Modern evolutionary synthesis
Encyclopedia
The modern evolutionary synthesis is a union of ideas from several biological
specialties which provides a widely accepted account of evolution
. It is also referred to as the new synthesis, the modern synthesis, the evolutionary synthesis, millennium synthesis and the neo-darwinian synthesis.
The synthesis, produced between 1936 and 1947, reflects the current consensus. The previous development of population genetics
, between 1918 and 1932, was a stimulus, as it showed that Mendelian genetics
was consistent with natural selection
and gradual evolution. The synthesis is still, to a large extent, the current paradigm
in evolutionary biology.
The modern synthesis solved difficulties and confusions caused by the specialisation and poor communication between biologists in the early years of the 20th century. At its heart was the question of whether Mendelian genetics could be reconciled with gradual evolution by means of natural selection. A second issue was whether the broad-scale changes (macroevolution
) seen by palaeontologists could be explained by changes seen in local populations (microevolution
).
The synthesis included evidence from biologists, trained in genetics, who studied populations in the field and in the laboratory. These studies were crucial to evolutionary theory. The synthesis drew together ideas from several branches of biology which had become separated, particularly genetics
, cytology
, systematics
, botany
, morphology
, ecology
and paleontology
.
Julian Huxley
invented the term, when he produced his book, Evolution: The Modern Synthesis
(1942). Other major figures in the modern synthesis include R. A. Fisher, Theodosius Dobzhansky
, J. B. S. Haldane
, Sewall Wright
, E. B. Ford, Ernst Mayr
, Bernhard Rensch
, Sergei Chetverikov
, George Gaylord Simpson
, and G. Ledyard Stebbins
.
The idea that speciation
occurs after populations are reproductively isolated has been much debated. In plants, polyploidy
must be included in any view of speciation. Formulations such as 'evolution consists primarily of changes in the frequencies of alleles
between one generation and another' were proposed rather later. The traditional view is that developmental biology ('evo-devo') played little part in the synthesis, but an account of Gavin de Beer
's work by Stephen J. Gould suggests he may be an exception.
's On the Origin of Species was successful in convincing most biologists that evolution had occurred, but was less successful in convincing them that natural selection was its primary mechanism. In the 19th and early 20th centuries, variations of Lamarckism
, orthogenesis
('progressive' evolution), and saltationism (evolution by jumps) were discussed as alternatives. Also, Darwin did not offer a precise explanation of how new species arise. As part of the disagreement about whether natural selection alone was sufficient to explain speciation
, George Romanes
coined the term neo-Darwinism
to refer to the version of evolution advocated by Alfred Russel Wallace
and August Weismann
with its heavy dependence on natural selection. Weismann and Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin had not ruled out.
Weismann's idea was that the relationship between the hereditary material, which he called the germ plasm
(de: Keimplasma), and the rest of the body (the soma
) was a one-way relationship: the germ-plasm formed the body, but the body did not influence the germ-plasm, except indirectly in its participation in a population subject to natural selection. Weismann was translated into English, and though he was influential, it took many years for the full significance of his work to be appreciated. Later, after the completion of the modern synthesis, the term neo-Darwinism came to be associated with its core concept: evolution, driven by natural selection acting on variation produced by genetic mutation, and genetic recombination
(chromosomal crossover
s).
's work was re-discovered by Hugo de Vries
and Carl Correns
in 1900. News of this reached William Bateson
in England, who reported on the paper during a presentation to the Royal Horticultural Society in May 1900. It showed that the contributions of each parent retained their integrity rather than blending with the contribution of the other parent. This reinforced a division of thought, which was already present in the 1890s.
The two schools were:
The relevance of Mendelism to evolution was unclear and hotly debated, especially by Bateson, who opposed the biometric ideas of his former teacher Weldon. Many scientists believed the two theories substantially contradicted each other. This debate between the biometricians and the Mendelians continued for some 20 years and was only solved by the development of population genetics
.
T. H. Morgan began his career in genetics as a saltationist, and started out trying to demonstrate that mutations could produce new species in fruit flies. However, the experimental work at his lab with Drosophila melanogaster
, which helped establish the link between Mendelian genetics and the chromosomal theory of inheritance, demonstrated that rather than creating new species in a single step, mutations increased the genetic variation in the population.
. R.A. Fisher, J.B.S. Haldane, and Sewall Wright
provided critical contributions. In 1918, Fisher produced the paper "The Correlation Between Relatives on the Supposition of Mendelian Inheritance
", which showed how the continuous variation measured by the biometricians could be the result of the action of many discrete genetic loci
. In this and subsequent papers culminating in his 1930 book The Genetical Theory of Natural Selection
, Fisher was able to show how Mendelian genetics was, contrary to the thinking of many early geneticists, completely consistent with the idea of evolution driven by natural selection
. During the 1920s, a series of papers by J.B.S. Haldane applied mathematical analysis to real world examples of natural selection such as the evolution of industrial melanism in peppered moths
. Haldane established that natural selection could work in the real world at a faster rate than even Fisher had assumed.
