, testing hypotheses, and developing theories that illustrate and describe its causes.
Comparison of the genetic sequence
of organisms has revealed that organisms that are phylogenetically
close have a higher degree of sequence similarity than organisms that are phylogenetically distant. Further evidence for common descent comes from genetic detritus such as pseudogene
s, regions of DNA
that are orthologous to a gene in a related organism
, but are no longer active and appear to be undergoing a steady process of degeneration.
Fossils are important for estimating when various lineages developed in geologic time. As fossilization is an uncommon occurrence, usually requiring hard body parts and death near a site where sediment
s are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Evidence
of organisms prior to the development of hard body parts such as shells, bones and teeth is especially scarce, but exists in the form of ancient microfossils, as well as impressions of various soft-bodied organisms. The comparative study of the anatomy
of groups of animals shows structural features that are fundamentally similar or homologous, demonstrating phylogenetic and ancestral relationships with other organism, most especially when compared with fossils of ancient extinct
organisms. Vestigial structures and comparisons in embryonic development
are largely a contributing factor in anatomical resemblance in concordance with common descent. Since metabolic
processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms’ physiology
and biochemistry
. Many lineages diverged at different stages of development, so it is possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor. Universal biochemical organization and molecular variance patterns in all organisms also show a direct correlation with common descent.
Further evidence comes from the field of biogeography
because evolution with common descent provides the best and most thorough explanation for a variety of facts concerning the geographical distribution of plants and animals across the world. This is especially obvious in the field of island biogeography
. Combined with the theory of plate tectonics
common descent provides a way to combine facts about the current distribution of species with evidence from the fossil record to provide a logically consistent explanation of how the distribution of living organisms has changed over time.
The development and spread of antibiotic resistant
bacteria, like the spread of pesticide resistant forms of plants and insects provides evidence that evolution due to natural selection
is an ongoing process in the natural world. Alongside this, are observed instances of the separation of populations of species into sets of new species (speciation
). Speciation has been observed directly and indirectly in the lab and in nature. Multiple forms of such have been described and documented as examples for individual modes of speciation. Furthermore, evidence of common descent extends from direct laboratory experimentation with the artificial selection of organisms—historically and currently—and other controlled experiments involving many of the topics in the article. This article explains the different types of evidence for evolution with common descent along with many specialized examples of each.
One of the strongest evidences for common descent comes from the study of gene sequences. Comparative sequence analysis
examines the relationship between the DNA sequences of different species, producing several lines of evidence that confirm Darwin's original hypothesis of common descent. If the hypothesis of common descent is true, then species that share a common ancestor will have inherited that ancestor's DNA sequence. They will have inherited mutations unique to that ancestor. More closely related species will have a greater fraction of identical sequence and will have shared substitutions when compared to more distantly related species.
The simplest and most powerful evidence is provided by phylogenetic reconstruction
. Such reconstructions, especially when done using slowly evolving protein sequences, are often quite robust and can be used to reconstruct a great deal of the evolutionary history of modern organisms (and even in some instances such as the recovered gene sequences of mammoth
s, Neanderthal
s or T. rex
, the evolutionary history of extinct organisms). These reconstructed phylogenies recapitulate the relationships established through morphological and biochemical studies. The most detailed reconstructions have been performed on the basis of the mitochondrial genomes shared by all eukaryotic
organisms, which are short and easy to sequence; the broadest reconstructions have been performed either using the sequences of a few very ancient proteins or by using ribosomal RNA
sequence.
Phylogenetic relationships also extend to a wide variety of nonfunctional sequence elements, including repeats, transposons, pseudogenes, and mutations in protein-coding sequences that do not result in changes in amino-acid sequence. While a minority of these elements might later be found to harbor function, in aggregate they demonstrate that identity must be the product of common descent rather than common function.
, or RNA
for viruses), transcribed into RNA
, then translated into protein
s (that is, polymers of amino acid
s) by highly conserved ribosome
s. Perhaps most tellingly, the Genetic Code
(the "translation table" between DNA and amino acids) is the same for almost every organism, meaning that a piece of DNA
in a bacterium
codes for the same amino acid as in a human cell
. ATP
is used as energy currency by all extant life. A deeper understanding of developmental biology
shows that common morphology is, in fact, the product of shared genetic elements. For example, although camera-like eyes are believed to have evolved independently on many separate occasions, they share a common set of light-sensing proteins (opsin
s), suggesting a common point of origin for all sighted creatures. Another noteworthy example is the familiar vertebrate body plan, whose structure is controlled by the homeobox (Hox) family of genes.
trees are typically congruent with traditional taxonomy
, and are often used to strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee
, 1.6% from gorilla
s, and 6.6% from baboon
s. Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other ape
s. The sequence of the 16S ribosomal RNA
gene, a vital gene encoding a part of the ribosome
, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by Carl Woese
, resulted in the three-domain system
, arguing for two major splits in the early evolution of life. The first split led to modern Bacteria
and the subsequent split led to modern Archaea
and Eukaryotes.
es (or ERVs) are remnant sequences in the genome left from ancient viral infections in an organism. The retroviruses (or virogenes) are always passed on
to the next generation of that organism which received the infection. This leaves the virogene left in the genome. Because this event is rare and random, finding identical chromosomal positions of a virogene in two different species suggests common ancestry. See examples of humans and cats below.
evidence also supports the universal ancestry of life. Vital protein
s, such as the ribosome
, DNA polymerase
, and RNA polymerase
, are found in everything from the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions. Higher organisms have evolved additional protein subunit
s, largely affecting the regulation and protein-protein interaction
of the core. Other overarching similarities between all lineages of extant organisms, such as DNA
, RNA
, amino acids, and the lipid bilayer
, give support to the theory of common descent. Phylogenetic analyses of protein sequences from various organisms produce similar trees of relationship between all organisms. The chirality
of DNA, RNA, and amino acids is conserved across all known life. As there is no functional advantage to right- or left-handed molecular chirality, the simplest hypothesis is that the choice was made randomly by early organisms and passed on to all extant life through common descent. Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogene
s, "dead" genes which steadily accumulate mutations.
, are extra DNA in a genome that do not get transcribed into RNA to synthesize proteins. Some of this noncoding DNA has known functions, but much of it has no known function and is called "Junk DNA". This is an example of a vestige since replicating these genes uses energy, making it a waste in many cases. Pseudogenes make up 99% of the human genome (1% working DNA). A pseudogene can be produced when a coding gene accumulates mutations that prevent it from being transcribed, making it non-functional. But since it is not transcribed, it may disappear without affecting fitness, unless it has provided some new beneficial function as non-coding DNA. Non-functional pseudogenes may be passed on to later species, thereby labeling the later species as descended from the earlier species.
and gene duplication
, which facilitates rapid evolution by providing substantial quantities of genetic material under weak or no selective constraints; horizontal gene transfer
, the process of transferring genetic material to another cell that is not an organism's offspring, allowing for species to acquire beneficial genes from each other; and recombination
, capable of reassorting large numbers of different allele
s and of establishing reproductive isolation
. The Endosymbiotic theory
explains the origin of mitochondria
and plastid
s (e.g. chloroplast
s), which are organelle
s of eukaryotic cells, as the incorporation of an ancient prokaryotic
cell into ancient eukaryotic
cell. Rather than evolving eukaryotic
organelle
s slowly, this theory offers a mechanism for a sudden evolutionary leap by incorporating the genetic material and biochemical composition of a separate species. Evidence supporting this mechanism has been found in the protist
Hatena
: as a predator it engulfs a green algae
cell, which subsequently behaves as an endosymbiont
, nourishing Hatena, which in turn loses its feeding apparatus and behaves as an autotroph
.
Since metabolic
processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor or by detecting their physical manifestations. As an example, the appearance of oxygen
in the earth's atmosphere
is linked to the evolution of photosynthesis
.
) present another example of virogene sequences in common descent. The standard phylogenetic tree for Felidae have smaller cats (Felis chaus, Felis silvestris, Felis nigripes, and Felis catus) diverging from larger cats such as the subfamily Pantherinae
and other carnivores
. The small cats have an ERV where the larger cats do not suggesting that the gene was inserted into the ancestor of the small cats after the larger cats had diverged.
Evidence for the evolution of Homo sapiens from a common ancestor with chimpanzees is found in the number of chromosomes in humans as compared to all other members of Hominidae
. All Hominidae (with the exception of humans) have 24 pairs of chromosomes. Humans have only 23 pairs. Human chromosome 2 is a result of an end-to-end fusion of two ancestral chromosomes.
The evidence for this includes:
Chromosome 2 thus presents very strong evidence in favour of the common descent of humans and other ape
s. According to J. W. IJdo, "We conclude that the locus cloned in cosmids c8.1 and c29B is the relic of an ancient telomere-telomere fusion and marks the point at which two ancestral ape chromosomes fused to give rise to human chromosome 2."
Cytochrome c
in living cells. The variance of cytochrome c of different organisms is measured in the number of differing amino acids, each differing amino acid being a result of a base pair
substitution, a mutation
. If each differing amino acid is assumed to be the result of one base pair substitution, it can be calculated how long ago the two species diverged by multiplying the number of base pair substitutions by the estimated time it takes for a substituted base pair of the cytochrome c gene to be successfully passed on. For example, if the average time it takes for a base pair of the cytochrome c gene to mutate is N years, the number of amino acids making up the cytochrome c protein in monkeys differ by one from that of humans, this leads to the conclusion that the two species diverged N years ago.
The primary structure of cytochrome c consists of a chain of about 100 amino acid
s. Many higher order organisms possess a chain of 104 amino acids.
The cytochrome c molecule has been extensively studied for the glimpse it gives into evolutionary biology. Both chicken
and turkey
s have identical sequence homology (amino acid for amino acid), as do pig
s, cows and sheep. Both human
s and chimpanzee
s share the identical molecule, while rhesus monkeys share all but one of the amino acids: the 66th amino acid is isoleucine
in the former and threonine
in the latter.
What makes these homologous similarities particularly suggestive of common ancestry in the case of cytochrome C, in addition to the fact that the phylogenies derived from them match other phylogenies very well, is the high degree of functional redundancy of the cytochrome C molecule. The different existing configurations of amino acids do not significantly affect the functionality of the protein, which indicates that the base pair substitutions are not part of a directed design, but the result of random mutations that aren't subject to selection.
, Ronald Fisher
and J. B. S. Haldane
and extended via diffusion theory by Motoo Kimura
, allow predictions about the genetic structure of evolving populations. Direct examination of the genetic structure of modern populations via DNA sequencing has allowed verification of many of these predictions. For example, the Out of Africa theory of human origins, which states that modern humans developed in Africa and a small sub-population migrated out (undergoing a population bottleneck
), implies that modern populations should show the signatures of this migration pattern. Specifically, post-bottleneck populations (Europeans and Asians) should show lower overall genetic diversity and a more uniform distribution of allele frequencies compared to the African population. Both of these predictions are borne out by actual data from a number of studies.
of groups of animals or plants reveals that certain structural features are basically similar. For example, the basic structure of all flower
s consists of sepal
s, petal
s, stigma, style and ovary
; yet the size, colour, number
of parts and specific structure are different for each individual species.
s that do not even reach the ground, chicken's teeth, reemergence of sexual reproduction
in Hieracium pilosella and Crotoniidae
; and humans with tails, extra nipple
s, and large canine teeth
.
) during human development; the appearance of transitions from fish to amphibians to reptiles and then to mammals in all mammal embryos; development and degeneration of a yolk sac
; terrestrial frogs and salamanders passing through the larval stage within the egg—with features of typically aquatic larvae—but hatch ready for life on land; and the appearance of gill-like structures (pharyngeal arch) in vertebrate embryo development. Note that in fish the arches become gills while in humans, for example, they become the pharynx
.
When a group of organisms share a homologous structure which is specialized to perform a variety of functions in order to adapt different environmental conditions and modes of life are called adaptive radiation
. The gradual spreading of organisms with adaptive radiation is known as divergent evolution
.
is based on the fact that all organisms are related to each other in nested hierarchies based on shared characteristics. Most existing species can be organized rather easily in a nested hierarchical classification. This is evident from the Linnaean classification scheme. Based on shared derived characters, closely related organisms can be placed in one group (such as a genus), several genera can be grouped together into one family, several families can be grouped together into an order, etc. The existence of these nested hierarchies was recognized by many biologists before Darwin, but he showed that his theory of evolution with its branching pattern of common descent could explain them. Darwin described how common descent could provide a logical basis for classification:
s) of flies
and mosquitos, wings of flightless birds such as ostrich
es, and the leaves
of some xerophyte
s (e.g. cactus
) and parasitic plant
s (e.g. dodder
). However, vestigial structures may have their original function replaced with another. For example, the halteres
in dipterist
s help balance the insect while in flight and the wings of ostriches are used in mating
rituals.
s possess internally reduced hind parts such as the pelvis and hind legs (Fig. 5a). Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs. On October 28, 2006, a four-finned bottlenose dolphin was caught and studied due to its extra set of hind limbs. These legged Cetacea
display an example of an atavism predicted from their common ancestry.
s, a hypopharynx
(floor of mouth), a pair of maxillae, and a labium. (Fig. 5b) Evolution has caused enlargement and modification of these structures in some species, while it has caused the reduction and loss of them in other species. The modifications enable the insects to exploit a variety of food materials:
s: The anterior pair of legs may be modified as analogues of antennae, particularly in whip scorpions
, which walk on six legs. These developments provide support for the theory that complex modifications often arise by duplication of components, with the duplicates modified in different directions.
and ornithischia
. They are classified as one or the other in accordance with what the fossils demonstrate. Figure 5c, shows that early saurischians resembled early ornithischians. The pattern of the pelvis
in all species of dinosaurs is an example of homologous structures. Each order of dinosaur has slightly differing pelvis bones providing evidence of common descent. Additionally, modern birds show a similarity to ancient saurischian pelvic structures indicating the evolution of birds from dinosaurs. This can also be seen in Figure 5c as the Aves branch off the Theropoda
suborder.
s (i.e. from amphibian
s to mammal
s). It can even be traced back to the fin
s of certain fossil fishes from which the first amphibians evolved such as tiktaalik
. The limb has a single proximal bone (humerus
), two distal bones (radius
and ulna
), a series of carpals (wrist
bones), followed by five series of metacarpals (palm
bones) and phalanges (digits). Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaptation to different environments and modes of life. This phenomenon is shown in the forelimbs of mammals. For example:
is a fourth branch of the vagus nerve
, which is a cranial nerve. In mammals, its path is unusually long. As a part of the vagus nerve, it comes from the brain, passes through the neck down to heart, rounds the dorsal aorta
and returns up to the larynx
, again through the neck. (Fig. 5e)
This path is suboptimal even for humans, but for giraffes it becomes even more suboptimal. Due to the lengths of their necks, the recurrent laryngeal nerve may be up to 4m long (13 ft), despite its optimal route being a distance of just several inches.
The indirect route of this nerve is the result of evolution of mammals from fish, which had no neck and had a relatively short nerve that innervated one gill slit and passed near the gill arch. Since then, gills have evolved into lungs and the gill arch has become the dorsal aorta in mammals.
is part of the male anatomy of many vertebrates; it transports sperm from the epididymis
in anticipation of ejaculation
. In humans, the vas deferens routes up from the testicle
, looping over the ureter
, and back down to the urethra
and penis
. It has been suggested that this is due to the descent of the testicles during the course of human evolution—likely associated with temperature. As the testicles descended, the vas deferens lengthened to accommodate the accidental “hook” over the ureter.
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When organisms die, they often decompose
rapidly or are consumed by scavenger
s, leaving no permanent evidences of their existence. However, occasionally, some organisms are preserved. The remains or traces
of organisms from a past geologic age
embedded in rocks
by natural processes are called fossil
s. They are extremely important for understanding the evolutionary history of life
on Earth, as they provide direct evidence of evolution and detailed information on the ancestry of organisms. Paleontology
is the study of past life based on fossil records and their relations to different geologic time periods.
For fossilization to take place, the traces and remains of organisms must be quickly buried so that weathering
and decomposition do not occur. Skeletal structures or other hard parts of the organisms are the most commonly occurring form of fossilized remains (Paul, 1998), (Behrensmeyer, 1980) and (Martin, 1999). There are also some trace "fossils" showing moulds
, cast or imprints of some previous organisms.
As an animal dies, the organic materials gradually decay, such that the bone
s become porous. If the animal is subsequently buried in mud
, mineral
salts will infiltrate into the bones and gradually fill up the pores. The bones will harden into stones and be preserved as fossils. This process is known as petrification
. If dead animals are covered by wind-blown sand
, and if the sand is subsequently turned into mud by heavy rain
or flood
s, the same process of mineral infiltration may occur. Apart from petrification, the dead bodies of organisms may be well preserved in ice
, in hardened resin
of coniferous
trees (amber
), in tar, or in anaerobic, acid
ic peat
. Fossilization can sometimes be a trace, an impression of a form. Examples include leaves and footprints, the fossils of which are made in layers that then harden.
. Sedimentary rock is formed by layers of silt
or mud on top of each other; thus, the resulting rock contains a series of horizontal layers, or strata
. Each layer contains fossils which are typical for a specific time period during which they were made. The lowest strata contain the oldest rock and the earliest fossils, while the highest strata contain the youngest rock and more recent fossils.
A succession
of animals and plants can also be seen from fossil discoveries. By studying the number and complexity of different fossils at different stratigraphic
levels, it has been shown that older fossil-bearing rocks contain fewer types of fossilized organisms, and they all have a simpler structure, whereas younger rocks contain a greater variety of fossils, often with increasingly complex structures.