Sewall Wright focused on combinations of genes that interacted as complexes, and the effects of inbreeding on small relatively isolated populations, which could exhibit genetic drift
. In a 1932 paper he introduced the concept of an adaptive landscape in which phenomena such as cross breeding and genetic drift in small populations could push them away from adaptive peaks, which would in turn allow natural selection
to push them towards new adaptive peaks. Wright's model would appeal to field naturalists such as Theodosius Dobzhansky and Ernst Mayr who were becoming aware of the importance of geographical isolation in real world populations. The work of Fisher, Haldane and Wright founded the discipline of population genetics
. This is the precursor of the modern synthesis, which is an even broader coalition of ideas. One limitation of the modern synthesis version of population genetics is that it treats one gene locus at a time, neglecting genetic linkage
and resulting linkage disequilibrium
between loci.
, a Ukrainian emigrant, who had been a postdoctoral worker in Morgan's fruit fly lab, was one of the first to apply genetics to natural populations. He worked mostly with Drosophila pseudoobscura
. He says pointedly: "Russia has a variety of climates from the Arctic to sub-tropical... Exclusively laboratory workers who neither possess nor wish to have any knowledge of living beings in nature were and are in a minority". Not surprisingly, there were other Russian geneticists with similar ideas, though for some time their work was known to only a few in the West. His 1937 work Genetics and the Origin of Species
was a key step in bridging the gap between population geneticists and field naturalists. It presented the conclusions reached by Fisher, Haldane, and especially Wright in their highly mathematical papers in a form that was easily accessible to others. It also emphasized that real world populations had far more genetic variability than the early population geneticists had assumed in their models, and that genetically distinct sub-populations were important. Dobzhansky argued that natural selection worked to maintain genetic diversity as well as driving change. Dobzhansky had been influenced by his exposure in the 1920s to the work of a Russian geneticist named Sergei Chetverikov
who had looked at the role of recessive genes in maintaining a reservoir of genetic variability in a population before his work was shut down by the rise of Lysenkoism
in the Soviet Union
.
Edmund Brisco Ford's work complemented that of Dobzhansky. It was as a result of Ford's work, as well as his own, that Dobzhansky changed the emphasis in the third edition of his famous text from drift to selection. Ford was an experimental naturalist who wanted to test natural selection in nature. He virtually invented the field of research known as ecological genetics
. His work on natural selection in wild populations of butterflies and moths was the first to show that predictions made by R.A. Fisher were correct. He was the first to describe and define genetic polymorphism, and to predict that human blood group polymorphisms
might be maintained in the population by providing some protection against disease.
Ernst Mayr
's key contribution to the synthesis was Systematics and the Origin of Species
, published in 1942. Mayr emphasized the importance of allopatric speciation
, where geographically isolated sub-populations diverge so far that reproductive isolation
occurs. He was skeptical of the reality of sympatric speciation
believing that geographical isolation was a prerequisite for building up intrinsic (reproductive) isolating mechanisms. Mayr also introduced the biological species concept that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations. Before he left Germany
for the United States
in 1930, Mayr had been influenced by the work of German biologist Bernhard Rensch
. In the 1920s Rensch, who like Mayr did field work in Indonesia
, analyzed the geographic distribution of polytypic
species and complexes of closely related species paying particular attention to how variations between different populations correlated with local environmental factors such as differences in climate. In 1947, Rensch published Neuere Probleme der Abstammungslehre: die Transspezifische Evolution (English translation 1959: Evolution above the Species level). This looked at how the same evolutionary mechanisms involved in speciation might be extended to explain the origins of the differences between the higher level taxa. His writings contributed to the rapid acceptance of the synthesis in Germany.
George Gaylord Simpson
was responsible for showing that the modern synthesis was compatible with paleontology in his book Tempo and Mode in Evolution
published in 1944. Simpson's work was crucial because so many paleontologists had disagreed, in some cases vigorously, with the idea that natural selection was the main mechanism of evolution. It showed that the trends of linear progression (in for example the evolution of the horse
) that earlier paleontologists had used as support for neo-Lamarckism and orthogenesis
did not hold up under careful examination. Instead the fossil record was consistent with the irregular, branching, and non-directional pattern predicted by the modern synthesis.
The botanist G. Ledyard Stebbins
was another major contributor to the synthesis. His major work, Variation and Evolution in Plants
, was published in 1950. It extended the synthesis to encompass botany including the important effects of hybridization and polyploidy
in some kinds of plants.
, George C. Williams
, John Maynard Smith
and others led to the development of a gene-centric view of evolution in the 1960s. The synthesis as it exists now has extended the scope of the Darwinian idea of natural selection to include subsequent scientific discoveries and concepts unknown to Darwin, such as DNA
and genetics
, which allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection
, altruism
, and speciation
.
In The Selfish Gene
, author Richard Dawkins
asserts the gene is the only true unit of selection
. (Dawkins also attempts to apply evolutionary theory to non-biological entities, such as cultural "memes
", imagined to be subject to selective forces analogous to those affecting biological entities.)
Others, such as Stephen J. Gould, reject the notion that genetic entities are subject to anything other than genetic or chemical forces, (as well as the idea evolution acts on "populations" per se), reasserting the centrality of the individual organism as the true unit of selection, whose specific phenotype is directly subject to evolutionary pressures.