For many years, geologists could only roughly estimate the ages of various strata and the fossils found. They did so, for instance, by estimating the time for the formation of sedimentary rock layer by layer. Today, by measuring the proportions of radioactive
and stable elements
in a given rock, the ages of fossils can be more precisely dated by scientists. This technique is known as radiometric dating
.
Throughout the fossil record, many species that appear at an early stratigraphic level disappear at a later level. This is interpreted in evolutionary terms as indicating the times at which species originated and became extinct. Geographical regions and climatic conditions have varied throughout the Earth's history
. Since organisms are adapted to particular environments, the constantly changing conditions favoured species which adapted to new environments through the mechanism of natural selection
.
Despite the relative rarity of suitable conditions for fossilization, approximately 250,000 fossil species are known. The number of individual fossils this represents varies greatly from species to species, but many millions of fossils have been recovered: for instance, more than three million fossils from the last Ice Age
have been recovered from the La Brea Tar Pits
in Los Angeles. Many more fossils are still in the ground, in various geological formations known to contain a high fossil density, allowing estimates of the total fossil content of the formation to be made. An example of this occurs in South Africa's Beaufort Formation (part of the Karoo Supergroup
, which covers most of South Africa), which is rich in vertebrate fossils, including therapsids (reptile/mammal transitional forms
). It has been estimated that this formation contains 800 billion vertebrate fossils.
s are found that show intermediate forms in what had previously been a gap in knowledge, they are often popularly referred to as "missing links".
There is a gap of about 100 million years between the beginning of the Cambrian
period and the end of the Ordovician
period. The early Cambrian period was the period from which numerous fossils of sponges, cnidaria
ns (e.g., jellyfish
), echinoderm
s (e.g., eocrinoids), molluscs
(e.g., snail
s) and arthropod
s (e.g., trilobite
s) are found. The first animal that possessed the typical features of vertebrate
s, the Arandaspis
, was dated to have existed in the later Ordovician period. Thus few, if any, fossils of an intermediate type between invertebrate
s and vertebrates have been found, although likely candidates include the Burgess Shale
animal, Pikaia gracilens, and its Maotianshan shales
relatives, Myllokunmingia
, Yunnanozoon
, Haikouella lanceolata, and Haikouichthys
.
Some of the reasons for the incompleteness of fossil records are:
Due to an almost-complete fossil record found in North America
n sedimentary deposits from the early Eocene
to the present, the horse
provides one of the best examples of evolutionary history (phylogeny).
This evolutionary sequence starts with a small animal called Hyracotherium
(commonly referred to as Eohippus) which lived in North America about 54 million years ago, then spread across to Europe
and Asia
. Fossil remains of Hyracotherium show it to have differed from the modern horse in three important respects: it was a small animal (the size of a fox
), lightly built and adapted for running; the limbs were short and slender, and the feet elongated so that the digits were almost vertical, with four digits in the forelimb
s and three digits in the hindlimbs; and the incisor
s were small, the molar
s having low crowns with rounded cusp
s covered in enamel
.
The probable course of development of horses from Hyracotherium to Equus (the modern horse) involved at least 12 genera
and several hundred species
. The major trends seen in the development of the horse to changing environmental conditions may be summarized as follows:
Fossilized plants found in different strata show that the marsh
y, wooded country in which Hyracotherium lived became gradually drier. Survival now depended on the head being in an elevated position for gaining a good view of the surrounding countryside, and on a high turn of speed for escape from predators
, hence the increase in size and the replacement of the splayed-out foot by the hoofed foot. The drier, harder ground would make the original splayed-out foot unnecessary for support. The changes in the teeth can be explained by assuming that the diet changed from soft vegetation
to grass
. A dominant genus from each geological period has been selected to show the slow alteration of the horse lineage from its ancestral to its modern form.
s and island
s (biogeography
) can provide evidence of common descent and shed light on patterns of speciation
.
are capable of supporting a particular species in one geographic area, then one might assume that the same species would be found in a similar habitat in a similar geographic area, e.g. in Africa
and South America
. This is not the case. Plant and animal species are discontinuously distributed throughout the world:
Even greater differences can be found if Australia
is taken into consideration, though it occupies the same latitude
as much of South America and Africa. Marsupial
s like kangaroo
s, bandicoots, and quoll
s make up about half of Australia's indigenous mammal species. By contrast, marsupials are today totally absent from Africa and form a smaller portion of the mammalian fauna of South America, where opossums, shrew opossum
s, and the monito del monte
occur. The only living representatives of primitive egg-laying mammals (monotreme
s) are the echidna
s and the platypus
. The short-beaked echidna (Tachyglossus aculeatus) and its subspecies populate Australia, Tasmania
, New Guinea
, and Kangaroo Island
while the long-beaked echidna (Zaglossus bruijni) lives only in New Guinea. The platypus lives in the waters of eastern Australia. They have been introduced to Tasmania, King Island, and Kangaroo Island. These Monotremes are totally absent in the rest of the world. On the other hand, Australia is missing many groups of placental mammals that are common on other continents (carnivora
ns, artiodactyls, shrew
s, squirrel
s, lagomorphs), although it does have indigenous bat
s and murine
rodents; many other placentals, such as rabbit
s and fox
es, have been introduced there by humans.
Other animal distribution examples include bear
s, located on all continents excluding Africa, Australia and Antarctica, and the polar bear only located solely in the Arctic Circle and adjacent land masses. Penguins are located only around the South Pole despite similar weather conditions at the North Pole. Families of sirenians are distributed exclusively around the earth’s waters, where manatees are located in western Africa waters, northern South American waters, and West Indian waters only while the related family, the Dugong
s, are located only in Oceanic
waters north of Australia, and the coasts surrounding the Indian Ocean
Additionally, the now extinct Steller's Sea Cow
resided in the Bering Sea
.
The same kinds of fossils are found from areas known to be adjacent to one another in the past but which, through the process of continental drift
, are now in widely divergent geographic locations. For example, fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica, which can be dated to the Paleozoic
Era, at which time these regions were united as a single landmass called Gondwana
. Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates.
has played an important and historic role in the development of evolutionary biology. For purposes of biogeography
, islands are divided into two classes. Continental islands are islands like Great Britain
, and Japan
that have at one time or another been part of a continent. Oceanic islands, like the Hawaiian islands
, the Galapagos islands
and St. Helena, on the other hand are islands that have formed in the ocean and never been part of any continent. Oceanic islands have distributions of native plants and animals that are unbalanced in ways that make them distinct from the biota
s found on continents or continental islands. Oceanic islands do not have native terrestrial mammals (they do sometimes have bats and seals), amphibians, or fresh water fish. In some cases they have terrestrial reptiles (such as the iguanas and giant tortoises of the Galapagos islands) but often (for example Hawaii) they do not. This despite the fact that when species such as rats, goats, pigs, cats, mice, and cane toad
s, are introduced to such islands by humans they often thrive. Starting with Charles Darwin
, many scientists have conducted experiments and made observations that have shown that the types of animals and plants found, and not found, on such islands are consistent with the theory that these islands were colonized accidentally by plants and animals that were able to reach them. Such accidental colonization could occur by air, such as plant seeds carried by migratory birds, or bats and insects being blown out over the sea by the wind, or by floating from a continent or other island by sea, as for example by some kinds of plant seeds like coconuts that can survive immersion in salt water, and reptiles that can survive for extended periods on rafts of vegetation carried to sea by storms.
, lemurs of Madagascar
, the Komodo dragon
of Komodo
, the Dragon’s blood tree of Socotra
, Tuatara
of New Zealand, and others. However many such endemic species are related to species found on other nearby islands or continents; the relationship of the animals found on the Galapagos Islands to those found in South America is a well-known example. All of these facts, the types of plants and animals found on oceanic islands, the large number of endemic species found on oceanic islands, and the relationship of such species to those living on the nearest continents, are most easily explained if the islands were colonized by species from nearby continents that evolved into the endemic species now found there.
Other types of endemism do not have to include, in the strict sense, islands. Islands can mean isolated lakes or remote and isolated areas. Examples of these would include the highlands of Ethiopia
, Lake Baikal
, Fynbos
of South Africa
, forests of New Caledonia
, and others. Examples of endemic organisms living in isolated areas include the Kagu
of New Caledonia, cloud rat
s of the Luzon tropical pine forests
of the Philippines
, the boojum tree (Fouquieria columnaris) of the Baja California peninsula
, the Baikal Seal and the omul
of Lake Baikal.
s, members of the sunflower family on the Juan Fernandez Archipelago and wood weevils on St. Helena are called adaptive radiations because they are best explained by a single species colonizing an island (or group of islands) and then diversifying to fill available ecological niches. Such radiations can be spectacular; 800 species of the fruit fly family Drosophila
, nearly half the world's total, are endemic to the Hawaiian islands. Another illustrative example from Hawaii is the Silversword alliance
, which is a group of thirty species found only on those islands. Members range from the Silverswords that flower spectacularly on high volcanic slopes to trees, shrubs, vines and mats that occur at various elevations from mountain top to sea level, and in Hawaiian habitats that vary from deserts to rainforests. Their closest relatives outside Hawaii, based on molecular studies, are tarweeds found on the west coast of North America. These tarweeds have sticky seeds that facilitate distribution by migrant birds. Additionally, nearly all of the species on the island can be crossed and the hybrids are often fertile, and they have been hybridized experimentally with two of the west coast tarweed species as well. Continental islands have less distinct biota, but those that have been long separated from any continent also have endemic species and adaptive radiations, such as the 75 lemur
species of Madagascar
, and the eleven extinct moa
species of New Zealand
.
is an extinct species of seed fern plants from the Permian
. Glossopteris appears in the fossil record around the beginning of the Permian on the ancient continent of Gondwana
. Continental drift explains the current biogeography of the tree. Present day Glossopteris fossils are found in Permian strata in southeast South America, southeast Africa, all of Madagascar, northern India, all of Australia, all of New Zealand, and scattered on the southern and northern edges of Antarctica. During the Permian, these continents were connected as Gondwana (see figure 6a) in agreement with magnetic striping, other fossil distributions, and glacial scratches pointing away from the temperate climate of the South Pole during the Permian.
and that they were connected by land that is now part of Antarctica. Therefore combining the two theories scientists predicted that marsupials migrated from what is now South America across what is now Antarctica to what is now Australia between 40 and 30 million years ago. This hypothesis led paleontologists to Antarctica to look for marsupial fossils of the appropriate age. After years of searching they found, starting in 1982, fossils on Seymour Island
off the coast of the Antarctic Peninsula
of more than a dozen marsupial species that lived 35-40 million years ago.
provides an example of how fossil evidence can be used to reconstruct migration and subsequent evolution. The fossil record indicates that the evolution of camelid
s started in North America (see figure 6c), from which 6 million years ago they migrated across the Bering Strait into Asia and then to Africa, and 3.5 million years ago through the Isthmus of Panama into South America. Once isolated, they evolved along their own lines, giving rise to the Bactrian camel
and Dromedary
in Asia and Africa and the llama and its relatives
in South America. Camelids then went extinct in North America at the end of the last ice age
.
in the field and the laboratory. Scientists have observed and documented a multitude of events where natural selection is in action. The most well known examples are antibiotic resistance in the medical field along with better-known laboratory experiments documenting evolution's occurrence. Natural selection is tantamount to common descent in the fact that long-term occurrence and selection pressures can lead to the diversity of life on earth as found today. All adaptations—documented and undocumented changes concerned—are caused by natural selection (and a few other minor processes). The examples below are only a small fraction of the actual experiments and observations.
, like the spread of pesticide
resistant forms of plants and insects is evidence for evolution of species, and of change within species. Thus the appearance of vancomycin
resistant Staphylococcus aureus, and the danger it poses to hospital patients is a direct result of evolution through natural selection. The rise of Shigella
strains resistant to the synthetic antibiotic class of sulfonamides also demonstrates the generation of new information as an evolutionary process. Similarly, the appearance of DDT
resistance in various forms of Anopheles
mosquitoes, and the appearance of myxomatosis
resistance in breeding rabbit populations in Australia, are all evidence of the existence of evolution in situations of evolutionary selection pressure in species in which generations occur rapidly.
set up an experiment shortly before 1880, subjecting microbes to heat with the aim of forcing adaptive changes. His experiment ran for around seven years, and his published results were acclaimed, but he did not resume the experiment after the apparatus failed.
The E. coli long-term evolution experiment that began in 1988 under the leadership of Richard Lenski
is still in progress, and has shown adaptations including the evolution of a strain of E. coli that was able to grow on citric acid in the growth media.
has significant over-representation of an immune variant of the prion protein gene G127V versus non-immune alleles. Scientists postulate one of the reasons for the rapid selection of this genetic variant is the lethality of the disease in non-immune persons. Other reported evolutionary trends in other populations include a lengthening of the reproductive period, reduction in cholesterol levels, blood glucose and blood pressure.
is the inability to metabolize
lactose
, because of a lack of the required enzyme lactase
in the digestive system. The normal mammalian condition is for the young of a species to experience reduced lactase
production at the end of the weaning
period (a species-specific length of time). In humans, in non-dairy consuming societies, lactase production usually drops about 90% during the first four years of life, although the exact drop over time varies widely. However, certain human populations have a mutation on chromosome 2 which eliminates the shutdown in lactase production, making it possible for members of these populations to continue consumption of raw milk and other fresh and fermented dairy products throughout their lives without difficulty. This appears to be an evolutionarily recent adaptation to dairy consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral lifestyle.
are a strain of Flavobacterium
that is capable of digesting certain byproducts of nylon 6
manufacture. There is scientific consensus that the capacity to synthesize nylonase most probably developed as a single-step mutation that survived because it improved the fitness of the bacteria possessing the mutation. This is seen as a good example of evolution through mutation and natural selection that has been observed as it occurs.
dumped polychlorinated biphenyls (PCBs) in the Hudson River
from 1947 through 1976, tomcods living in the river were found to have evolved an increased resistance to the compound's toxic effects. At first the tomcod population was devastated, but it recovered. Scientists identified the genetic mutation that conferred the resistance. The mutated form was found to be present in 99 per cent of the surviving tomcods in the river, compared to fewer than 10 percent of the tomcods from other waters.
in England.
to convert gamma radiation into chemical energy for growth and were first discovered in 2007 as black molds growing inside and around the Chernobyl Nuclear Power Plant
. Research at the Albert Einstein College of Medicine
showed that three melanin-containing fungi, Cladosporium sphaerospermum, Wangiella dermatitidis, and Cryptococcus neoformans
, increased in biomass
and accumulated acetate
faster in an environment in which the radiation
level was 500 times higher than in the normal environment.
is wildlife
that is able to live or thrive in urban
environments. These types of environments can exert selection pressures on organism, often leading to new adaptations. For example, the weed Crepis sancta
, found in France, has two types of seed, heavy and fluffy. The heavy ones land nearby to the parent plant, whereas the fluffy seeds float further away on the wind. In urban environments, seeds that float far will often land on infertile concrete. Within about 5-12 generations, the weed has been found to evolve to produce significantly more heavy seeds than its rural relatives do. Other examples of urban wildlife are rock pigeon
s and species of crows adapting to city environments around the world; African penguins in Simons Town; baboon
s in South Africa
; and a variety of insects living in human habitations.
is the evolutionary process by which new biological species arise. Speciation can occur from a variety of different causes and are classified in various forms (e.g. allopatric, sympatric, polyploidization, etc.). Scientists have observed numerous examples of speciation in the laboratory and in nature, however, evolution has produced far more species than an observer would consider necessary. For example, there are well over 350,000 described species of beetles. Great examples of observed speciation come from the observations of island biogeography and the process of adaptive radiation, both explained in an earlier section. The examples shown below provide strong evidence for common descent and are only a small fraction of the instances observed.
, commonly referred to as Blackcaps, lives in Germany and flies southwest to Spain while a smaller group flies northwest to Great Britain during the winter. Gregor Rolshausen from the University of Freiburg
found that the genetic separation of the two populations is already in progress. The differences found have arisen in about 30 generations. With DNA sequencing, the individuals can be assigned to a correct group with an 85% accuracy. Stuart Bearhop from the University of Exeter
reported that birds wintering in England tend to mate only among themselves, and not usually with those wintering in the Mediterranean. It is still inference to say that the populations will become two different species, but researchers expect it due to the continued genetic and geographic separation.
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William R. Rice and George W. Salt found experimental evidence of sympatric speciation
in the common fruit fly
. They collected a population of Drosophila melanogaster from Davis, California
and placed the pupae into a habitat maze. Newborn flies had to investigate the maze to find food. The flies had three choices to take in finding food. Light and dark (phototaxis
), up and down (geotaxis), and the scent of acetaldehyde
and the scent of ethanol (chemotaxis
) were the three options. This eventually divided the flies into 42 spatio-temporal habitats.
They then cultured two strains that chose opposite habitats. One of the strains emerged early, immediately flying upward in the dark attracted to the acetaldehyde
. The other strain emerged late and immediately flew downward, attracted to light and ethanol. Pupae from the two strains were then placed together in the maze and allowed to mate at the food site. They then were collected. A selective penalty was imposed on the female flies that switched habitats. This entailed that none of their gametes would pass on to the next generation. After 25 generations of this mating test, it showed reproductive isolation between the two strains. They repeated the experiment again without creating the penalty against habitat switching and the result was the same; reproductive isolation was produced.
. Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after apple
s, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns
. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as the fact that six out of thirteen allozyme
loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4-6% hybridization rate) suggests that speciation is occurring.
is a species of mosquito
in the genus Culex
found in the London Underground
. It evolved from the overground species Culex pipiens.