In 1972, the notion of gradualism in evolution was challenged by a theory of "punctuated equilibrium
" put forward by Gould and Niles Eldredge
, proposing evolutionary changes could occur in relatively rapid spurts, when selective pressures were heightened, punctuating long periods of morphological stability, as well-adapted organisms coped successfully in their respective environments.
Discovery in the 1980's of Hox genes and regulators conserved across multiple phyletic divisions began the process of addressing basic theoretical problems relating to gradualism, incremental change, and sources of novelty in evolution. Suddenly, evolutionary theorists could answer the charge that spontaneous random mutations should result overwhelmingly in deleterious changes to a fragile, monolithic genome: Mutations in homeobox
regulation could safely--yet dramatically--alter morphology at a high level, without damaging coding for specific organs or tissues.
This, in turn, provided the means to model hypothetical genomic changes expressed in the phenotypes of long-extinct species, like the recently discovered "fish with hands"' Tiktaalik
.
As these recent discoveries suggest, the synthesis continues to undergo regular review, drawing on insights offered by both new biotechnologies and new paleontological discoveries. (See also Current research in evolutionary biology
).
's geology, but our present understanding of Earth history includes some critical advances made during the last half-century.
Conclusion: Our present knowledge of earth history strongly suggests that large-scale geophysical events influenced macroevolution
and megaevolution. These terms refer to evolution above the species level, including such events as mass extinctions, adaptive radiation
, and the major transitions in evolution.
was discovered in lichen
and in plant roots (rhizobia
in root nodules) in the 19th century, the idea arose that the process might have occurred more widely, and might be important in evolution. Anton de Bary
invented the concept of symbiosis; several Russian biologists promoted the idea; Edwin Wilson
mentioned it in his text The Cell; as did Ivan Emmanuel Wallin in his Symbionticism and the origin of species; and there was a brief mention by Julian Huxley
in 1930; all in vain because sufficient evidence was lacking. Symbiosis as a major evolutionary force was not discussed at all in the evolutionary synthesis.
The role of symbiosis in cell evolution was revived partly by Joshua Lederberg
, and finally brought to light by Lynn Margulis
in a series of papers and books. It turns out that some cell organelles are of microbial origin: mitochondria and chloroplasts definitely, cilia, flagella and centrioles possibly, and perhaps the nuclear membrane and much of the chromosome structure as well. What is now clear is that the evolution of eukaryote
cells is either caused by, or at least profoundly influenced by, symbiosis with bacterial and archaea
n cells in the Proterozoic
.
The origin of the eukaryote cell by symbiosis in several stages was not part of the evolutionary synthesis. It is, at least on first sight, an example of megaevolution by big jumps. However, what symbiosis provided was a copious supply of heritable variation from microorganisms, which was fine-tuned over a long period to produce the cell structure we see today. This part of the process is consistent with evolution by natural selection.
, RNA
) provides evidence of descent, and permits us to work out genealogical trees covering the whole of life, since now there are data on every major group of living organisms. This project, begun in a tentative way in the 1960s, has become a search for the universal tree or the universal ancestor, a phrase of Carl Woese
. The tree that results has some unusual features, especially in its roots. There are two domains of prokaryotes: bacteria
and archaea
, both of which contributed genetic material to the eukaryotes, mainly by means of symbiosis
. Also, since bacteria can pass genetic material to other bacteria, their relationships look more like a web than a tree. Once eukaryotes were established, their sexual reproduction produced the traditional branching tree-like pattern, the only diagram Darwin put in the Origin. The last universal ancestor (LUA
) would be a prokaryotic cell before the split between the bacteria and archaea. LUA is defined as most recent organism from which all organisms now living on Earth descend (some 3.5 to 3.8 billion years ago, in the Archean
era).
This technique may be used to clarify relationships within any group of related organisms. It is now a standard procedure, and examples are published regularly. April 2009 sees the publication of a tree covering all the animal phyla, derived from sequences from 150 genes in 77 taxa.
Early attempts to identify relationships between major groups were made in the 19th century by Ernst Haeckel
, and by comparative anatomists such as Thomas Henry Huxley and E. Ray Lankester. Enthusiasm waned: it was often difficult to find evidence to adjudicate between different opinions. Perhaps for that reason, the evolutionary synthesis paid surprisingly little attention to this activity. It is certainly a lively field of research today.
played a modest role in the evolutionary synthesis, mostly about evolution by changes in developmental timing (allometry and heterochrony
). Man himself was, according to Bolk
, a typical case of evolution by retention of juvenile characteristics (neoteny
). He listed many characters where "Man, in his bodily development, is a primate
foetus that has become sexually mature". Unfortunately, his interpretation of these ideas was non-Darwinian, but his list of characters is both interesting and convincing.
Modern interest in Evo-devo springs from clear proof that development is closely controlled by special genetic systems, and the hope that comparison of these systems will tell us much about the evolutionary history of different groups. In a series of experiments with the fruit-fly Drosophila
, Edward B. Lewis
was able to identify a complex of genes whose proteins bind to the cis-regulatory regions of target genes. The latter then activate or repress systems of cellular processes that accomplish the final development of the organism. Furthermore, the sequence of these control genes show co-linearity: the order of the loci in the chromosome parallels the order in which the loci are expressed along the anterior-posterior axis of the body. Not only that, but this cluster of master control genes programs the development of all higher organisms. Each of the genes contains a homeobox
, a remarkably conserved DNA sequence. This suggests the complex itself arose by gene duplication. In his Nobel
lecture, Lewis said "Ultimately, comparisons of the [control complexes] throughout the animal kingdom should provide a picture of how the organisms, as well as the [control genes] have evolved".
The term deep homology
was coined to describe the common origin of genetic regulatory apparatus used to build morphologically and phylogenetically disparate animal features. It applies when a complex genetic regulatory system is inherited from a common ancestor, as it is in the evolution of vertebrate and invertebrate eyes. The phenomenon is implicated in many cases of parallel evolution
.
A great deal of evolution may take place by changes in the control of development. This may be relevant to punctuated equilibrium
theory, for in development a few changes to the control system could make a significant difference to the adult organism. An example is the giant panda
, whose place in the Carnivora
was long uncertain. Apparently, the giant panda's evolution required the change of only a few genetic messages (5 or 6 perhaps), yet the phenotypic and lifestyle change from a standard bear is considerable. The transition could therefore be effected relatively swiftly.
period. These soft-bodied fossils are the first record of multicellular life. The interpretation of this fauna is still in flux.
Many outstanding discoveries have been made, and some of these have implications for evolutionary theory. The discovery of feathered dinosaurs
and early birds from the Lower Cretaceous of Liaoning, N.E. China have convinced most students that birds did evolve from coelurosaurian theropod dinosaurs. Less well known, but perhaps of equal evolutionary significance, are the studies on early insect flight, on stem tetrapods from the Upper Devonian, and the early stages of whale evolution.
Recent work has shed light on the evolution of flatfish
(pleuronectiformes), such as plaice
, sole
, turbot
and halibut
. Flatfish are interesting because they are one of the few vertebrate groups with external asymmetry. Their young are perfectly symmetrical, but the head is remodelled during a metamorphosis, which entails the migration of one eye to the other side, close to the other eye. Some species have both eyes on the left (turbot), some on the right (halibut, sole); all living and fossil flatfish to date show an 'eyed' side and a 'blind' side. The lack of an intermediate condition in living and fossil flatfish species had led to debate about the origin of such a striking adaptation. The case was considered by Lamark, who thought flatfish precursors would have lived in shallow water for a long period, and by Darwin
, who predicted a gradual migration of the eye, mirroring the metamorphosis of the living forms. Darwin's long-time critic St. George Mivart thought that the intermediate stages could have no selective value, and in the 6th edition of the Origin, Darwin made a concession to the possibility of acquired traits. Many years later the geneticist Richard Goldschmidt
put the case forward as an example of evolution by saltation
, bypassing intermediate forms.
A recent examination of two fossil species from the Eocene
has provided the first clear picture of flatfish evolution. The discovery of stem flatfish with incomplete orbital migration refutes Goldschmidt's ideas, and demonstrates that "the assembly of the flatfish bodyplan occurred in a gradual, stepwise fashion". There are no grounds for thinking that incomplete orbital migration was maladaptive, because stem forms with this condition ranged over two geological stages, and are found in localities which also yield flatfish with the full cranial asymmetry. The evolution of flatfish falls squarely within the evolutionary synthesis.
(HGT) (or Lateral gene transfer) is any process in which an organism gets genetic
material from another organism without being the offspring of that organism.
Most thinking in genetics
has focused on vertical transfer, but there is a growing awareness that horizontal gene transfer is a significant phenomenon. Amongst single-celled organisms it may be the dominant form of genetic transfer. Artificial horizontal gene transfer is a form of genetic engineering
.
Richardson and Palmer (2007) state: "Horizontal gene transfer (HGT) has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryote
s. However, the prevalence and importance of HGT in the evolution of multicellular eukaryotes remain unclear".
The bacterial means of HGT are:
Some examples of HGT in metazoa are now known. Genes in bdelloid rotifers have been found which appear to have originated in bacteria
, fungi, and plant
s. This suggests they arrived by horizontal gene transfer
. The capture and use of exogenous (~foreign) genes may represent an important force in bdelloid evolution. The team led by Matthew S. Meselson at Harvard University
has also shown that, despite the lack of sexual reproduction
, bdelloid rotifers do engage in genetic (DNA
) transfer within a species
or clade
. The method used is not known at present.
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
specialties which provides a widely accepted account of evolution
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...
. It is also referred to as the new synthesis, the modern synthesis, the evolutionary synthesis, millennium synthesis and the neo-darwinian synthesis.
The synthesis, produced between 1936 and 1947, reflects the current consensus. The previous development of population genetics
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
, between 1918 and 1932, was a stimulus, as it showed that Mendelian genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
was consistent with 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....
and gradual evolution. The synthesis is still, to a large extent, the current paradigm
Paradigm
The word paradigm has been used in science to describe distinct concepts. It comes from Greek "παράδειγμα" , "pattern, example, sample" from the verb "παραδείκνυμι" , "exhibit, represent, expose" and that from "παρά" , "beside, beyond" + "δείκνυμι" , "to show, to point out".The original Greek...
in evolutionary biology.