This mosquito, although first discovered in the London Underground system, has been found in underground systems around the world. It is suggested that it may have adapted to human-made underground systems since the last century from local above-ground Culex pipiens, although more recent evidence suggests that it is a southern mosquito variety related to Culex pipiens that has adapted to the warm underground spaces of northern cities.
The species have very different behaviours, are extremely difficult to mate, and with different allele frequency, consistent with genetic drift during a founder event. More specifically, this mosquito, Culex pipiens molestus, breeds all-year round, is cold intolerant, and bites rats, mice, and humans, in contrast to the above ground species Culex pipiens that is cold tolerant, hibernates in the winter, and bites only birds. When the two varieties were cross-bred the eggs were infertile suggesting reproductive isolation.
The fundamental results still stands: the genetic data indicate that the molestus form in the London Underground mosquito appeared to have a common ancestry, rather than the population at each station being related to the nearest above-ground population (i.e. the pipiens form). Byrne and Nichols' working hypothesis was that adaptation to the underground environment had occurred locally in London once only.
These widely separated populations are distinguished by very minor genetic differences, which suggest that the molestus form developed: a single mtDNA difference shared among the underground populations of ten Russian cities; a single fixed microsatellite difference in populations spanning Europe, Japan, Australia, the middle East and Atlantic islands.
—is a small fish that lives in the Sulfur Caves
of Mexico. Michael Tobler from the Texas A&M University
has studied the fish for years and found that two distinct populations of mollies—the dark interior fish and the bright surface water fish—are becoming more genetically divergent. The populations have no obvious barrier separating the two; however, it was found that the mollies are hunted by a large water bug (Belostoma spp). Tobler collected the bug and both types of mollies, placed them in large plastic bottles, and put them back in the cave. After a day, it was found that, in the light, the cave-adapted fish endured the most damage, with four out of every five stab-wounds from the water bugs sharp mouthparts. In the dark, the situation was the opposite. The mollies’ senses can detect a predator’s threat in their own habitats, but not in the other ones. Moving from one habitat to the other significantly increases the risk of dying. Tobler plans on further experiments, but believes that it is a good example of the rise of a new species.
Kirsten Bomblies
et al. from the Max Planck Institute for Developmental Biology
discovered that two genes passed down by each parent of the thale cress plant, Arabidopsis thaliana
. When the genes are passed down, it ignites a reaction in the hybrid plant that turns its own immune system against it. In the parents, the genes were not detrimental, but they evolved separately to react defectively when combined.
To test this, Bomblies crossed 280 genetically different strains of Arabidopsis in 861 distinct ways and found that 2 per cent of the resulting hybrids were necrotic. Along with allocating the same indicators, the 20 plants also shared a comparable collection of genetic activity in a group of 1,080 genes. In almost all of the cases, Bomblies discovered that only two genes were required to cause the autoimmune response. Bomblies looked at one hybrid in detail and found that one of the two genes belonged to the NB-LRR class
, a common group of disease resistance genes involved in recognizing new infections. When Bomblies removed the problematic gene, the hybrids developed normally.
Over successive generations, these incompatibilities could create divisions between different plant strains, reducing their chances of successful mating and turning distinct strains into separate species.
(Ursus maritimus). The polar bear is related to the brown bear
(Ursus arctos) but they can still interbreed and produce fertile offspring. However, it has acquired significant physiological differences from the brown bear. These differences allow the polar bear to comfortably survive in conditions that the brown bear could not including the ability to swim sixty miles or more at a time in freezing waters, to blend in with the snow, and to stay warm in the arctic environment. Additionally, the elongation of the neck makes it easier to keep their heads above water while swimming and the oversized webbed feet that act as paddles when swimming. The polar bear has also evolved small papillae and vacuole-like suction cups on the soles to make them less likely to slip on the ice; feet covered with heavy matting to protect the bottoms from intense cold and provide traction; smaller ears to reduce the loss of heat; eyelids that act like sunglasses; accommodations for their all-meat diet; a large stomach capacity to enable opportunistic feeding; and the ability to fast for up to nine months while recycling their urea.
includes all intergeneric hybrids between the genera Raphanus
(radish) and Brassica
(cabbages, etc.).
The Raphanobrassica is an allopolyploid cross between the radish
(Raphanus sativus) and cabbage
(Brassica oleracea). Plants of this parentage are now known as radicole. Two other fertile forms of Raphanobrassica are known. Raparadish, an allopolyploid hybrid between Raphanus sativus and Brassica rapa is grown as a fodder crop. "Raphanofortii" is the allopolyploid hybrid between Brassica tournefortii
and Raphanus caudatus.
The Raphanobrassica is a fascinating plant, because (in spite of its hybrid nature), it is not sterile. This has led some botanists to propose that the accidental hybridization of a flower by pollen of another species in nature could be a mechanism of speciation common in higher plants.
are one example where hybrid speciation
has been observed. In the early 20th century, humans introduced three species of goatsbeard into North America. These species, the western salsify (Tragopogon dubius), the meadow salsify (Tragopogon pratensis), and the oyster plant (Tragopogon porrifolius), are now common weeds in urban wastelands. In the 1950s, botanists found two new species in the regions of Idaho
and Washington, where the three already known species overlapped. One new species, Tragopogon miscellus, is a tetraploid hybrid of T. dubius and T. pratensis. The other new species, Tragopogon mirus, is also an allopolyploid, but its ancestors were T. dubius and T. porrifolius. These new species are usually referred to as "the Ownbey hybrids" after the botanist who first described them. The T. mirus population grows mainly by reproduction of its own members, but additional episodes of hybridization continue to add to the T. mirus population.
T. dubius and T. pratensis mated in Europe but were never able to hybridize. A study published in March 2011 found that when these two plants were introduced to North America in the 1920s, they mated and doubled the number of chromosomes in there hybrid Tragopogon miscellus allowing for a “reset” of its genes, which in turn, allows for greater genetic variation. Professor Doug Soltis of the University of Florida
said, “We caught evolution in the act…New and diverse patterns of gene expression may allow the new species to rapidly adapt in new environments”. This observable event of speciation through hybridization further advances the evidence for the common descent of organisms and the time frame in which the new species arose in its new environment. The hybridizations have been reproduced artificially in laboratories from 2004 to present day.
) grow alongside each other. Sometime in the early 20th century, an accidental doubling of the number of chromosomes in an S. × baxteri plant led to the formation of a new fertile species.
Senecio squalidus
(also known as Oxford ragwort) and the self-compatible Senecio vulgaris (also known as Common groundsel). Like S. vulgaris, S. eboracensis is self-compatible, however, it shows little or no natural crossing with its parent species, and is therefore reproductively isolated, indicating that strong breed barriers exist between this new hybrid and its parents.
It resulted from a backcrossing
of the F1 hybrid
of its parents to S. vulgaris. S. vulgaris is native to Britain, while S. squalidus was introduced from Sicily in the early 18th century; therefore, S. eboracensis has speciated from those two species within the last 300 years.
Other hybrids descended from the same two parents are known. Some are infertile, such as S. x baxteri. Other fertile hybrids are also known, including S. vulgaris var. hibernicus, now common in Britain, and the allohexaploid
S. cambrensis
, which according to molecular evidence probably originated independently at least three times in different locations. Morphological and genetic evidence support the status of S. eboracensis as separate from other known hybrids.
demonstrates the diversity that can exist among organisms that share a relatively recent common ancestor. In artificial selection, one species is bred selectively at each generation, allowing only those organisms that exhibit desired characteristics to reproduce. These characteristics become increasingly well developed in successive generations. Artificial selection was successful long before science discovered the genetic basis. Examples of artificial selection would be dog breeding
, genetically modified food
, flower breeding, cultivation of foods such as wild cabbage
, and others.
allows the iteration
of self changing complex system
s to be studied, allowing a mathematical understanding of the nature of the processes behind evolution; providing evidence for the hidden causes of known evolutionary events. The evolution of specific cellular mechanisms like spliceosome
s that can turn the cell's genome into a vast workshop of billions of interchangeable parts that can create tools that create tools that create tools that create us can be studied for the first time in an exact way.
"It has taken more than five decades, but the electronic computer is now powerful enough to simulate evolution" assisting bioinformatics
in its attempt to solve biological problems.
Computational evolutionary biology has enabled researchers to trace the evolution of a large number of organisms by measuring changes in their DNA, rather than through physical taxonomy or physiological observations alone. It has compared entire genomes permitting the study of more complex evolutionary events, such as gene duplication
, horizontal gene transfer
, and the prediction of factors important in speciation. It has also helped build complex computational models of populations to predict the outcome of the system over time and track and share information on an increasingly large number of species and organisms.
Future endeavors are to reconstruct a now more complex tree of life.
Christoph Adami, a professor at the Keck Graduate Institute made this point in Evolution of biological complexity:
David J. Earl and Michael W. Deem—professors at Rice University
made this point in Evolvability is a selectable trait:
"Computer simulations of the evolution of linear sequences have demonstrated the importance of recombination of blocks of sequence rather than point mutagenesis alone. Repeated cycles of point mutagenesis, recombination, and selection should allow in vitro molecular evolution of complex sequences, such as proteins." Evolutionary molecular engineering, also called directed evolution or in vitro molecular evolution involves the iterated cycle of mutation, multiplication with recombination, and selection of the fittest of individual molecules (proteins, DNA, and RNA). Natural evolution can be relived showing us possible paths from catalytic cycles based on proteins to based on RNA to based on DNA.
developed an artificial life
computer program with the ability to detail the evolution of complex systems. The system uses values set to determine random mutations and allows for the effect of natural selection to conserve beneficial traits. The program was dubbed Avida and starts with an artificial petri dish where organisms reproduce and perform mathematical calculations to acquire rewards of more computer time for replication. The program randomly adds mutations to copies of the artificial organisms to allow for natural selection. As the artificial life reproduced, different lines adapted and evolved depending on their set environments. The beneficial side to the program is that it parallels that of real life at rapid speeds.
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
and provided a wealth of information on the natural processes by which the variety and diversity of life on Earth developed. This evidence supports the modern evolutionary synthesis
Modern evolutionary synthesis
The modern evolutionary synthesis is a union of ideas from several biological specialties which provides a widely accepted account of evolution...
, the current scientific theory
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...
that explains how and why life changes over time. Evolutionary biologists document the fact of common descent
Common descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
: making testable predictions, testing hypotheses, and developing theories that illustrate and describe its causes.
Comparison of the genetic sequence
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
of organisms has revealed that organisms that are phylogenetically
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
close have a higher degree of sequence similarity than organisms that are phylogenetically distant. Further evidence for common descent comes from genetic detritus such as pseudogene
Pseudogene
Pseudogenes are dysfunctional relatives of known genes that have lost their protein-coding ability or are otherwise no longer expressed in the cell...
s, regions of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
that are orthologous to a gene in a related organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
, but are no longer active and appear to be undergoing a steady process of degeneration.
Fossils are important for estimating when various lineages developed in geologic time. As fossilization is an uncommon occurrence, usually requiring hard body parts and death near a site where sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Evidence
Scientific evidence
Scientific evidence has no universally accepted definition but generally refers to evidence which serves to either support or counter a scientific theory or hypothesis. Such evidence is generally expected to be empirical and properly documented in accordance with scientific method such as is...
of organisms prior to the development of hard body parts such as shells, bones and teeth is especially scarce, but exists in the form of ancient microfossils, as well as impressions of various soft-bodied organisms. The comparative study of the anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
of groups of animals shows structural features that are fundamentally similar or homologous, demonstrating phylogenetic and ancestral relationships with other organism, most especially when compared with fossils of ancient extinct
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
organisms. Vestigial structures and comparisons in embryonic development
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
are largely a contributing factor in anatomical resemblance in concordance with common descent. Since metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms’ physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
and biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
. Many lineages diverged at different stages of development, so it is possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor. Universal biochemical organization and molecular variance patterns in all organisms also show a direct correlation with common descent.
Further evidence comes from the field of biogeography
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
because evolution with common descent provides the best and most thorough explanation for a variety of facts concerning the geographical distribution of plants and animals across the world. This is especially obvious in the field of island biogeography
Island biogeography
Island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of natural communities. The theory was developed to explain species richness of actual islands...
. Combined with the theory of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
common descent provides a way to combine facts about the current distribution of species with evidence from the fossil record to provide a logically consistent explanation of how the distribution of living organisms has changed over time.
The development and spread of antibiotic resistant
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...
bacteria, like the spread of pesticide resistant forms of plants and insects provides evidence that evolution due to natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
is an ongoing process in the natural world. Alongside this, are observed instances of the separation of populations of species into sets of new species (speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
). Speciation has been observed directly and indirectly in the lab and in nature. Multiple forms of such have been described and documented as examples for individual modes of speciation. Furthermore, evidence of common descent extends from direct laboratory experimentation with the artificial selection of organisms—historically and currently—and other controlled experiments involving many of the topics in the article. This article explains the different types of evidence for evolution with common descent along with many specialized examples of each.
Evidence from comparative physiology and biochemistry
Genetics
Sequence alignment
In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural, or evolutionary relationships between the sequences. Aligned sequences of nucleotide or amino acid residues are...
examines the relationship between the DNA sequences of different species, producing several lines of evidence that confirm Darwin's original hypothesis of common descent. If the hypothesis of common descent is true, then species that share a common ancestor will have inherited that ancestor's DNA sequence. They will have inherited mutations unique to that ancestor. More closely related species will have a greater fraction of identical sequence and will have shared substitutions when compared to more distantly related species.
The simplest and most powerful evidence is provided by phylogenetic reconstruction
Computational phylogenetics
Computational phylogenetics is the application of computational algorithms, methods and programs to phylogenetic analyses. The goal is to assemble a phylogenetic tree representing a hypothesis about the evolutionary ancestry of a set of genes, species, or other taxa...
. Such reconstructions, especially when done using slowly evolving protein sequences, are often quite robust and can be used to reconstruct a great deal of the evolutionary history of modern organisms (and even in some instances such as the recovered gene sequences of mammoth
Mammoth
A mammoth is any species of the extinct genus Mammuthus. These proboscideans are members of Elephantidae, the family of elephants and mammoths, and close relatives of modern elephants. They were often equipped with long curved tusks and, in northern species, a covering of long hair...
s, Neanderthal
Neanderthal
The Neanderthal is an extinct member of the Homo genus known from Pleistocene specimens found in Europe and parts of western and central Asia...
s or T. rex
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...
, the evolutionary history of extinct organisms). These reconstructed phylogenies recapitulate the relationships established through morphological and biochemical studies. The most detailed reconstructions have been performed on the basis of the mitochondrial genomes shared by all eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
organisms, which are short and easy to sequence; the broadest reconstructions have been performed either using the sequences of a few very ancient proteins or by using ribosomal RNA
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
sequence.
Phylogenetic relationships also extend to a wide variety of nonfunctional sequence elements, including repeats, transposons, pseudogenes, and mutations in protein-coding sequences that do not result in changes in amino-acid sequence. While a minority of these elements might later be found to harbor function, in aggregate they demonstrate that identity must be the product of common descent rather than common function.
Universal biochemical organisation and molecular variance patterns
All known extant organisms are based on the same fundamental biochemical organisation: genetic information encoded as nucleic acid (DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, or RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
for viruses), transcribed into RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, then translated into protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s (that is, polymers of amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s) by highly conserved ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s. Perhaps most tellingly, the Genetic Code
Genetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....
(the "translation table" between DNA and amino acids) is the same for almost every organism, meaning that a piece of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
in a bacterium
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
codes for the same amino acid as in a human cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
is used as energy currency by all extant life. A deeper understanding of developmental biology
Evolutionary developmental biology
Evolutionary developmental biology is a field of biology that compares the developmental processes of different organisms to determine the ancestral relationship between them, and to discover how developmental processes evolved...
shows that common morphology is, in fact, the product of shared genetic elements. For example, although camera-like eyes are believed to have evolved independently on many separate occasions, they share a common set of light-sensing proteins (opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
s), suggesting a common point of origin for all sighted creatures. Another noteworthy example is the familiar vertebrate body plan, whose structure is controlled by the homeobox (Hox) family of genes.
DNA sequencing
Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and the resulting phylogeneticPhylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
trees are typically congruent with traditional taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
, and are often used to strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
, 1.6% from gorilla
Gorilla
Gorillas are the largest extant species of primates. They are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Africa. Gorillas are divided into two species and either four or five subspecies...
s, and 6.6% from baboon
Baboon
Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger...
s. Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other ape
Ape
Apes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. The apes are native to Africa and South-east Asia, although in relatively recent times humans have spread all over the world...
s. The sequence of the 16S ribosomal RNA
16S ribosomal RNA
16S ribosomal RNA is a component of the 30S subunit of prokaryotic ribosomes. It is approximately 1.5kb in length...
gene, a vital gene encoding a part of the ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by Carl Woese
Carl Woese
Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...
, resulted in the three-domain system
Three-domain system
The three-domain system is a biological classification introduced by Carl Woese in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria...
, arguing for two major splits in the early evolution of life. The first split led to modern Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and the subsequent split led to modern Archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
and Eukaryotes.
Endogenous retroviruses
Endogenous retrovirusEndogenous retrovirus
Endogenous retroviruses are sequences in the genome thought to be derived from ancient viral infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome....
es (or ERVs) are remnant sequences in the genome left from ancient viral infections in an organism. The retroviruses (or virogenes) are always passed on
Inheritance
Inheritance is the practice of passing on property, titles, debts, rights and obligations upon the death of an individual. It has long played an important role in human societies...
to the next generation of that organism which received the infection. This leaves the virogene left in the genome. Because this event is rare and random, finding identical chromosomal positions of a virogene in two different species suggests common ancestry. See examples of humans and cats below.
Proteins
The proteomicProteome
The proteome is the entire set of proteins expressed by a genome, cell, tissue or organism. More specifically, it is the set of expressed proteins in a given type of cells or an organism at a given time under defined conditions. The term is a portmanteau of proteins and genome.The term has been...
evidence also supports the universal ancestry of life. Vital protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, such as the ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
, DNA polymerase
DNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
, and RNA polymerase
RNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
, are found in everything from the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions. Higher organisms have evolved additional protein subunit
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
s, largely affecting the regulation and protein-protein interaction
Protein-protein interaction
Protein–protein interactions occur when two or more proteins bind together, often to carry out their biological function. Many of the most important molecular processes in the cell such as DNA replication are carried out by large molecular machines that are built from a large number of protein...
of the core. Other overarching similarities between all lineages of extant organisms, such as DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, amino acids, and the lipid bilayer
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
, give support to the theory of common descent. Phylogenetic analyses of protein sequences from various organisms produce similar trees of relationship between all organisms. The chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
of DNA, RNA, and amino acids is conserved across all known life. As there is no functional advantage to right- or left-handed molecular chirality, the simplest hypothesis is that the choice was made randomly by early organisms and passed on to all extant life through common descent. Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogene
Pseudogene
Pseudogenes are dysfunctional relatives of known genes that have lost their protein-coding ability or are otherwise no longer expressed in the cell...
s, "dead" genes which steadily accumulate mutations.
Pseudogenes
Pseudogenes, also known as noncoding DNANoncoding DNA
In genetics, noncoding DNA describes components of an organism's DNA sequences that do not encode for protein sequences. In many eukaryotes, a large percentage of an organism's total genome size is noncoding DNA, although the amount of noncoding DNA, and the proportion of coding versus noncoding...
, are extra DNA in a genome that do not get transcribed into RNA to synthesize proteins. Some of this noncoding DNA has known functions, but much of it has no known function and is called "Junk DNA". This is an example of a vestige since replicating these genes uses energy, making it a waste in many cases. Pseudogenes make up 99% of the human genome (1% working DNA). A pseudogene can be produced when a coding gene accumulates mutations that prevent it from being transcribed, making it non-functional. But since it is not transcribed, it may disappear without affecting fitness, unless it has provided some new beneficial function as non-coding DNA. Non-functional pseudogenes may be passed on to later species, thereby labeling the later species as descended from the earlier species.
Other mechanisms
There is also a large body of molecular evidence for a number of different mechanisms for large evolutionary changes, among them: genomeGenome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
and gene duplication
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
, which facilitates rapid evolution by providing substantial quantities of genetic material under weak or no selective constraints; horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
, the process of transferring genetic material to another cell that is not an organism's offspring, allowing for species to acquire beneficial genes from each other; and recombination
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
, capable of reassorting large numbers of different allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s and of establishing reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
. The Endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
explains the origin of mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
and plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s (e.g. chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s), which are organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s of eukaryotic cells, as the incorporation of an ancient prokaryotic
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
cell into ancient eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cell. Rather than evolving eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s slowly, this theory offers a mechanism for a sudden evolutionary leap by incorporating the genetic material and biochemical composition of a separate species. Evidence supporting this mechanism has been found in the protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
Hatena
Hatena
Hatena arenicola is a species of single-celled eukaryotes described in 2006. The species is a flagellate, and can resemble a plant at one stage of its life, in which it carries a photosynthesizing alga inside itself, or an animal, acting as predator in another stage of its life...
: as a predator it engulfs a green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
cell, which subsequently behaves as an endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
, nourishing Hatena, which in turn loses its feeding apparatus and behaves as an autotroph
Autotroph
An autotroph, or producer, is an organism that produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions . They are the producers in a food chain, such as plants on land or algae in water...
.
Since metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor or by detecting their physical manifestations. As an example, the appearance of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
is linked to the evolution of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
.
Feline endogenous retroviruses
Cats (FelidaeFelidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...
) present another example of virogene sequences in common descent. The standard phylogenetic tree for Felidae have smaller cats (Felis chaus, Felis silvestris, Felis nigripes, and Felis catus) diverging from larger cats such as the subfamily Pantherinae
Pantherinae
Pantherinae is the subfamily of the family Felidae, which includes the genera Panthera, Uncia and Neofelis.The divergence of Pantherinae from Felinae has been ranked between six and ten million years ago. DNA analysis suggests that the snow leopard Uncia uncia is basal to the entire Pantherinae and...
and other carnivores
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
. The small cats have an ERV where the larger cats do not suggesting that the gene was inserted into the ancestor of the small cats after the larger cats had diverged.
Chromosome 2 in humans
Hominidae
The Hominidae or include them .), as the term is used here, form a taxonomic family, including four extant genera: chimpanzees , gorillas , humans , and orangutans ....
. All Hominidae (with the exception of humans) have 24 pairs of chromosomes. Humans have only 23 pairs. Human chromosome 2 is a result of an end-to-end fusion of two ancestral chromosomes.
The evidence for this includes:
- The correspondence of chromosome 2 to two ape chromosomes. The closest human relative, the common chimpanzeeCommon ChimpanzeeThe common chimpanzee , also known as the robust chimpanzee, is a great ape. Colloquially, the common chimpanzee is often called the chimpanzee , though technically this term refers to both species in the genus Pan: the common chimpanzee and the closely related bonobo, formerly called the pygmy...
, has near-identical DNA sequences to human chromosome 2, but they are found in two separate chromosomes. The same is true of the more distant gorillaGorillaGorillas are the largest extant species of primates. They are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Africa. Gorillas are divided into two species and either four or five subspecies...
and orangutanOrangutanOrangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping...
. - The presence of a vestigialVestigial structureVestigiality describes homologous characters of organisms that have seemingly lost all or most of their original function in a species through evolution. These may take various forms such as anatomical structures, behaviors and biochemical pathways. Some of these disappear early in embryonic...
centromereCentromereA centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
. Normally a chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere. - The presence of vestigial telomereTelomereA telomere is a region of repetitive DNA sequences at the end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos "end" and merοs "part"...
s. These are normally found only at the ends of a chromosome, but in chromosome 2 there are additional telomere sequences in the middle.
Chromosome 2 thus presents very strong evidence in favour of the common descent of humans and other ape
Ape
Apes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. The apes are native to Africa and South-east Asia, although in relatively recent times humans have spread all over the world...
s. According to J. W. IJdo, "We conclude that the locus cloned in cosmids c8.1 and c29B is the relic of an ancient telomere-telomere fusion and marks the point at which two ancestral ape chromosomes fused to give rise to human chromosome 2."
Cytochrome c
A classic example of biochemical evidence for evolution is the variance of the ubiquitous (i.e. all living organisms have it, because it performs very basic life functions) proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
Cytochrome c
Cytochrome c
The Cytochrome complex, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins. Cytochrome c is a highly soluble protein, unlike other cytochromes, with a solubility of about 100 g/L and is an...
in living cells. The variance of cytochrome c of different organisms is measured in the number of differing amino acids, each differing amino acid being a result of a base pair
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
substitution, a mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
. If each differing amino acid is assumed to be the result of one base pair substitution, it can be calculated how long ago the two species diverged by multiplying the number of base pair substitutions by the estimated time it takes for a substituted base pair of the cytochrome c gene to be successfully passed on. For example, if the average time it takes for a base pair of the cytochrome c gene to mutate is N years, the number of amino acids making up the cytochrome c protein in monkeys differ by one from that of humans, this leads to the conclusion that the two species diverged N years ago.
The primary structure of cytochrome c consists of a chain of about 100 amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s. Many higher order organisms possess a chain of 104 amino acids.
The cytochrome c molecule has been extensively studied for the glimpse it gives into evolutionary biology. Both chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
and turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
s have identical sequence homology (amino acid for amino acid), as do pig
Pig
A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...
s, cows and sheep. Both human
Human
Humans are the only living species in the Homo genus...
s and chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
s share the identical molecule, while rhesus monkeys share all but one of the amino acids: the 66th amino acid is isoleucine
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....
in the former and threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
in the latter.
What makes these homologous similarities particularly suggestive of common ancestry in the case of cytochrome C, in addition to the fact that the phylogenies derived from them match other phylogenies very well, is the high degree of functional redundancy of the cytochrome C molecule. The different existing configurations of amino acids do not significantly affect the functionality of the protein, which indicates that the base pair substitutions are not part of a directed design, but the result of random mutations that aren't subject to selection.
Human endogenous retroviruses
Humans contain many ERVs that comprise nearly 8% of the genome. Humans and chimps share seven different instances of virogenes while all primates share similar retroviruses congruent with phylogeny.Recent African origin of modern humans
Mathematical models of evolution, pioneered by the likes of Sewall WrightSewall Wright
Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of...
, Ronald Fisher
Ronald Fisher
Sir Ronald Aylmer Fisher FRS was an English statistician, evolutionary biologist, eugenicist and geneticist. Among other things, Fisher is well known for his contributions to statistics by creating Fisher's exact test and Fisher's equation...
and J. B. S. Haldane
J. B. S. Haldane
John Burdon Sanderson Haldane FRS , known as Jack , was a British-born geneticist and evolutionary biologist. A staunch Marxist, he was critical of Britain's role in the Suez Crisis, and chose to leave Oxford and moved to India and became an Indian citizen...
and extended via diffusion theory by Motoo Kimura
Motoo Kimura
was a Japanese biologist best known for introducing the neutral theory of molecular evolution in 1968. He became one of the most influential theoretical population geneticists. He is remembered in genetics for his innovative use of diffusion equations to calculate the probability of fixation of...
, allow predictions about the genetic structure of evolving populations. Direct examination of the genetic structure of modern populations via DNA sequencing has allowed verification of many of these predictions. For example, the Out of Africa theory of human origins, which states that modern humans developed in Africa and a small sub-population migrated out (undergoing a population bottleneck
Population bottleneck
A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....
), implies that modern populations should show the signatures of this migration pattern. Specifically, post-bottleneck populations (Europeans and Asians) should show lower overall genetic diversity and a more uniform distribution of allele frequencies compared to the African population. Both of these predictions are borne out by actual data from a number of studies.
Evidence from comparative anatomy
Comparative study of the anatomyComparative anatomy
Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny .-Description:...
of groups of animals or plants reveals that certain structural features are basically similar. For example, the basic structure of all flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s consists of sepal
Sepal
A sepal is a part of the flower of angiosperms . Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petals when the flower is in bud...
s, petal
Petal
Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...
s, stigma, style and ovary
Gynoecium
Gynoecium is most commonly used as a collective term for all carpels in a flower. A carpel is the ovule and seed producing reproductive organ in flowering plants. Carpels are derived from ovule-bearing leaves which evolved to form a closed structure containing the ovules...
; yet the size, colour, number
Number
A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers....
of parts and specific structure are different for each individual species.
Atavisms
An atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations ago. Atavisms occur because genes for previously existing phenotypical features are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them. Some examples of this are hind-legged snakes or whales (see specific example below); the extra toes of ungulateUngulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
s that do not even reach the ground, chicken's teeth, reemergence of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
in Hieracium pilosella and Crotoniidae
Crotoniidae
Crotoniidae are a family of mites of the Desmonomata group that may be the first animal lineage to have abandoned sexual reproduction and then reevolved it...
; and humans with tails, extra nipple
Nipple
In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
s, and large canine teeth
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...
.
Evolutionary developmental biology and embryonic development
Evolutionary developmental biology is the biological field that compares the developmental process of different organisms to determine ancestral relationships between species. A large variety of organism’s genomes contain a small fraction of genes that control the organisms development. Hox genes are an example of these types of nearly universal genes in organisms pointing to an origin of common ancestry. Embryological evidence comes from the development of organisms at the embryological level with the comparison of different organisms embryos similarity. Remains of ancestral traits often appear and disappear in different stages of the embryological development process. Examples include such as hair growth and loss (lanugoLanugo
Lanugo is fine, downy hair as a type of fur. It is often found in teratomas .-Fetal development:Lanugo grows on fetuses as a normal part of gestation, but is usually shed and replaced by vellus hair at about 33 to 36 weeks of gestational age...
) during human development; the appearance of transitions from fish to amphibians to reptiles and then to mammals in all mammal embryos; development and degeneration of a yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
; terrestrial frogs and salamanders passing through the larval stage within the egg—with features of typically aquatic larvae—but hatch ready for life on land; and the appearance of gill-like structures (pharyngeal arch) in vertebrate embryo development. Note that in fish the arches become gills while in humans, for example, they become the pharynx
Human pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
.
Homologous structures and divergent (adaptive) evolution
If widely separated groups of organisms are originated from a common ancestry, they are expected to have certain basic features in common. The degree of resemblance between two organisms should indicate how closely related they are in evolution:- Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups which have a lot in common;
- In deciding how closely related two animals are, a comparative anatomist looks for structureStructureStructure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...
s that are fundamentally similar, even though they may serve different functions in the adultAdultAn adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age....
. Such structures are described as homologousHomology (biology)Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
and suggest a common origin. - In cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic developmentEmbryogenesisEmbryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
. A similar developmental origin suggests they are the same structure, and thus likely to be derived from a common ancestor.
When a group of organisms share a homologous structure which is specialized to perform a variety of functions in order to adapt different environmental conditions and modes of life are called adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
. The gradual spreading of organisms with adaptive radiation is known as divergent evolution
Divergent evolution
Divergent evolution is the accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations allowing differentiated fixation...
.
Nested hierarchies and classification
TaxonomyBiological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
is based on the fact that all organisms are related to each other in nested hierarchies based on shared characteristics. Most existing species can be organized rather easily in a nested hierarchical classification. This is evident from the Linnaean classification scheme. Based on shared derived characters, closely related organisms can be placed in one group (such as a genus), several genera can be grouped together into one family, several families can be grouped together into an order, etc. The existence of these nested hierarchies was recognized by many biologists before Darwin, but he showed that his theory of evolution with its branching pattern of common descent could explain them. Darwin described how common descent could provide a logical basis for classification:
Vestigial structures
A strong and direct evidence for common descent comes from vestigial structures. Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in the ancestral species, are called vestigial organs. They are usually degenerated or underdeveloped. The existence of vestigial organs can be explained in terms of changes in the environment or modes of life of the species. Those organs are typically functional in the ancestral species but are now either nonfunctional or re-purposed. Examples are the pelvic girdles of whales, haltere (hind wingWing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
s) of flies
Fly
True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...
and mosquitos, wings of flightless birds such as ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...
es, and the leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
of some xerophyte
Xerophyte
A xerophyte or xerophytic organism is a plant which has adapted to survive in an environment that lacks water, such as a desert. Xerophytic plants may have adapted shapes and forms or internal functions that reduce their water loss or store water during long periods of dryness...
s (e.g. cactus
Cactus
A cactus is a member of the plant family Cactaceae. Their distinctive appearance is a result of adaptations to conserve water in dry and/or hot environments. In most species, the stem has evolved to become photosynthetic and succulent, while the leaves have evolved into spines...
) and parasitic plant
Parasitic plant
A parasitic plant is one that derives some or all of its sustenance from another plant. About 4,100 species in approximately 19 families of flowering plants are known. Parasitic plants have a modified root, the haustorium, that penetrates the host plant and connects to the xylem, phloem, or...
s (e.g. dodder
Cuscuta
Cuscuta is a genus of about 100-170 species of yellow, orange or red parasitic plants. Formerly treated as the only genus in the family Cuscutaceae, recent genetic research by the Angiosperm Phylogeny Group has shown that it is correctly placed in the morning glory family, Convolvulaceae...
). However, vestigial structures may have their original function replaced with another. For example, the halteres
Halteres
Halteres are small knobbed structures modified from the hind wings in some two-winged insects. They are flapped rapidly and function as gyroscopes, informing the insect about rotation of the body during flight....
in dipterist
Fly
True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...
s help balance the insect while in flight and the wings of ostriches are used in mating
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
rituals.
Specific examples
Hind structures in whales
WhaleWhale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s possess internally reduced hind parts such as the pelvis and hind legs (Fig. 5a). Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs. On October 28, 2006, a four-finned bottlenose dolphin was caught and studied due to its extra set of hind limbs. These legged Cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...
display an example of an atavism predicted from their common ancestry.
Insect mouthparts
Many different species of insects have mouthparts derived from the same embryonic structures, indicating that the mouthparts are modifications of a common ancestor's original features. These include a labrum (upper lip), a pair of mandibleMandible (insect)
Insect mandibles are a pair of appendages near the insect’s mouth, and the most anterior of the three pairs of oral appendages . Their function is typically to grasp, crush, or cut the insect’s food, or to defend against predators or rivals...
s, a hypopharynx
Hypopharynx
In human anatomy, the hypopharynx is the bottom part of the pharynx, and is the part of the throat that connects to the esophagus....
(floor of mouth), a pair of maxillae, and a labium. (Fig. 5b) Evolution has caused enlargement and modification of these structures in some species, while it has caused the reduction and loss of them in other species. The modifications enable the insects to exploit a variety of food materials:
Other arthropod appendages
Insect mouthparts and antennae are considered homologues of insect legs. Parallel developments are seen in some arachnidArachnid
Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...
s: The anterior pair of legs may be modified as analogues of antennae, particularly in whip scorpions
Uropygid
Thelyphonida is an arachnid order comprising invertebrates commonly known as vinegaroons . They are often called uropygids in the scientific community after the former order Uropygi...