The modern synthesis solved difficulties and confusions caused by the specialisation and poor communication between biologists in the early years of the 20th century. At its heart was the question of whether Mendelian genetics could be reconciled with gradual evolution by means of natural selection. A second issue was whether the broad-scale changes (macroevolution
Macroevolution
Macroevolution is evolution on a scale of separated gene pools. Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes within a species or population.The process of speciation may fall...
) seen by palaeontologists could be explained by changes seen in local populations (microevolution
Microevolution
Microevolution is the changes in allele frequencies that occur over time within a population. This change is due to four different processes: mutation, selection , gene flow, and genetic drift....
).
The synthesis included evidence from biologists, trained in genetics, who studied populations in the field and in the laboratory. These studies were crucial to evolutionary theory. The synthesis drew together ideas from several branches of biology which had become separated, particularly genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, cytology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
, systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
, botany
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
, morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
, ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
and 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...
.
Julian Huxley
Julian Huxley
Sir Julian Sorell Huxley FRS was an English evolutionary biologist, humanist and internationalist. He was a proponent of natural selection, and a leading figure in the mid-twentieth century evolutionary synthesis...
invented the term, when he produced his book, Evolution: The Modern Synthesis
Evolution: The Modern Synthesis
Evolution: The Modern Synthesis, a 1942 book by Julian Huxley , is one of the most important books of the modern evolutionary synthesis.- Publication history :Allen & Unwin, London...
(1942). Other major figures in the modern synthesis include R. A. Fisher, Theodosius Dobzhansky
Theodosius Dobzhansky
Theodosius Grygorovych Dobzhansky ForMemRS was a prominent geneticist and evolutionary biologist, and a central figure in the field of evolutionary biology for his work in shaping the unifying modern evolutionary synthesis...
, 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...
, Sewall Wright
Sewall 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...
, E. B. Ford, Ernst Mayr
Ernst Mayr
Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
, Bernhard Rensch
Bernhard Rensch
Bernhard Rensch was a German evolutionary biologist, and ornithologist who did field work in Indonesia and India. He is probably best known as one of the architects of the modern evolutionary synthesis, which he popularised in Germany...
, Sergei Chetverikov
Sergei Chetverikov
Sergei Sergeevich Chetverikov was one of the early contributors to the development of the field of genetics...
, George Gaylord Simpson
George Gaylord Simpson
George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...
, and G. Ledyard Stebbins
G. Ledyard Stebbins
George Ledyard Stebbins, Jr. was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century. Stebbins received his Ph.D. in botany from Harvard University in 1931. He went on to the University of California, Berkeley, where his work...
.
Summary of the modern synthesis
The modern synthesis bridged the gap between experimental geneticists and naturalists, and between palaeontologists. It states that:- All evolutionary phenomena can be explained in a way consistent with known genetic mechanisms and the observational evidence of naturalists.
- Evolution is gradual: small genetic changes, recombination ordered by natural selection. Discontinuities amongst species (or other taxa) are explained as originating gradually through geographical separation and extinction (not saltationSaltation (biology)In biology, ""saltation"" is a sudden change from one generation to the next, that is large, or very large, in comparison with the usual variation of an organism...
). - Natural selectionNatural selectionNatural 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 by far the main mechanism of change; even slight advantages are important when continued. The object of selection is the phenotypePhenotypeA phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
in its surrounding environment. - The role of genetic driftGenetic driftGenetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
is equivocal. Though strongly supported initially by Dobzhansky, it was downgraded later as results from ecological genetics were obtained. - Thinking in terms of populationPopulationA population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
s, rather than individuals, is primary: the genetic diversityGenetic diversityGenetic diversity, the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....
existing in natural populations is a key factor in evolution. The strength of natural selection in the wild is greater than previously expected; the effect of ecological factors such as nicheEcological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
occupation and the significance of barriers to gene flow are all important. - In palaeontology, the ability to explain historical observations by extrapolation from microevolutionMicroevolutionMicroevolution is the changes in allele frequencies that occur over time within a population. This change is due to four different processes: mutation, selection , gene flow, and genetic drift....
to macroevolutionMacroevolutionMacroevolution is evolution on a scale of separated gene pools. Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes within a species or population.The process of speciation may fall...
is proposed. Historical contingency means explanations at different levels may exist. Gradualism does not mean constant rate of change.
The idea that 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...
occurs after populations are reproductively isolated has been much debated. In plants, polyploidy
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...
must be included in any view of speciation. Formulations such as 'evolution consists primarily of changes in the frequencies of alleles
Allele frequency
Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...
between one generation and another' were proposed rather later. The traditional view is that developmental biology ('evo-devo') played little part in the synthesis, but an account of Gavin de Beer
Gavin de Beer
Sir Gavin Rylands de Beer FRS was a British evolutionary embryologist. He was Director of the British Museum , President of the Linnean Society, and received the Royal Society's Darwin Medal for his studies on evolution.-Biography:...
's work by Stephen J. Gould suggests he may be an exception.
Developments leading up to the synthesis
1859–1899
Charles DarwinCharles 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...
's On the Origin of Species was successful in convincing most biologists that evolution had occurred, but was less successful in convincing them that natural selection was its primary mechanism. In the 19th and early 20th centuries, variations of Lamarckism
Lamarckism
Lamarckism is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring . It is named after the French biologist Jean-Baptiste Lamarck , who incorporated the action of soft inheritance into his evolutionary theories...
, orthogenesis
Orthogenesis
Orthogenesis, orthogenetic evolution, progressive evolution or autogenesis, is the hypothesis that life has an innate tendency to evolve in a unilinear fashion due to some internal or external "driving force". The hypothesis is based on essentialism and cosmic teleology and proposes an intrinsic...
('progressive' evolution), and saltationism (evolution by jumps) were discussed as alternatives. Also, Darwin did not offer a precise explanation of how new species arise. As part of the disagreement about whether natural selection alone was sufficient to explain 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...
, George Romanes
George Romanes
George John Romanes FRS was a Canadian-born English evolutionary biologist and physiologist who laid the foundation of what he called comparative psychology, postulating a similarity of cognitive processes and mechanisms between humans and other animals.He was the youngest of Charles Darwin's...
coined the term neo-Darwinism
Neo-Darwinism
Neo-Darwinism is the 'modern synthesis' of Darwinian evolution through natural selection with Mendelian genetics, the latter being a set of primary tenets specifying that evolution involves the transmission of characteristics from parent to child through the mechanism of genetic transfer, rather...
to refer to the version of evolution advocated by Alfred Russel Wallace
Alfred Russel Wallace
Alfred Russel Wallace, OM, FRS was a British naturalist, explorer, geographer, anthropologist and biologist...
and August Weismann
August Weismann
Friedrich Leopold August Weismann was a German evolutionary biologist. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin...
with its heavy dependence on natural selection. Weismann and Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin had not ruled out.
Weismann's idea was that the relationship between the hereditary material, which he called the germ plasm
Germ plasm
Germ plasm or polar plasm is a zone found in the cytoplasm of the egg cells of some model organisms , which contains determinants that will give rise to the germ cell lineage. As the zygote undergoes mitotic divisions the germ plasm is ultimately restricted to a few cells of the embryo...
(de: Keimplasma), and the rest of the body (the soma
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...
) was a one-way relationship: the germ-plasm formed the body, but the body did not influence the germ-plasm, except indirectly in its participation in a population subject to natural selection. Weismann was translated into English, and though he was influential, it took many years for the full significance of his work to be appreciated. Later, after the completion of the modern synthesis, the term neo-Darwinism came to be associated with its core concept: evolution, driven by natural selection acting on variation produced by genetic mutation, and genetic 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...
(chromosomal crossover
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
s).
1900–1915
Gregor MendelGregor Mendel
Gregor Johann Mendel was an Austrian scientist and Augustinian friar who gained posthumous fame as the founder of the new science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance...
's work was re-discovered by Hugo de Vries
Hugo de Vries
Hugo Marie de Vries ForMemRS was a Dutch botanist and one of the first geneticists. He is known chiefly for suggesting the concept of genes, rediscovering the laws of heredity in the 1890s while unaware of Gregor Mendel's work, for introducing the term "mutation", and for developing a mutation...
and Carl Correns
Carl Correns
Carl Erich Correns was a German botanist and geneticist, who is notable primarily for his independent discovery of the principles of heredity, and for his rediscovery of Gregor Mendel's earlier paper on that subject, which he achieved simultaneously but independently of the botanists Erich...
in 1900. News of this reached William Bateson
William Bateson
William Bateson was an English geneticist and a Fellow of St. John's College, Cambridge...
in England, who reported on the paper during a presentation to the Royal Horticultural Society in May 1900. It showed that the contributions of each parent retained their integrity rather than blending with the contribution of the other parent. This reinforced a division of thought, which was already present in the 1890s.
The two schools were:
- Saltationism (large mutations or jumps), favored by early Mendelians who viewed hard inheritance as incompatible with natural selection
- Biometric schoolBiostatisticsBiostatistics is the application of statistics to a wide range of topics in biology...
: led by Karl PearsonKarl PearsonKarl Pearson FRS was an influential English mathematician who has been credited for establishing the disciplineof mathematical statistics....
and Walter WeldonWalter Frank Raphael WeldonWalter Frank Raphael Weldon DSc FRS generally called Raphael Weldon, was an English evolutionary biologist and a founder of biometry...
, argued vigorously against it, saying that empirical evidence indicated that variation was continuous in most organisms, not discrete as Mendelism predicted.
The relevance of Mendelism to evolution was unclear and hotly debated, especially by Bateson, who opposed the biometric ideas of his former teacher Weldon. Many scientists believed the two theories substantially contradicted each other. This debate between the biometricians and the Mendelians continued for some 20 years and was only solved by the development of population genetics
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
.
T. H. Morgan began his career in genetics as a saltationist, and started out trying to demonstrate that mutations could produce new species in fruit flies. However, the experimental work at his lab with Drosophila melanogaster
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...
, which helped establish the link between Mendelian genetics and the chromosomal theory of inheritance, demonstrated that rather than creating new species in a single step, mutations increased the genetic variation in the population.