, which walk on six legs. These developments provide support for the theory that complex modifications often arise by duplication of components, with the duplicates modified in different directions.
Pelvic structure of dinosaurs
Similar to the pentadactyl limb in mammals, the earliest dinosaurs split into two distinct orders—the saurischiaSaurischia
Saurischia meaning 'lizard' and ischion meaning 'hip joint') is one of the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure...
and ornithischia
Ornithischia
Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...
. They are classified as one or the other in accordance with what the fossils demonstrate. Figure 5c, shows that early saurischians resembled early ornithischians. The pattern of the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
in all species of dinosaurs is an example of homologous structures. Each order of dinosaur has slightly differing pelvis bones providing evidence of common descent. Additionally, modern birds show a similarity to ancient saurischian pelvic structures indicating the evolution of birds from dinosaurs. This can also be seen in Figure 5c as the Aves branch off the Theropoda
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
suborder.
Pentadactyl limb
The pattern of limb bones called pentadactyl limb is an example of homologous structures (Fig. 5d). It is found in all classes of tetrapodTetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s (i.e. from amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s to mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s). It can even be traced back to the fin
Fin
A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, . The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices...
s of certain fossil fishes from which the first amphibians evolved such as tiktaalik
Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...
. The limb has a single proximal bone (humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
), two distal bones (radius
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
and ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
), a series of carpals (wrist
Wrist
In human anatomy, the wrist is variously defined as 1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;...
bones), followed by five series of metacarpals (palm
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
bones) and phalanges (digits). Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaptation to different environments and modes of life. This phenomenon is shown in the forelimbs of mammals. For example:
- In the monkeyMonkeyA monkey is a primate, either an Old World monkey or a New World monkey. There are about 260 known living species of monkey. Many are arboreal, although there are species that live primarily on the ground, such as baboons. Monkeys are generally considered to be intelligent. Unlike apes, monkeys...
, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees. - In the pigPigA pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...
, the first digit is lost, and the second and fifth digits are reduced. The remaining two digits are longer and stouter than the rest and bear a hoof for supporting the body. - In the horse, the forelimbs are adapted for support and running by great elongation of the third digit bearing a hoof.
- The moleMole (animal)Moles are small cylindrical mammals adapted to a subterranean lifestyle. They have velvety fur; tiny or invisible ears and eyes; and short, powerful limbs with large paws oriented for digging. The term is especially and most properly used for the true moles, those of the Talpidae family in the...
has a pair of short, spade-like forelimbs for burrowing. - The anteaterAnteaterAnteaters, also known as antbear, are the four mammal species of the suborder Vermilingua commonly known for eating ants and termites. Together with the sloths, they compose the order Pilosa...
uses its enlarged third digit for tearing down antAntAnts are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...
hills and termiteTermiteTermites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...
nests. - In the whaleWhaleWhale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
, the forelimbs become flipperFlipper (anatomy)A flipper is a typically flat limb evolved for movement through water. Various creatures have evolved flippers, for example penguins , cetaceans A flipper is a typically flat limb evolved for movement through water. Various creatures have evolved flippers, for example penguins (also called...
s for steering and maintaining equilibrium during swimming. - In the batBatBats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
, the forelimbs have turned into wingWingA wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
s for flying by great elongation of four digits, while the hook-like first digit remains free for hanging from treeTreeA tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s.
Recurrent laryngeal nerve in giraffes
The recurrent laryngeal nerveRecurrent laryngeal nerve
The recurrent laryngeal nerve is a branch of the vagus nerve that supplies motor function and sensation to the larynx . It travels within the endoneurium...
is a fourth branch of the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
, which is a cranial nerve. In mammals, its path is unusually long. As a part of the vagus nerve, it comes from the brain, passes through the neck down to heart, rounds the dorsal aorta
Dorsal aorta
Each primitive aorta receives anteriorly a vein—the vitelline vein—from the yolk-sac, and is prolonged backward on the lateral aspect of the notochord under the name of the dorsal aorta....
and returns up to the larynx
Larynx
The larynx , commonly called the voice box, is an organ in the neck of amphibians, reptiles and mammals involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume...
, again through the neck. (Fig. 5e)
This path is suboptimal even for humans, but for giraffes it becomes even more suboptimal. Due to the lengths of their necks, the recurrent laryngeal nerve may be up to 4m long (13 ft), despite its optimal route being a distance of just several inches.
The indirect route of this nerve is the result of evolution of mammals from fish, which had no neck and had a relatively short nerve that innervated one gill slit and passed near the gill arch. Since then, gills have evolved into lungs and the gill arch has become the dorsal aorta in mammals.
Route of the vas deferens
Similar to the laryngeal nerve in giraffes, the vas deferensVas deferens
The vas deferens , also called ductus deferens, , is part of the male anatomy of many vertebrates; they transport sperm from the epididymis in anticipation of ejaculation....
is part of the male anatomy of many vertebrates; it transports sperm from the epididymis
Epididymis
The epididymis is part of the male reproductive system and is present in all male amniotes. It is a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens. A similar, but probably non-homologous, structure is found in cartilaginous...
in anticipation of ejaculation
Ejaculation
Ejaculation is the ejecting of semen from the male reproductory tract, and is usually accompanied by orgasm. It is usually the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. In rare cases ejaculation occurs because of prostatic disease...
. In humans, the vas deferens routes up from the testicle
Testicle
The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...
, looping over the ureter
Ureter
In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....
, and back down to the urethra
Urethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...
and penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...
. It has been suggested that this is due to the descent of the testicles during the course of human evolution—likely associated with temperature. As the testicles descended, the vas deferens lengthened to accommodate the accidental “hook” over the ureter.
Evidence from paleontology
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
rapidly or are consumed by scavenger
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...
s, leaving no permanent evidences of their existence. However, occasionally, some organisms are preserved. The remains or traces
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
of organisms from a past geologic age
Geologic time scale
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...
embedded in rocks
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
by natural processes are called fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s. They are extremely important for understanding the evolutionary history of life
Timeline of evolution
This timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
on Earth, as they provide direct evidence of evolution and detailed information on the ancestry of organisms. Paleontology
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
is the study of past life based on fossil records and their relations to different geologic time periods.
For fossilization to take place, the traces and remains of organisms must be quickly buried so that weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
and decomposition do not occur. Skeletal structures or other hard parts of the organisms are the most commonly occurring form of fossilized remains (Paul, 1998), (Behrensmeyer, 1980) and (Martin, 1999). There are also some trace "fossils" showing moulds
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
, cast or imprints of some previous organisms.
As an animal dies, the organic materials gradually decay, such that the bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s become porous. If the animal is subsequently buried in mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
, mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
salts will infiltrate into the bones and gradually fill up the pores. The bones will harden into stones and be preserved as fossils. This process is known as petrification
Petrifaction
In geology, petrifaction, petrification or silicification is the process by which organic material is converted into stone by impregnation with silica. It is a rare form of fossilization...
. If dead animals are covered by wind-blown sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, and if the sand is subsequently turned into mud by heavy rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
or flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
s, the same process of mineral infiltration may occur. Apart from petrification, the dead bodies of organisms may be well preserved in ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
, in hardened resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
of coniferous
Pinophyta
The conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority being...
trees (amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...
), in tar, or in anaerobic, acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ic peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
. Fossilization can sometimes be a trace, an impression of a form. Examples include leaves and footprints, the fossils of which are made in layers that then harden.
Fossil record
It is possible to find out how a particular group of organisms evolved by arranging its fossil records in a chronological sequence. Such a sequence can be determined because fossils are mainly found in sedimentary rockSedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
. Sedimentary rock is formed by layers of silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
or mud on top of each other; thus, the resulting rock contains a series of horizontal layers, or strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
. Each layer contains fossils which are typical for a specific time period during which they were made. The lowest strata contain the oldest rock and the earliest fossils, while the highest strata contain the youngest rock and more recent fossils.
A succession
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
of animals and plants can also be seen from fossil discoveries. By studying the number and complexity of different fossils at different stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
levels, it has been shown that older fossil-bearing rocks contain fewer types of fossilized organisms, and they all have a simpler structure, whereas younger rocks contain a greater variety of fossils, often with increasingly complex structures.
For many years, geologists could only roughly estimate the ages of various strata and the fossils found. They did so, for instance, by estimating the time for the formation of sedimentary rock layer by layer. Today, by measuring the proportions of radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
and stable elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
in a given rock, the ages of fossils can be more precisely dated by scientists. This technique is known as radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
.
Throughout the fossil record, many species that appear at an early stratigraphic level disappear at a later level. This is interpreted in evolutionary terms as indicating the times at which species originated and became extinct. Geographical regions and climatic conditions have varied throughout the Earth's history
History of Earth
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...
. Since organisms are adapted to particular environments, the constantly changing conditions favoured species which adapted to new environments through the mechanism of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
.
Extent of the fossil record
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
have been recovered from the La Brea Tar Pits
La Brea Tar Pits
The La Brea Tar Pits are a cluster of tar pits around which Hancock Park was formed, in the urban heart of Los Angeles. Asphaltum or tar has seeped up from the ground in this area for tens of thousands of years. The tar is often covered with water...
in Los Angeles. Many more fossils are still in the ground, in various geological formations known to contain a high fossil density, allowing estimates of the total fossil content of the formation to be made. An example of this occurs in South Africa's Beaufort Formation (part of the Karoo Supergroup
Karoo Supergroup
The Karoo Supergroup is the largest stratigraphic unit in Southern Africa, covering almost two thirds of the present land surface, including central Cape Province, almost all of Orange Free State, western Natal, much of south-east Transvaal, Zambia, Zimbabwe and Malawi...
, which covers most of South Africa), which is rich in vertebrate fossils, including therapsids (reptile/mammal transitional forms
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
). It has been estimated that this formation contains 800 billion vertebrate fossils.
Limitations
The fossil record is an important source for scientists when tracing the evolutionary history of organisms. However, because of limitations inherent in the record, there are not fine scales of intermediate forms between related groups of species. This lack of continuous fossils in the record is a major limitation in tracing the descent of biological groups. When transitional fossilTransitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
s are found that show intermediate forms in what had previously been a gap in knowledge, they are often popularly referred to as "missing links".
There is a gap of about 100 million years between the beginning of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period and the end of the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
period. The early Cambrian period was the period from which numerous fossils of sponges, cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns (e.g., jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
), echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s (e.g., eocrinoids), molluscs
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
(e.g., snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s) and arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s (e.g., trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s) are found. The first animal that possessed the typical features of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, the Arandaspis
Arandaspis
Arandaspis prionotolepis is an extinct species of jawless fish that lived in the Ordovician period, about 480 to 470 million years ago. It is the oldest known vertebrate....
, was dated to have existed in the later Ordovician period. Thus few, if any, fossils of an intermediate type between invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s and vertebrates have been found, although likely candidates include the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
animal, Pikaia gracilens, and its Maotianshan shales
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
relatives, Myllokunmingia
Myllokunmingia
Myllokunmingia is a chordate from the Lower Cambrian Maotianshan shales of China, thought to be a vertebrate, although this is not conclusively proven. It is 28 mm long and 6 mm high....
, Yunnanozoon
Yunnanozoon
Yunnanozoon lividum is a suspected a hemichordate or chordate from the Lower Cambrian, Chengjiang biota of Yunnan province, China....
, Haikouella lanceolata, and Haikouichthys
Haikouichthys
Haikouichthys is an extinct genus of craniate believed to have lived c. 530 million years ago, during the Cambrian explosion of multicellular life...
.
Some of the reasons for the incompleteness of fossil records are:
- In general, the probability that an organism becomes fossilized is very low;
- Some species or groups are less likely to become fossils because they are soft-bodied;
- Some species or groups are less likely to become fossils because they live (and die) in conditions that are not favourable for fossilization;
- Many fossils have been destroyed through erosion and tectonic movements;
- Most fossils are fragmentary;
- Some evolutionary change occurs in populations at the limits of a species' ecological range, and as these populations are likely to be small, the probability of fossilization is lower (see punctuated equilibriumPunctuated equilibriumPunctuated equilibrium is a theory in evolutionary biology which proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis...
); - Similarly, when environmental conditions change, the population of a species is likely to be greatly reduced, such that any evolutionary change induced by these new conditions is less likely to be fossilized;
- Most fossils convey information about external form, but little about how the organism functioned;
- Using present-day biodiversityBiodiversityBiodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
as a guide, this suggests that the fossils unearthed represent only a small fraction of the large number of species of organisms that lived in the past.
Evolution of the horse
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n sedimentary deposits from the early Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
to the present, the horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
provides one of the best examples of evolutionary history (phylogeny).
This evolutionary sequence starts with a small animal called Hyracotherium
Hyracotherium
Hyracotherium , also known as Eohippus or the dawn horse, is an extinct genus of very small perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene...
(commonly referred to as Eohippus) which lived in North America about 54 million years ago, then spread across to Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
. Fossil remains of Hyracotherium show it to have differed from the modern horse in three important respects: it was a small animal (the size of a fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
), lightly built and adapted for running; the limbs were short and slender, and the feet elongated so that the digits were almost vertical, with four digits in the forelimb
Forelimb
A forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....
s and three digits in the hindlimbs; and the incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s were small, the molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s having low crowns with rounded cusp
Cusp (dentistry)
A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
s covered in enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
.
The probable course of development of horses from Hyracotherium to Equus (the modern horse) involved at least 12 genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
and several hundred species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. The major trends seen in the development of the horse to changing environmental conditions may be summarized as follows:
- Increase in size (from 0.4 m to 1.5 m — from 15in to 60in);
- Lengthening of limbs and feet;
- Reduction of lateral digits;
- Increase in length and thickness of the third digit;
- Increase in width of incisorIncisorIncisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s; - Replacement of premolarPremolarThe premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
s by molarMolar (tooth)Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s; and - Increases in tooth length, crown height of molars.
Fossilized plants found in different strata show that the marsh
Marsh
In geography, a marsh, or morass, is a type of wetland that is subject to frequent or continuous flood. Typically the water is shallow and features grasses, rushes, reeds, typhas, sedges, other herbaceous plants, and moss....
y, wooded country in which Hyracotherium lived became gradually drier. Survival now depended on the head being in an elevated position for gaining a good view of the surrounding countryside, and on a high turn of speed for escape from predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
, hence the increase in size and the replacement of the splayed-out foot by the hoofed foot. The drier, harder ground would make the original splayed-out foot unnecessary for support. The changes in the teeth can be explained by assuming that the diet changed from soft vegetation
Vegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
to grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
. A dominant genus from each geological period has been selected to show the slow alteration of the horse lineage from its ancestral to its modern form.
Evidence from geographical distribution
Data about the presence or absence of species on various continentContinent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s and island
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
s (biogeography
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
) can provide evidence of common descent and shed light on patterns of speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
.
Continental distribution
All organisms are adapted to their environment to a greater or lesser extent. If the abiotic and biotic factors within a habitatHabitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
are capable of supporting a particular species in one geographic area, then one might assume that the same species would be found in a similar habitat in a similar geographic area, e.g. in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. This is not the case. Plant and animal species are discontinuously distributed throughout the world:
- Africa has Old World monkeyOld World monkeyThe Old World monkeys or Cercopithecidae are a group of primates, falling in the superfamily Cercopithecoidea in the clade Catarrhini. The Old World monkeys are native to Africa and Asia today, inhabiting a range of environments from tropical rain forest to savanna, shrubland and mountainous...
s, apeApeApes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. The apes are native to Africa and South-east Asia, although in relatively recent times humans have spread all over the world...
s, elephantElephantElephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
s, leopardLeopardThe leopard , Panthera pardus, is a member of the Felidae family and the smallest of the four "big cats" in the genus Panthera, the other three being the tiger, lion, and jaguar. The leopard was once distributed across eastern and southern Asia and Africa, from Siberia to South Africa, but its...
s, giraffeGiraffeThe giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant...
s, and hornbillHornbillHornbills are a family of bird found in tropical and subtropical Africa, Asia and Melanesia. They are characterized by a long, down-curved bill which is frequently brightly-colored and sometimes has a casque on the upper mandible. Both the common English and the scientific name of the family...
s. - South America has New World monkeyNew World monkeyNew World monkeys are the five families of primates that are found in Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Platyrrhini parvorder and the Ceboidea superfamily, which are essentially synonymous since...
s, cougars, jaguarJaguarThe jaguar is a big cat, a feline in the Panthera genus, and is the only Panthera species found in the Americas. The jaguar is the third-largest feline after the tiger and the lion, and the largest in the Western Hemisphere. The jaguar's present range extends from Southern United States and Mexico...
s, slothSlothSloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae , part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.They are arboreal residents of the jungles of Central and South...
s, llamaLlamaThe llama is a South American camelid, widely used as a meat and pack animal by Andean cultures since pre-Hispanic times....
s, and toucanToucanToucans are members of the family Ramphastidae of near passerine birds from the Neotropics. The family is most closely related to the American barbets. They are brightly marked and have large, often colorful bills. The family includes five genera and about forty different species...
s. - Deserts in North and South America have native cactiCacti-See also:* RRDtool The underlying software upon which Cacti is built* MRTG The original Multi Router Traffic Grapher from which RRDtool was "extracted".* Munin -External links:******...
, but deserts in Africa, Asia, and Australia have succulent native euphorbsEuphorbiaceaeEuphorbiaceae, the Spurge family are a large family of flowering plants with 300 genera and around 7,500 species. Most are herbs, but some, especially in the tropics, are also shrubs or trees. Some are succulent and resemble cacti....
that resemble cacti but are very different, even though in some cases cacti have done very well (for example in Australian deserts) when introduced by humans.
Even greater differences can be found if Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
is taken into consideration, though it occupies the same latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
as much of South America and Africa. Marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s like kangaroo
Kangaroo
A kangaroo is a marsupial from the family Macropodidae . In common use the term is used to describe the largest species from this family, especially those of the genus Macropus, Red Kangaroo, Antilopine Kangaroo, Eastern Grey Kangaroo and Western Grey Kangaroo. Kangaroos are endemic to the country...
s, bandicoots, and quoll
Quoll
The quoll, or native cat, is a carnivorous marsupial native to mainland Australia, New Guinea and Tasmania. It is primarily nocturnal and spends most of the day in its den. There are six species of quoll; four are found in Australia and two in New Guinea...
s make up about half of Australia's indigenous mammal species. By contrast, marsupials are today totally absent from Africa and form a smaller portion of the mammalian fauna of South America, where opossums, shrew opossum
Shrew opossum
The order Paucituberculata contains the six surviving species of shrew opossum: small, shrew-like marsupials which are confined to the Andes mountains of South America. It is thought that the order diverged from the ancestral marsupial line very early. As recently as 20 million years ago, there...
s, and the monito del monte
Monito del Monte
The Monito del Monte The Monito del Monte The Monito del Monte (Spanish for "little mountain monkey", Dromiciops gliroides, is a diminutive marsupial native only to southwestern South America (Chile and Argentina). It is the only extant species in the ancient order Microbiotheria, and the sole New...
occur. The only living representatives of primitive egg-laying mammals (monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s) are the echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
s and the platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
. The short-beaked echidna (Tachyglossus aculeatus) and its subspecies populate Australia, Tasmania
Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...
, New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...
, and Kangaroo Island
Kangaroo Island
Kangaroo Island is Australia's third-largest island after Tasmania and Melville Island. It is southwest of Adelaide at the entrance of Gulf St Vincent. Its closest point to the mainland is off Cape Jervis, on the tip of the Fleurieu Peninsula in the state of South Australia. The island is long...
while the long-beaked echidna (Zaglossus bruijni) lives only in New Guinea. The platypus lives in the waters of eastern Australia. They have been introduced to Tasmania, King Island, and Kangaroo Island. These Monotremes are totally absent in the rest of the world. On the other hand, Australia is missing many groups of placental mammals that are common on other continents (carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns, artiodactyls, shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...
s, squirrel
Squirrel
Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots , flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa and have been introduced to Australia...
s, lagomorphs), although it does have indigenous bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s and murine
Murinae
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. This subfamily is larger than all mammal families except the Cricetidae and Muridae, and is larger than all mammal orders except the bats and the remainder of the...
rodents; many other placentals, such as rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
s and fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
es, have been introduced there by humans.
Other animal distribution examples include bear
Bear
Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...
s, located on all continents excluding Africa, Australia and Antarctica, and the polar bear only located solely in the Arctic Circle and adjacent land masses. Penguins are located only around the South Pole despite similar weather conditions at the North Pole. Families of sirenians are distributed exclusively around the earth’s waters, where manatees are located in western Africa waters, northern South American waters, and West Indian waters only while the related family, the Dugong
Dugong
The dugong is a large marine mammal which, together with the manatees, is one of four living species of the order Sirenia. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow , was hunted to extinction in the 18th century...
s, are located only in Oceanic
Oceania
Oceania is a region centered on the islands of the tropical Pacific Ocean. Conceptions of what constitutes Oceania range from the coral atolls and volcanic islands of the South Pacific to the entire insular region between Asia and the Americas, including Australasia and the Malay Archipelago...
waters north of Australia, and the coasts surrounding the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
Additionally, the now extinct Steller's Sea Cow
Steller's Sea Cow
Steller's sea cow was a large herbivorous marine mammal. In historical times, it was the largest member of the order Sirenia, which includes its closest living relative, the dugong , and the manatees...
resided in the Bering Sea
Bering Sea
The Bering Sea is a marginal sea of the Pacific Ocean. It comprises a deep water basin, which then rises through a narrow slope into the shallower water above the continental shelves....
.
The same kinds of fossils are found from areas known to be adjacent to one another in the past but which, through the process of continental drift
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
, are now in widely divergent geographic locations. For example, fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica, which can be dated to the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
Era, at which time these regions were united as a single landmass called Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
. Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates.
Island biogeography
Types of species found on islands
Evidence from island biogeographyIsland biogeography
Island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of natural communities. The theory was developed to explain species richness of actual islands...
has played an important and historic role in the development of evolutionary biology. For purposes of biogeography
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
, islands are divided into two classes. Continental islands are islands like Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
, and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
that have at one time or another been part of a continent. Oceanic islands, like the Hawaiian islands
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
, the Galapagos islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
and St. Helena, on the other hand are islands that have formed in the ocean and never been part of any continent. Oceanic islands have distributions of native plants and animals that are unbalanced in ways that make them distinct from the biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...
s found on continents or continental islands. Oceanic islands do not have native terrestrial mammals (they do sometimes have bats and seals), amphibians, or fresh water fish. In some cases they have terrestrial reptiles (such as the iguanas and giant tortoises of the Galapagos islands) but often (for example Hawaii) they do not. This despite the fact that when species such as rats, goats, pigs, cats, mice, and cane toad
Cane Toad
The Cane Toad , also known as the Giant Neotropical Toad or Marine Toad, is a large, terrestrial true toad which is native to Central and South America, but has been introduced to various islands throughout Oceania and the Caribbean...
s, are introduced to such islands by humans they often thrive. Starting with Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
, many scientists have conducted experiments and made observations that have shown that the types of animals and plants found, and not found, on such islands are consistent with the theory that these islands were colonized accidentally by plants and animals that were able to reach them. Such accidental colonization could occur by air, such as plant seeds carried by migratory birds, or bats and insects being blown out over the sea by the wind, or by floating from a continent or other island by sea, as for example by some kinds of plant seeds like coconuts that can survive immersion in salt water, and reptiles that can survive for extended periods on rafts of vegetation carried to sea by storms.
Endemism
Many of the species found on remote islands are endemic to a particular island or group of islands, meaning they are found nowhere else on earth. Examples of species endemic to islands include many flightless birds of New ZealandNew Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, lemurs of Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, the Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...
of Komodo
Komodo (island)
Komodo is one of the 17,508 islands that make up the Republic of Indonesia. The island has a surface area of 390 km² and over 2000 inhabitants. The inhabitants of the island are descendants of former convicts who were exiled to the island and who have mixed themselves with the Bugis from...
, the Dragon’s blood tree of Socotra
Socotra
Socotra , also spelt Soqotra, is a small archipelago of four islands in the Indian Ocean. The largest island, also called Socotra, is about 95% of the landmass of the archipelago. It lies some east of the Horn of Africa and south of the Arabian Peninsula. The island is very isolated and through...
, Tuatara
Tuatara
The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...
of New Zealand, and others. However many such endemic species are related to species found on other nearby islands or continents; the relationship of the animals found on the Galapagos Islands to those found in South America is a well-known example. All of these facts, the types of plants and animals found on oceanic islands, the large number of endemic species found on oceanic islands, and the relationship of such species to those living on the nearest continents, are most easily explained if the islands were colonized by species from nearby continents that evolved into the endemic species now found there.
Other types of endemism do not have to include, in the strict sense, islands. Islands can mean isolated lakes or remote and isolated areas. Examples of these would include the highlands of Ethiopia
Ethiopia
Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...
, Lake Baikal
Lake Baikal
Lake Baikal is the world's oldest at 30 million years old and deepest lake with an average depth of 744.4 metres.Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the...
, Fynbos
Fynbos
Fynbos is the natural shrubland or heathland vegetation occurring in a small belt of the Western Cape of South Africa, mainly in winter rainfall coastal and mountainous areas with a Mediterranean climate...
of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, forests of New Caledonia
New Caledonia
New Caledonia is a special collectivity of France located in the southwest Pacific Ocean, east of Australia and about from Metropolitan France. The archipelago, part of the Melanesia subregion, includes the main island of Grande Terre, the Loyalty Islands, the Belep archipelago, the Isle of...
, and others. Examples of endemic organisms living in isolated areas include the Kagu
Kagu
The Kagu or Cagou is a crested, long-legged, and bluish-grey bird endemic to the dense mountain forests of New Caledonia. It is the only surviving member of the genus Rhynochetos and the family Rhynochetidae, although a second species has been described from the fossil record...
of New Caledonia, cloud rat
Cloud rat
The cloud rats are a group of arboreal and folivorous nocturnal rodents native to the forests of the Philippines. There are six known species of cloud rat. These animals have not been studied extensively. The Filipino Ecosystems Research and Development Bureau has begun breeding Northern Luzon...
s of the Luzon tropical pine forests
Luzon tropical pine forests
The Luzon tropical pine forests are a tropical coniferous forest ecoregion of the Philippines in the western Pacific Ocean. These pine forests are home to a large number of the island's endemic plants and animals.-Location and description:...
of the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, the boojum tree (Fouquieria columnaris) of the Baja California peninsula
Baja California Peninsula
The Baja California peninsula , is a peninsula in northwestern Mexico. Its land mass separates the Pacific Ocean from the Gulf of California. The Peninsula extends from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south.The total area of the Baja California...
, the Baikal Seal and the omul
Omul
The omul, Coregonus migratorius, also known as Baikal omul , is a whitefish species of the salmon family endemic to Lake Baikal in Siberia, Russia. It is considered a delicacy and is the object of one of the largest commercial fisheries on Lake Baikal...
of Lake Baikal.
Adaptive radiations
Oceanic islands are frequently inhabited by clusters of closely related species that fill a variety of ecological niches, often niches that are filled by very different species on continents. Such clusters, like the Finches of the Galapagos, Hawaiian honeycreeperHawaiian honeycreeper
Hawaiian honeycreepers are small, passerine birds endemic to Hawaii. Some authorities still categorize this group as a family Drepanididae, but in recent years, most authorities consider them a subfamily, Drepanidinae, of Fringillidae, the finch family...
s, members of the sunflower family on the Juan Fernandez Archipelago and wood weevils on St. Helena are called adaptive radiations because they are best explained by a single species colonizing an island (or group of islands) and then diversifying to fill available ecological niches. Such radiations can be spectacular; 800 species of the fruit fly family Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, nearly half the world's total, are endemic to the Hawaiian islands. Another illustrative example from Hawaii is the Silversword alliance
Silversword alliance
Silversword alliance refers to an adaptive radiation of over 50 species in the composite or sunflower family, Asteraceae. The group is endemic to Hawaii, and is derived from a single immigrant to the islands...
, which is a group of thirty species found only on those islands. Members range from the Silverswords that flower spectacularly on high volcanic slopes to trees, shrubs, vines and mats that occur at various elevations from mountain top to sea level, and in Hawaiian habitats that vary from deserts to rainforests. Their closest relatives outside Hawaii, based on molecular studies, are tarweeds found on the west coast of North America. These tarweeds have sticky seeds that facilitate distribution by migrant birds. Additionally, nearly all of the species on the island can be crossed and the hybrids are often fertile, and they have been hybridized experimentally with two of the west coast tarweed species as well. Continental islands have less distinct biota, but those that have been long separated from any continent also have endemic species and adaptive radiations, such as the 75 lemur
Lemur
Lemurs are a clade of strepsirrhine primates endemic to the island of Madagascar. They are named after the lemures of Roman mythology due to the ghostly vocalizations, reflective eyes, and the nocturnal habits of some species...
species of Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, and the eleven extinct moa
Moa
The moa were eleven species of flightless birds endemic to New Zealand. The two largest species, Dinornis robustus and Dinornis novaezelandiae, reached about in height with neck outstretched, and weighed about ....
species of New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
.
Ring Species
In biology, a ring species is a connected series of neighboring populations that can interbreed with relatively closely related populations, but for which there exist at least two "end" populations in the series that are too distantly related to interbreed. Often such non-breeding-though-genetically-connected populations co-exist in the same region thus creating a "ring". Ring species provide important evidence of evolution in that they illustrate what happens over time as populations genetically diverge, and are special because they represent in living populations what normally happens over time between long deceased ancestor populations and living populations. If any of the populations intermediate between the two ends of the ring were gone they would not be a continuous line of reproduction and each side would be a different species.Specific examples
Distribution of Glossopteris
The combination of continental drift and evolution can sometimes be used to make predictions about what will be found in the fossil record. GlossopterisGlossopteris
Glossopteris is the largest and best-known genus of the extinct order of seed ferns known as Glossopteridales ....
is an extinct species of seed fern plants from the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
. Glossopteris appears in the fossil record around the beginning of the Permian on the ancient continent of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
. Continental drift explains the current biogeography of the tree. Present day Glossopteris fossils are found in Permian strata in southeast South America, southeast Africa, all of Madagascar, northern India, all of Australia, all of New Zealand, and scattered on the southern and northern edges of Antarctica. During the Permian, these continents were connected as Gondwana (see figure 6a) in agreement with magnetic striping, other fossil distributions, and glacial scratches pointing away from the temperate climate of the South Pole during the Permian.
Distribution of marsupials
The history of marsupials also provides an example of how the theories of evolution and continental drift can be combined to make predictions about what will be found in the fossil record. The earliest marsupial fossils are about 80 million years old and found in North America; by 40 million years ago fossils show that they could be found throughout South America, but there is no evidence of them in Australia, where they now predominate, until about 30 million years ago. The theory of evolution predicts that the Australian marsupials must be descended from the older ones found in the Americas. The theory of continental drift says that between 30 and 40 million years ago South America and Australia were still part of the Southern hemisphere super continent of GondwanaGondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
and that they were connected by land that is now part of Antarctica. Therefore combining the two theories scientists predicted that marsupials migrated from what is now South America across what is now Antarctica to what is now Australia between 40 and 30 million years ago. This hypothesis led paleontologists to Antarctica to look for marsupial fossils of the appropriate age. After years of searching they found, starting in 1982, fossils on Seymour Island
Seymour Island
Seymour Island is an island in the chain of 16 major islands around the tip of the Graham Land on the Antarctic Peninsula. Graham Land is closer to continental land mass than any other part of that Antarctica. It lies within the section of the island chain that resides off the west side of the...
off the coast of the Antarctic Peninsula
Antarctic Peninsula
The Antarctic Peninsula is the northernmost part of the mainland of Antarctica. It extends from a line between Cape Adams and a point on the mainland south of Eklund Islands....
of more than a dozen marsupial species that lived 35-40 million years ago.
Migration, isolation, and distribution of the Camel
The history of the camelCamel
A camel is an even-toed ungulate within the genus Camelus, bearing distinctive fatty deposits known as humps on its back. There are two species of camels: the dromedary or Arabian camel has a single hump, and the bactrian has two humps. Dromedaries are native to the dry desert areas of West Asia,...
provides an example of how fossil evidence can be used to reconstruct migration and subsequent evolution. The fossil record indicates that the evolution of camelid
Camelid
Camelids are members of the biological family Camelidae, the only living family in the suborder Tylopoda. Dromedaries, Bactrian Camels, llamas, alpacas, vicuñas, and guanacos are in this group....
s started in North America (see figure 6c), from which 6 million years ago they migrated across the Bering Strait into Asia and then to Africa, and 3.5 million years ago through the Isthmus of Panama into South America. Once isolated, they evolved along their own lines, giving rise to the Bactrian camel
Bactrian camel
The Bactrian camel is a large, even-toed ungulate native to the steppes of central Asia. It is presently restricted in the wild to remote regions of the Gobi and Taklamakan Deserts of Mongolia and Xinjiang. A small number of wild Bactrian camels still roam the Mangystau Province of southwest...
and Dromedary
Dromedary
The dromedary or Arabian camel is a large, even-toed ungulate with one hump on its back. Its native range is unclear, but it was probably the Arabian Peninsula. The domesticated form occurs widely in North Africa and the Middle East...
in Asia and Africa and the llama and its relatives
Lama (genus)
Lama is the modern genus name for two South American camelids, the wild guanaco and the domesticated llama. This genus is closely allied to the wild vicuña and domesticated alpaca of the genus Vicugna. Before the Spanish conquest of the Americas, llamas and alpacas were the only domesticated...
in South America. Camelids then went extinct in North America at the end of the last ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
.
Evidence from observed natural selection
Examples for the evidence for evolution often stems from direct observation of natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
in the field and the laboratory. Scientists have observed and documented a multitude of events where natural selection is in action. The most well known examples are antibiotic resistance in the medical field along with better-known laboratory experiments documenting evolution's occurrence. Natural selection is tantamount to common descent in the fact that long-term occurrence and selection pressures can lead to the diversity of life on earth as found today. All adaptations—documented and undocumented changes concerned—are caused by natural selection (and a few other minor processes). The examples below are only a small fraction of the actual experiments and observations.