The foundation of population genetics
The first step towards the synthesis was the development of population geneticsPopulation genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
. R.A. Fisher, J.B.S. Haldane, and Sewall Wright
Sewall 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...
provided critical contributions. In 1918, Fisher produced the paper "The Correlation Between Relatives on the Supposition of Mendelian Inheritance
The Correlation Between Relatives on the Supposition of Mendelian Inheritance
The Correlation Between Relatives on the Supposition of Mendelian Inheritance is a scientific paper by R.A. Fisher which was published in the Philosophical Transactions of the Royal Society of Edinburgh in 1918,...
", which showed how the continuous variation measured by the biometricians could be the result of the action of many discrete genetic loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
. In this and subsequent papers culminating in his 1930 book The Genetical Theory of Natural Selection
The Genetical Theory of Natural Selection
The Genetical Theory of Natural Selection is a book by R.A. Fisher first published in 1930 by Clarendon. It is one of the most important books of the modern evolutionary synthesis and is commonly cited in biology books.-Editions:...
, Fisher was able to show how Mendelian genetics was, contrary to the thinking of many early geneticists, completely consistent with the idea of evolution driven by 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....
. During the 1920s, a series of papers by J.B.S. Haldane applied mathematical analysis to real world examples of natural selection such as the evolution of industrial melanism in peppered moths
Peppered moth evolution
The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon...
. Haldane established that natural selection could work in the real world at a faster rate than even Fisher had assumed.
Sewall Wright focused on combinations of genes that interacted as complexes, and the effects of inbreeding on small relatively isolated populations, which could exhibit genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
. In a 1932 paper he introduced the concept of an adaptive landscape in which phenomena such as cross breeding and genetic drift in small populations could push them away from adaptive peaks, which would in turn allow 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....
to push them towards new adaptive peaks. Wright's model would appeal to field naturalists such as Theodosius Dobzhansky and Ernst Mayr who were becoming aware of the importance of geographical isolation in real world populations. The work of Fisher, Haldane and Wright founded the discipline of population genetics
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
. This is the precursor of the modern synthesis, which is an even broader coalition of ideas. One limitation of the modern synthesis version of population genetics is that it treats one gene locus at a time, neglecting genetic linkage
Genetic linkage
Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...
and resulting linkage disequilibrium
Linkage disequilibrium
In population genetics, linkage disequilibrium is the non-random association of alleles at two or more loci, not necessarily on the same chromosome. It is also referred to as to as gametic phase disequilibrium , or simply gametic disequilibrium...
between loci.
The modern synthesis
Theodosius DobzhanskyTheodosius Dobzhansky
Theodosius Grygorovych Dobzhansky ForMemRS was a prominent geneticist and evolutionary biologist, and a central figure in the field of evolutionary biology for his work in shaping the unifying modern evolutionary synthesis...
, a Ukrainian emigrant, who had been a postdoctoral worker in Morgan's fruit fly lab, was one of the first to apply genetics to natural populations. He worked mostly with Drosophila pseudoobscura
Drosophila pseudoobscura
Drosophila pseudoobscura is a species of fruit fly, used extensively in lab studies of speciation.In 2005, D. pseudoobscura was the second Drosophila species to have its genome sequenced, after the model organism Drosophila melanogaster....
. He says pointedly: "Russia has a variety of climates from the Arctic to sub-tropical... Exclusively laboratory workers who neither possess nor wish to have any knowledge of living beings in nature were and are in a minority". Not surprisingly, there were other Russian geneticists with similar ideas, though for some time their work was known to only a few in the West. His 1937 work Genetics and the Origin of Species
Genetics and the Origin of Species
Genetics and the Origin of Species is a 1937 book by the twentieth century Ukrainian-American evolutionary biologist Theodosius Dobzhansky and one of the important books of the modern evolutionary synthesis. The book describes the Modern Synthesis of Evolution Theory, also known as Synthetic...
was a key step in bridging the gap between population geneticists and field naturalists. It presented the conclusions reached by Fisher, Haldane, and especially Wright in their highly mathematical papers in a form that was easily accessible to others. It also emphasized that real world populations had far more genetic variability than the early population geneticists had assumed in their models, and that genetically distinct sub-populations were important. Dobzhansky argued that natural selection worked to maintain genetic diversity as well as driving change. Dobzhansky had been influenced by his exposure in the 1920s to the work of a Russian geneticist named Sergei Chetverikov
Sergei Chetverikov
Sergei Sergeevich Chetverikov was one of the early contributors to the development of the field of genetics...
who had looked at the role of recessive genes in maintaining a reservoir of genetic variability in a population before his work was shut down by the rise of Lysenkoism
Lysenkoism
Lysenkoism, or Lysenko-Michurinism, also denotes the biological inheritance principle which Trofim Lysenko subscribed to and which derive from theories of the heritability of acquired characteristics, a body of biological inheritance theory which departs from Mendelism and that Lysenko named...
in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
.
Edmund Brisco Ford's work complemented that of Dobzhansky. It was as a result of Ford's work, as well as his own, that Dobzhansky changed the emphasis in the third edition of his famous text from drift to selection. Ford was an experimental naturalist who wanted to test natural selection in nature. He virtually invented the field of research known as ecological genetics
Ecological genetics
Ecological genetics is the study of genetics in natural populations.This contrasts with classical genetics, which works mostly on crosses between laboratory strains, and DNA sequence analysis, which studies genes at the molecular level....