Antibiotic and pesticide resistance
The development and spread of antibiotic resistant bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, like the spread of pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...
resistant forms of plants and insects is evidence for evolution of species, and of change within species. Thus the appearance of vancomycin
Vancomycin
Vancomycin INN is a glycopeptide antibiotic used in the prophylaxis and treatment of infections caused by Gram-positive bacteria. It has traditionally been reserved as a drug of "last resort", used only after treatment with other antibiotics had failed, although the emergence of...
resistant Staphylococcus aureus, and the danger it poses to hospital patients is a direct result of evolution through natural selection. The rise of Shigella
Shigella
Shigella is a genus of Gram-negative, nonspore forming, non-motile, rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella causes disease in primates, but not in other mammals. It is only naturally found in humans and apes. During...
strains resistant to the synthetic antibiotic class of sulfonamides also demonstrates the generation of new information as an evolutionary process. Similarly, the appearance of DDT
DDT
DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
resistance in various forms of Anopheles
Anopheles
Anopheles is a genus of mosquito. There are approximately 460 recognized species: while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus Plasmodium, which cause malaria in humans in endemic areas...
mosquitoes, and the appearance of myxomatosis
Myxomatosis
Myxomatosis is a disease that affects rabbits and is caused by the Myxoma virus. It was first observed in Uruguay in laboratory rabbits in the late 19th century. It was introduced into Australia in 1950 in an attempt to control the rabbit population...
resistance in breeding rabbit populations in Australia, are all evidence of the existence of evolution in situations of evolutionary selection pressure in species in which generations occur rapidly.
E. coli long-term evolution experiment
Experimental evolution uses controlled experiments to test hypotheses and theories of evolution. In one early example, William DallingerWilliam Dallinger
Rev. Dr. William Henry Dallinger F.R.S. was a British minister in the Wesleyan Methodist Church. He was also an accomplished scientist, being the first to study the complete lifecycle of unicellular organisms under the microscope and studying the adaptation of such organisms to temperature.He made...
set up an experiment shortly before 1880, subjecting microbes to heat with the aim of forcing adaptive changes. His experiment ran for around seven years, and his published results were acclaimed, but he did not resume the experiment after the apparatus failed.
The E. coli long-term evolution experiment that began in 1988 under the leadership of Richard Lenski
Richard Lenski
Richard E. Lenski is an American evolutionary biologist. He is the son of sociologist Gerhard Lenski. He earned his BA from Oberlin College in 1976, and his PhD from the University of North Carolina in 1982...
is still in progress, and has shown adaptations including the evolution of a strain of E. coli that was able to grow on citric acid in the growth media.
Humans
Natural selection is being observed in contemporary human populations, with recent findings demonstrating the population which is at risk of the severe debilitating disease kuruKuru (disease)
Kuru is an incurable degenerative neurological disorder that is a type of transmissible spongiform encephalopathy, caused by a prion found in humans...
has significant over-representation of an immune variant of the prion protein gene G127V versus non-immune alleles. Scientists postulate one of the reasons for the rapid selection of this genetic variant is the lethality of the disease in non-immune persons. Other reported evolutionary trends in other populations include a lengthening of the reproductive period, reduction in cholesterol levels, blood glucose and blood pressure.
Lactose intolerance in humans
Lactose intoleranceLactose intolerance
Lactose intolerance, also called lactase deficiency or hypolactasia, is the inability to digest and metabolize lactose, a sugar found in milk...
is the inability to metabolize
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
, because of a lack of the required enzyme lactase
Lactase
Lactase , a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers...
in the digestive system. The normal mammalian condition is for the young of a species to experience reduced lactase
Lactase
Lactase , a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers...
production at the end of the weaning
Weaning
Weaning is the process of gradually introducing a mammal infant, either human or animal, to what will be its adult diet and withdrawing the supply of its mother's milk.The process takes place only in mammals, as only mammals produce milk...
period (a species-specific length of time). In humans, in non-dairy consuming societies, lactase production usually drops about 90% during the first four years of life, although the exact drop over time varies widely. However, certain human populations have a mutation on chromosome 2 which eliminates the shutdown in lactase production, making it possible for members of these populations to continue consumption of raw milk and other fresh and fermented dairy products throughout their lives without difficulty. This appears to be an evolutionarily recent adaptation to dairy consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral lifestyle.
Nylon-eating bacteria
Nylon-eating bacteriaNylon-eating bacteria
Nylon-eating bacteria are a strain of Flavobacterium that is capable of digesting certain byproducts of nylon 6 manufacture. This strain of Flavobacterium, Sp...
are a strain of Flavobacterium
Flavobacterium
Flavobacterium is a genus of Gram-negative, non-motile and motile, rod-shaped bacteria that consists of ten recognized species, as well as three newly proposed species . Flavobacteria are found in soil and fresh water in a variety of environments...
that is capable of digesting certain byproducts of nylon 6
Nylon 6
Nylon 6 or polycaprolactam is a polymer developed by Paul Schlack at IG Farben to reproduce the properties of nylon 6,6 without violating the patent on its production. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization. This makes...
manufacture. There is scientific consensus that the capacity to synthesize nylonase most probably developed as a single-step mutation that survived because it improved the fitness of the bacteria possessing the mutation. This is seen as a good example of evolution through mutation and natural selection that has been observed as it occurs.
PCB tolerance
After General ElectricGeneral Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
dumped polychlorinated biphenyls (PCBs) in the Hudson River
Hudson River
The Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York...
from 1947 through 1976, tomcods living in the river were found to have evolved an increased resistance to the compound's toxic effects. At first the tomcod population was devastated, but it recovered. Scientists identified the genetic mutation that conferred the resistance. The mutated form was found to be present in 99 per cent of the surviving tomcods in the river, compared to fewer than 10 percent of the tomcods from other waters.
Peppered moth
One classic example of adaptation in response to selection pressure is the case of the peppered moth. The color of the moth has gone from light to dark to light again over the course of a few hundred years due to the appearance and later disappearance of pollution from the Industrial RevolutionIndustrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...
in England.
Radiotrophic fungus
Radiotrophic fungi are fungi which appear to use the pigment melaninMelanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
to convert gamma radiation into chemical energy for growth and were first discovered in 2007 as black molds growing inside and around the Chernobyl Nuclear Power Plant
Chernobyl Nuclear Power Plant
The Chernobyl Nuclear Power Plant or Chornobyl Nuclear Power Plant is a decommissioned nuclear power station near the city of Pripyat, Ukraine, northwest of the city of Chernobyl, from the Ukraine–Belarus border, and about north of Kiev. Reactor 4 was the site of the Chernobyl disaster in...
. Research at the Albert Einstein College of Medicine
Albert Einstein College of Medicine
Albert Einstein College of Medicine is a graduate school of Yeshiva University. It is a not-for-profit, private, nonsectarian medical school located on the Jack and Pearl Resnick Campus in the Morris Park neighborhood of the borough of the Bronx of New York City...
showed that three melanin-containing fungi, Cladosporium sphaerospermum, Wangiella dermatitidis, and Cryptococcus neoformans
Cryptococcus neoformans
Cryptococcus neoformans is an encapsulated yeast that can live in both plants and animals. Its teleomorph is Filobasidiella neoformans, a filamentous fungus belonging to the class Tremellomycetes. It is often found in pigeon excrement....
, increased in biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
and accumulated acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
faster in an environment in which the radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
level was 500 times higher than in the normal environment.
Urban wildlife
Urban wildlifeUrban wildlife
Urban wildlife is wildlife that can live or thrive in urban environments. Some urban wildlife, such as house mice, are synanthropic, ecologically associated with humans. Different types of urban area support different kinds of wildlife...
is wildlife
Wildlife
Wildlife includes all non-domesticated plants, animals and other organisms. Domesticating wild plant and animal species for human benefit has occurred many times all over the planet, and has a major impact on the environment, both positive and negative....
that is able to live or thrive in urban
Urban area
An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...
environments. These types of environments can exert selection pressures on organism, often leading to new adaptations. For example, the weed Crepis sancta
Crepis
Crepis, commonly known in some parts of the world as hawksbeard or hawk's-beard , is a genus of about 200 annual and perennial flowering plants of the family Asteraceae superficially resembling the dandelion, the most conspicuous difference being that Crepis usually has branching...
, found in France, has two types of seed, heavy and fluffy. The heavy ones land nearby to the parent plant, whereas the fluffy seeds float further away on the wind. In urban environments, seeds that float far will often land on infertile concrete. Within about 5-12 generations, the weed has been found to evolve to produce significantly more heavy seeds than its rural relatives do. Other examples of urban wildlife are rock pigeon
Rock Pigeon
The Rock Dove or Rock Pigeon, is a member of the bird family Columbidae . In common usage, this bird is often simply referred to as the "pigeon"....
s and species of crows adapting to city environments around the world; African penguins in Simons Town; baboon
Baboon
Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger...
s in South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
; and a variety of insects living in human habitations.
Evidence from observed speciation
SpeciationSpeciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
is the evolutionary process by which new biological species arise. Speciation can occur from a variety of different causes and are classified in various forms (e.g. allopatric, sympatric, polyploidization, etc.). Scientists have observed numerous examples of speciation in the laboratory and in nature, however, evolution has produced far more species than an observer would consider necessary. For example, there are well over 350,000 described species of beetles. Great examples of observed speciation come from the observations of island biogeography and the process of adaptive radiation, both explained in an earlier section. The examples shown below provide strong evidence for common descent and are only a small fraction of the instances observed.
Blackcap
The bird species, Sylvia atricapillabBlackcap
The Blackcap is a common and widespread sylviid warbler which breeds throughout temperate Europe, western Asia and northwestern Africa, and winters from northwestern Europe south to tropical Africa...
, commonly referred to as Blackcaps, lives in Germany and flies southwest to Spain while a smaller group flies northwest to Great Britain during the winter. Gregor Rolshausen from the University of Freiburg
University of Freiburg
The University of Freiburg , sometimes referred to in English as the Albert Ludwig University of Freiburg, is a public research university located in Freiburg im Breisgau, Baden-Württemberg, Germany.The university was founded in 1457 by the Habsburg dynasty as the...
found that the genetic separation of the two populations is already in progress. The differences found have arisen in about 30 generations. With DNA sequencing, the individuals can be assigned to a correct group with an 85% accuracy. Stuart Bearhop from the University of Exeter
University of Exeter
The University of Exeter is a public university in South West England. It belongs to the 1994 Group, an association of 19 of the United Kingdom's smaller research-intensive universities....
reported that birds wintering in England tend to mate only among themselves, and not usually with those wintering in the Mediterranean. It is still inference to say that the populations will become two different species, but researchers expect it due to the continued genetic and geographic separation.
Drosophila melanogaster
Sympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
in the common fruit fly
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
. They collected a population of Drosophila melanogaster from Davis, California
Davis, California
Davis is a city in Yolo County, California, United States. It is part of the Sacramento–Arden-Arcade–Roseville Metropolitan Statistical Area...
and placed the pupae into a habitat maze. Newborn flies had to investigate the maze to find food. The flies had three choices to take in finding food. Light and dark (phototaxis
Phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves in response to the stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis...
), up and down (geotaxis), and the scent of acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...
and the scent of ethanol (chemotaxis
Chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
) were the three options. This eventually divided the flies into 42 spatio-temporal habitats.
They then cultured two strains that chose opposite habitats. One of the strains emerged early, immediately flying upward in the dark attracted to the acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...
. The other strain emerged late and immediately flew downward, attracted to light and ethanol. Pupae from the two strains were then placed together in the maze and allowed to mate at the food site. They then were collected. A selective penalty was imposed on the female flies that switched habitats. This entailed that none of their gametes would pass on to the next generation. After 25 generations of this mating test, it showed reproductive isolation between the two strains. They repeated the experiment again without creating the penalty against habitat switching and the result was the same; reproductive isolation was produced.
Hawthorn fly
One example of evolution at work is the case of the hawthorn fly, Rhagoletis pomonella, also known as the apple maggot fly, which appears to be undergoing sympatric speciationSympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
. Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after apple
Apple
The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family . It is one of the most widely cultivated tree fruits, and the most widely known of the many members of genus Malus that are used by humans. Apple grow on small, deciduous trees that blossom in the spring...
s, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns
Crataegus
Crataegus , commonly called hawthorn or thornapple, is a large genus of shrubs and trees in the rose family, Rosaceae, native to temperate regions of the Northern Hemisphere in Europe, Asia and North America. The name hawthorn was originally applied to the species native to northern Europe,...
. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as the fact that six out of thirteen allozyme
Allozyme
Variant forms of an enzyme that are coded by different alleles at the same locus are called allozymes. These are opposed to isozymes, which are enzymes that perform the same function, but which are coded by genes located at different loci....
loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4-6% hybridization rate) suggests that speciation is occurring.
London Underground mosquito
The London Underground mosquitoLondon Underground mosquito
The London Underground mosquito is a species of mosquito in the genus Culex found in the London Underground. It is thought to have evolved from the overground species Culex pipiens recently. Biologists named the London Underground mosquito Culex pipiens molestus due to its voracious biting. Notably...
is a species of mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
in the genus Culex
Culex
Culex is a genus of mosquito, and is important in that several species serve as vectors of important diseases, such as West Nile virus, filariasis, Japanese encephalitis, St. Louis encephalitis and avian malaria....
found in the London Underground
London Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
. It evolved from the overground species Culex pipiens.
This mosquito, although first discovered in the London Underground system, has been found in underground systems around the world. It is suggested that it may have adapted to human-made underground systems since the last century from local above-ground Culex pipiens, although more recent evidence suggests that it is a southern mosquito variety related to Culex pipiens that has adapted to the warm underground spaces of northern cities.
The species have very different behaviours, are extremely difficult to mate, and with different allele frequency, consistent with genetic drift during a founder event. More specifically, this mosquito, Culex pipiens molestus, breeds all-year round, is cold intolerant, and bites rats, mice, and humans, in contrast to the above ground species Culex pipiens that is cold tolerant, hibernates in the winter, and bites only birds. When the two varieties were cross-bred the eggs were infertile suggesting reproductive isolation.
The fundamental results still stands: the genetic data indicate that the molestus form in the London Underground mosquito appeared to have a common ancestry, rather than the population at each station being related to the nearest above-ground population (i.e. the pipiens form). Byrne and Nichols' working hypothesis was that adaptation to the underground environment had occurred locally in London once only.
These widely separated populations are distinguished by very minor genetic differences, which suggest that the molestus form developed: a single mtDNA difference shared among the underground populations of ten Russian cities; a single fixed microsatellite difference in populations spanning Europe, Japan, Australia, the middle East and Atlantic islands.
Madeira House Mouse
The Madeira mice are species of mice, descended from the house mouse (Mus Musculus), that went through speciation after colonization of the island Madeira in the 1400's. Up to six distinct species are present, driven by Robertsonian translocations (fusions of different numbered chromosomes). This driving force is particularly interesting because the human genome evidences a Robertsonian translocation in its divergence from its ape and ape-like cousins.Mollies
The Shortfin Molly—Poecilia mexicanaPoecilia
Poecilia is a genus of euryhaline brackish water fish in family Poeciliidae of order Cyprinodontiformes. The type species is P. vivipara. Live-bearers, the Poecilia species are collectively known as mollies, with the exception of Endler's livebearer and the famous guppy...
—is a small fish that lives in the Sulfur Caves
Lechuguilla Cave
Lechuguilla Cave is, as of June 2011, the sixth longest cave known to exist in the world, and the deepest in the continental United States , but it is most famous for its unusual geology, rare formations, and pristine condition....
of Mexico. Michael Tobler from the Texas A&M University
Texas A&M University
Texas A&M University is a coeducational public research university located in College Station, Texas . It is the flagship institution of the Texas A&M University System. The sixth-largest university in the United States, A&M's enrollment for Fall 2011 was over 50,000 for the first time in school...
has studied the fish for years and found that two distinct populations of mollies—the dark interior fish and the bright surface water fish—are becoming more genetically divergent. The populations have no obvious barrier separating the two; however, it was found that the mollies are hunted by a large water bug (Belostoma spp). Tobler collected the bug and both types of mollies, placed them in large plastic bottles, and put them back in the cave. After a day, it was found that, in the light, the cave-adapted fish endured the most damage, with four out of every five stab-wounds from the water bugs sharp mouthparts. In the dark, the situation was the opposite. The mollies’ senses can detect a predator’s threat in their own habitats, but not in the other ones. Moving from one habitat to the other significantly increases the risk of dying. Tobler plans on further experiments, but believes that it is a good example of the rise of a new species.
Thale cress
Kirsten Bomblies
Kirsten Bomblies is a biological researcher. She was born in 1973 in Germany and grew up in Castle Rock, CO. She received a BA in Biochemistry and Biology from The University of Pennsylvania in 1996. Her research plumbs the genetic, biophysical and other processes that may give rise to new species....
et al. from the Max Planck Institute for Developmental Biology
Max Planck Institute for Developmental Biology
The Max Planck Institute for Developmental Biology is located in Tübingen, Germany. The main topics of scientific research conducted by the Max Planck Institute for Developmental Biology focus on the molecular mechanisms underlying spatial information within the embryo, communication between cells...
discovered that two genes passed down by each parent of the thale cress plant, Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
. When the genes are passed down, it ignites a reaction in the hybrid plant that turns its own immune system against it. In the parents, the genes were not detrimental, but they evolved separately to react defectively when combined.
To test this, Bomblies crossed 280 genetically different strains of Arabidopsis in 861 distinct ways and found that 2 per cent of the resulting hybrids were necrotic. Along with allocating the same indicators, the 20 plants also shared a comparable collection of genetic activity in a group of 1,080 genes. In almost all of the cases, Bomblies discovered that only two genes were required to cause the autoimmune response. Bomblies looked at one hybrid in detail and found that one of the two genes belonged to the NB-LRR class
Gene-for-gene relationship
The gene-for-gene relationship was discovered by Harold Henry Flor who was working with rust of flax . Flor was the first scientist to study the genetics of both the host and parasite and to integrate them into one genetic system...
, a common group of disease resistance genes involved in recognizing new infections. When Bomblies removed the problematic gene, the hybrids developed normally.
Over successive generations, these incompatibilities could create divisions between different plant strains, reducing their chances of successful mating and turning distinct strains into separate species.
Interspecies fertility or hybridization
Understood from laboratory studies and observed instances of speciation in nature, finding species that are able reproduce successfully or create hybrids between two different species infers that their relationship is close. In conjunction with this, hybridization has been found to be a precursor to the creation of new species by being a source of new genes for a species. The examples provided are only a small fraction of the observed instances of speciation through hybridization. Plants are often subject to the creation of a new species though hybridization.Polar bear
A specific example of large-scale evolution is the polar bearPolar Bear
The polar bear is a bear native largely within the Arctic Circle encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the world's largest land carnivore and also the largest bear, together with the omnivorous Kodiak Bear, which is approximately the same size...
(Ursus maritimus). The polar bear is related to the brown bear
Brown Bear
The brown bear is a large bear distributed across much of northern Eurasia and North America. It can weigh from and its largest subspecies, the Kodiak Bear, rivals the polar bear as the largest member of the bear family and as the largest land-based predator.There are several recognized...
(Ursus arctos) but they can still interbreed and produce fertile offspring. However, it has acquired significant physiological differences from the brown bear. These differences allow the polar bear to comfortably survive in conditions that the brown bear could not including the ability to swim sixty miles or more at a time in freezing waters, to blend in with the snow, and to stay warm in the arctic environment. Additionally, the elongation of the neck makes it easier to keep their heads above water while swimming and the oversized webbed feet that act as paddles when swimming. The polar bear has also evolved small papillae and vacuole-like suction cups on the soles to make them less likely to slip on the ice; feet covered with heavy matting to protect the bottoms from intense cold and provide traction; smaller ears to reduce the loss of heat; eyelids that act like sunglasses; accommodations for their all-meat diet; a large stomach capacity to enable opportunistic feeding; and the ability to fast for up to nine months while recycling their urea.
Raphanobrassica
RaphanobrassicaRaphanobrassica
Brassicoraphanus is the name for all the intergeneric hybrids between the genera Brassica and Raphanus . The name comes from the combination of the genus names...
includes all intergeneric hybrids between the genera Raphanus
Raphanus
Raphanus is a genus within the flowering plant family Brassicaceae. Two or three species are currently classified in Raphanus. They include the cultivated radish, Raphanus sativus and the common wild radish or jointed charlock, R. raphanistrum. Some authors accept the podding or rattail radish, R...
(radish) and Brassica
Brassica
Brassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
(cabbages, etc.).
The Raphanobrassica is an allopolyploid cross between the radish
Radish
The radish is an edible root vegetable of the Brassicaceae family that was domesticated in Europe, in pre-Roman times. They are grown and consumed throughout the world. Radishes have numerous varieties, varying in size, color and duration of required cultivation time...
(Raphanus sativus) and cabbage
Cabbage
Cabbage is a popular cultivar of the species Brassica oleracea Linne of the Family Brassicaceae and is a leafy green vegetable...
(Brassica oleracea). Plants of this parentage are now known as radicole. Two other fertile forms of Raphanobrassica are known. Raparadish, an allopolyploid hybrid between Raphanus sativus and Brassica rapa is grown as a fodder crop. "Raphanofortii" is the allopolyploid hybrid between Brassica tournefortii
Brassica tournefortii
The mustard species Brassica tournefortii is known by the common names Asian mustard, African mustard, and Sahara mustard, and is well-known as an invasive species, especially in California....
and Raphanus caudatus.
The Raphanobrassica is a fascinating plant, because (in spite of its hybrid nature), it is not sterile. This has led some botanists to propose that the accidental hybridization of a flower by pollen of another species in nature could be a mechanism of speciation common in higher plants.
Salsify
SalsifiesSalsify
Tragopogon, also known as salsify or goatsbeard, is a genus of flowering plants in the sunflower family Asteraceae that has over 140 species, including the vegetable known as salsify, as well as a number of common wild flowers, some of which are usually regarded as weeds.Salsifies are forbs growing...
are one example where hybrid speciation
Hybrid speciation
Hybrid speciation is the process wherein hybridization between two different closely related species leads to a distinct phenotype. This phenotype in very rare cases can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if...
has been observed. In the early 20th century, humans introduced three species of goatsbeard into North America. These species, the western salsify (Tragopogon dubius), the meadow salsify (Tragopogon pratensis), and the oyster plant (Tragopogon porrifolius), are now common weeds in urban wastelands. In the 1950s, botanists found two new species in the regions of Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
and Washington, where the three already known species overlapped. One new species, Tragopogon miscellus, is a tetraploid hybrid of T. dubius and T. pratensis. The other new species, Tragopogon mirus, is also an allopolyploid, but its ancestors were T. dubius and T. porrifolius. These new species are usually referred to as "the Ownbey hybrids" after the botanist who first described them. The T. mirus population grows mainly by reproduction of its own members, but additional episodes of hybridization continue to add to the T. mirus population.
T. dubius and T. pratensis mated in Europe but were never able to hybridize. A study published in March 2011 found that when these two plants were introduced to North America in the 1920s, they mated and doubled the number of chromosomes in there hybrid Tragopogon miscellus allowing for a “reset” of its genes, which in turn, allows for greater genetic variation. Professor Doug Soltis of the University of Florida
University of Florida
The University of Florida is an American public land-grant, sea-grant, and space-grant research university located on a campus in Gainesville, Florida. The university traces its historical origins to 1853, and has operated continuously on its present Gainesville campus since September 1906...
said, “We caught evolution in the act…New and diverse patterns of gene expression may allow the new species to rapidly adapt in new environments”. This observable event of speciation through hybridization further advances the evidence for the common descent of organisms and the time frame in which the new species arose in its new environment. The hybridizations have been reproduced artificially in laboratories from 2004 to present day.
Welsh groundsel
Welsh groundsel is an allopolyploid, a plant which contains sets of chromosomes originating from two different species. Its ancestor was Senecio × baxteri, an infertile hybrid which can arise spontaneously when the closely related groundsel (Senecio vulgaris) and Oxford ragwort (Senecio squalidusSenecio squalidus
Oxford Ragwort , is a member of the Senecio genus in the daisy family Asteraceae. It is a yellow-flowered herbaceous plant, native to mountainous, rocky or volcanic areas, that has managed to find other homes on man-made and natural piles of rocks, war-ruined neighborhoods and even on stone walls...
) grow alongside each other. Sometime in the early 20th century, an accidental doubling of the number of chromosomes in an S. × baxteri plant led to the formation of a new fertile species.
York groundsel
The York groundsel (Senecio eboracensis) is a hybrid species of the self-incompatibleSelf-incompatibility in plants
Self-incompatibility is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage outcrossing...
Senecio squalidus
Senecio squalidus
Oxford Ragwort , is a member of the Senecio genus in the daisy family Asteraceae. It is a yellow-flowered herbaceous plant, native to mountainous, rocky or volcanic areas, that has managed to find other homes on man-made and natural piles of rocks, war-ruined neighborhoods and even on stone walls...
(also known as Oxford ragwort) and the self-compatible Senecio vulgaris (also known as Common groundsel). Like S. vulgaris, S. eboracensis is self-compatible, however, it shows little or no natural crossing with its parent species, and is therefore reproductively isolated, indicating that strong breed barriers exist between this new hybrid and its parents.
It resulted from a backcrossing
Backcrossing
Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent...
of the F1 hybrid
F1 hybrid
F1 hybrid is a term used in genetics and selective breeding. F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types....
of its parents to S. vulgaris. S. vulgaris is native to Britain, while S. squalidus was introduced from Sicily in the early 18th century; therefore, S. eboracensis has speciated from those two species within the last 300 years.
Other hybrids descended from the same two parents are known. Some are infertile, such as S. x baxteri. Other fertile hybrids are also known, including S. vulgaris var. hibernicus, now common in Britain, and the allohexaploid
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
S. cambrensis
Senecio cambrensis
Senecio cambrensis is a flowering plant of the family Asteraceae. It is endemic to the United Kingdom and currently known only from North Wales...
, which according to molecular evidence probably originated independently at least three times in different locations. Morphological and genetic evidence support the status of S. eboracensis as separate from other known hybrids.
Evidence from artificial selection
Artificial selectionArtificial selection
Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive...
demonstrates the diversity that can exist among organisms that share a relatively recent common ancestor. In artificial selection, one species is bred selectively at each generation, allowing only those organisms that exhibit desired characteristics to reproduce. These characteristics become increasingly well developed in successive generations. Artificial selection was successful long before science discovered the genetic basis. Examples of artificial selection would be dog breeding
Dog breeding
Dog breeding is the practice of mating selected specimens with the intent to maintain or produce specific qualities and characteristics.Dogs reproduce without human interference, so their offsprings' characteristics are determined by natural selection. Domestic dogs may be intentionally bred by...
, genetically modified food
Genetically modified food
Genetically modified foods are foods derived from genetically modified organisms . Genetically modified organisms have had specific changes introduced into their DNA by genetic engineering techniques...
, flower breeding, cultivation of foods such as wild cabbage
Brassica oleracea
Brassica oleracea, or wild cabbage, is a species of Brassica native to coastal southern and western Europe, where its tolerance of salt and lime and its intolerance of competition from other plants typically restrict its natural occurrence to limestone sea cliffs, like the chalk cliffs on both...
, and others.
Evidence from computation and mathematical iteration
Computer scienceComputer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
allows the iteration
Iteration
Iteration means the act of repeating a process usually with the aim of approaching a desired goal or target or result. Each repetition of the process is also called an "iteration," and the results of one iteration are used as the starting point for the next iteration.-Mathematics:Iteration in...
of self changing complex system
Complex system
A complex system is a system composed of interconnected parts that as a whole exhibit one or more properties not obvious from the properties of the individual parts....
s to be studied, allowing a mathematical understanding of the nature of the processes behind evolution; providing evidence for the hidden causes of known evolutionary events. The evolution of specific cellular mechanisms like spliceosome
Spliceosome
A spliceosome is a complex of snRNA and protein subunits that removes introns from a transcribed pre-mRNA segment. This process is generally referred to as splicing.-Composition:...
s that can turn the cell's genome into a vast workshop of billions of interchangeable parts that can create tools that create tools that create tools that create us can be studied for the first time in an exact way.
"It has taken more than five decades, but the electronic computer is now powerful enough to simulate evolution" assisting bioinformatics
Bioinformatics
Bioinformatics is the application of computer science and information technology to the field of biology and medicine. Bioinformatics deals with algorithms, databases and information systems, web technologies, artificial intelligence and soft computing, information and computation theory, software...
in its attempt to solve biological problems.
Computational evolutionary biology has enabled researchers to trace the evolution of a large number of organisms by measuring changes in their DNA, rather than through physical taxonomy or physiological observations alone. It has compared entire genomes permitting the study of more complex evolutionary events, such as gene duplication
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
, horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
, and the prediction of factors important in speciation. It has also helped build complex computational models of populations to predict the outcome of the system over time and track and share information on an increasingly large number of species and organisms.
Future endeavors are to reconstruct a now more complex tree of life.
Christoph Adami, a professor at the Keck Graduate Institute made this point in Evolution of biological complexity:
- To make a case for or against a trend in the evolution of complexity in biological evolution, complexity needs to be both rigorously defined and measurable. A recent information-theoretic (but intuitively evident) definition identifies genomic complexity with the amount of information a sequence stores about its environment. We investigate the evolution of genomic complexity in populations of digital organisms and monitor in detail the evolutionary transitions that increase complexity. We show that, because natural selection forces genomes to behave as a natural "Maxwell DemonMaxwell's demonIn the philosophy of thermal and statistical physics, Maxwell's demon is a thought experiment created by the Scottish physicist James Clerk Maxwell to "show that the Second Law of Thermodynamics has only a statistical certainty." It demonstrates Maxwell's point by hypothetically describing how to...
", within a fixed environment, genomic complexity is forced to increase.
David J. Earl and Michael W. Deem—professors at Rice University
Rice University
William Marsh Rice University, commonly referred to as Rice University or Rice, is a private research university located on a heavily wooded campus in Houston, Texas, United States...
made this point in Evolvability is a selectable trait:
- Not only has life evolved, but life has evolved to evolve. That is, correlations within protein structure have evolved, and mechanisms to manipulate these correlations have evolved in tandem. The rates at which the various events within the hierarchy of evolutionary moves occur are not random or arbitrary but are selected by Darwinian evolution. Sensibly, rapid or extreme environmental change leads to selection for greater evolvability. This selection is not forbidden by causality and is strongest on the largest-scale moves within the mutational hierarchy. Many observations within evolutionary biology, heretofore considered evolutionary happenstance or accidents, are explained by selection for evolvability. For example, the vertebrate immune system shows that the variable environment of antigens has provided selective pressure for the use of adaptable codons and low-fidelity polymerases during somatic hypermutationSomatic hypermutationSomatic hypermutation is a mechanism inside cells that is part of the way the immune system adapts to the new foreign elements that confront it . SHM diversifies the receptors used by the immune system to recognize foreign elements and allows the immune system to adapt its response to new threats...
. A similar driving force for biased codon usage as a result of productively high mutation rates is observed in the hemagglutinin protein of influenza AInfluenzavirus AInfluenza A virus causes influenza in birds and some mammals and is the only species of Influenzavirus A. Influenzavirus A is a genus of the Orthomyxoviridae family of viruses. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon...
.
"Computer simulations of the evolution of linear sequences have demonstrated the importance of recombination of blocks of sequence rather than point mutagenesis alone. Repeated cycles of point mutagenesis, recombination, and selection should allow in vitro molecular evolution of complex sequences, such as proteins." Evolutionary molecular engineering, also called directed evolution or in vitro molecular evolution involves the iterated cycle of mutation, multiplication with recombination, and selection of the fittest of individual molecules (proteins, DNA, and RNA). Natural evolution can be relived showing us possible paths from catalytic cycles based on proteins to based on RNA to based on DNA.
Avida simulation
Richard Lenski, Charles Ofria, et al. at Michigan State UniversityMichigan State University
Michigan State University is a public research university in East Lansing, Michigan, USA. Founded in 1855, it was the pioneer land-grant institution and served as a model for future land-grant colleges in the United States under the 1862 Morrill Act.MSU pioneered the studies of packaging,...
developed an artificial life
Artificial life
Artificial life is a field of study and an associated art form which examine systems related to life, its processes, and its evolution through simulations using computer models, robotics, and biochemistry. The discipline was named by Christopher Langton, an American computer scientist, in 1986...
computer program with the ability to detail the evolution of complex systems. The system uses values set to determine random mutations and allows for the effect of natural selection to conserve beneficial traits. The program was dubbed Avida and starts with an artificial petri dish where organisms reproduce and perform mathematical calculations to acquire rewards of more computer time for replication. The program randomly adds mutations to copies of the artificial organisms to allow for natural selection. As the artificial life reproduced, different lines adapted and evolved depending on their set environments. The beneficial side to the program is that it parallels that of real life at rapid speeds.
See also
- Nothing in Biology Makes Sense Except in the Light of EvolutionNothing in Biology Makes Sense Except in the Light of Evolution"Nothing in Biology Makes Sense Except in the Light of Evolution" is a 1973 essay by the evolutionary biologist and Russian Orthodox Christian Theodosius Dobzhansky, criticising anti-evolution creationism and espousing theistic evolution...
Further reading
- Biological science, Oxford, 2002.
- Darwin, Charles November 24, 1859. On the Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London: John MurrayJohn Murray (publisher)John Murray is an English publisher, renowned for the authors it has published in its history, including Jane Austen, Sir Arthur Conan Doyle, Lord Byron, Charles Lyell, Johann Wolfgang von Goethe, Herman Melville, and Charles Darwin...
, Albemarle StreetAlbemarle StreetAlbemarle Street is a street in Mayfair in central London, off Piccadilly. It has historic associations with Lord Byron, whose publisher John Murray was based here, and Oscar Wilde, a member of the Albemarle Club, where an insult he received led to his suing for libel and to his eventual imprisonment...
. 502 pages. Reprinted: Gramercy (May 22, 1995). ISBN 0-517-12320-7 - Futuyma, D.J. 1998. Evolutionary Biology. 3rd ed. Sinauer Associates, Sunderland, Massachusetts. (dated 1998, published 1997) ISBN 0-87893-189-9
- Ho, YK (2004). Advanced-level Biology for Hong Kong, Manhattan Press. ISBN 962-990-635-X
External links
- National Academies Evolution Resources
- TalkOrigins ArchiveTalkOrigins ArchiveThe TalkOrigins Archive is a website that presents mainstream science perspectives on the antievolution claims of young-earth, old-earth, and "intelligent design" creationists...
- 29+ Evidences for Macroevolution: The Scientific Case for Common Descent - TalkOrigins ArchiveTalkOrigins ArchiveThe TalkOrigins Archive is a website that presents mainstream science perspectives on the antievolution claims of young-earth, old-earth, and "intelligent design" creationists...
- Transitional Vertebrate Fossils FAQ - Understanding Evolution: Your one-stop source for information on evolution
- National Academy Press: Teaching About Evolution and the Nature of Science
- Evolution — Provided by PBSPublic Broadcasting ServiceThe Public Broadcasting Service is an American non-profit public broadcasting television network with 354 member TV stations in the United States which hold collective ownership. Its headquarters is in Arlington, Virginia....
. - Evolution News from Genome News Network (GNN)
- Evolution by Natural Selection — An introduction to the logic of the theory of evolution by natural selection
- Howstuffworks.com — How Evolution Works
- 15 Evolutionary Gems