. His work on natural selection in wild populations of butterflies and moths was the first to show that predictions made by R.A. Fisher were correct. He was the first to describe and define genetic polymorphism, and to predict that human blood group polymorphisms
Human blood group systems
The International Society of Blood Transfusion currently recognises 30 major blood group systems . Thus, in addition to the ABO antigens and Rhesus antigens, many other antigens are expressed on the red blood cell surface membrane...
might be maintained in the population by providing some protection against disease.
Ernst Mayr
Ernst Mayr
Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
's key contribution to the synthesis was Systematics and the Origin of Species
Systematics and the Origin of Species
Systematics and the Origin of Species from the Viewpoint of a Zoologist is a book written by zoologist and evolutionary biologist Ernst Mayr that was first published in 1942...
, published in 1942. Mayr emphasized the importance of allopatric speciation
Allopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
, where geographically isolated sub-populations diverge so far that 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...
occurs. He was skeptical of the reality of sympatric speciation
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...
believing that geographical isolation was a prerequisite for building up intrinsic (reproductive) isolating mechanisms. Mayr also introduced the biological species concept that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations. Before he left Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
for the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
in 1930, Mayr had been influenced by the work of German biologist Bernhard Rensch
Bernhard Rensch
Bernhard Rensch was a German evolutionary biologist, and ornithologist who did field work in Indonesia and India. He is probably best known as one of the architects of the modern evolutionary synthesis, which he popularised in Germany...
. In the 1920s Rensch, who like Mayr did field work in Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
, analyzed the geographic distribution of polytypic
Polytypic
In zoology, polytypic refers to a taxonomic unit with more than one subgroup at the next lower level.-See also:*Linnaean taxonomy*monotypic*monotypic habitat...
species and complexes of closely related species paying particular attention to how variations between different populations correlated with local environmental factors such as differences in climate. In 1947, Rensch published Neuere Probleme der Abstammungslehre: die Transspezifische Evolution (English translation 1959: Evolution above the Species level). This looked at how the same evolutionary mechanisms involved in speciation might be extended to explain the origins of the differences between the higher level taxa. His writings contributed to the rapid acceptance of the synthesis in Germany.
George Gaylord Simpson
George Gaylord Simpson
George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...
was responsible for showing that the modern synthesis was compatible with paleontology in his book Tempo and Mode in Evolution
Tempo and Mode in Evolution
Tempo and Mode in Evolution was George Gaylord Simpson's seminal contribution to the evolutionary synthesis, which integrated the facts of paleontology with those of genetics and natural selection....
published in 1944. Simpson's work was crucial because so many paleontologists had disagreed, in some cases vigorously, with the idea that natural selection was the main mechanism of evolution. It showed that the trends of linear progression (in for example the evolution of the horse
Evolution of the horse
The evolution of the horse pertains to the phylogenetic ancestry of the modern horse from the small dog-sized, forest-dwelling Hyracotherium over geologic time scales...
) that earlier paleontologists had used as support for neo-Lamarckism and orthogenesis
Orthogenesis
Orthogenesis, orthogenetic evolution, progressive evolution or autogenesis, is the hypothesis that life has an innate tendency to evolve in a unilinear fashion due to some internal or external "driving force". The hypothesis is based on essentialism and cosmic teleology and proposes an intrinsic...
did not hold up under careful examination. Instead the fossil record was consistent with the irregular, branching, and non-directional pattern predicted by the modern synthesis.
The botanist G. Ledyard Stebbins
G. Ledyard Stebbins
George Ledyard Stebbins, Jr. was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century. Stebbins received his Ph.D. in botany from Harvard University in 1931. He went on to the University of California, Berkeley, where his work...
was another major contributor to the synthesis. His major work, Variation and Evolution in Plants
Variation and Evolution in Plants
Variation and Evolution in Plants is a book written by G. Ledyard Stebbins, published in 1950. It is one of the key publications embodying the modern evolutionary synthesis, as the first comprehensive publication to discuss the relationship between genetics and natural selection in plants. The book...
, was published in 1950. It extended the synthesis to encompass botany including the important effects of hybridization and polyploidy
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...
in some kinds of plants.
Further advances
The modern evolutionary synthesis continued to be developed and refined after the initial establishment in the 1930s and 1940s. The work of W. D. HamiltonW. D. Hamilton
William Donald Hamilton FRS was a British evolutionary biologist, widely recognised as one of the greatest evolutionary theorists of the 20th century....
, George C. Williams
George C. Williams
Professor George Christopher Williams was an American evolutionary biologist.Williams was a professor emeritus of biology at the State University of New York at Stony Brook. He was best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
, John Maynard Smith
John Maynard Smith
John Maynard Smith,His surname was Maynard Smith, not Smith, nor was it hyphenated. F.R.S. was a British theoretical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics under the well-known biologist J.B.S....
and others led to the development of a gene-centric view of evolution in the 1960s. The synthesis as it exists now has extended the scope of the Darwinian idea of natural selection to include subsequent scientific discoveries and concepts unknown to Darwin, 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...
and genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, which allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
, altruism
Altruism
Altruism is a concern for the welfare of others. It is a traditional virtue in many cultures, and a core aspect of various religious traditions, though the concept of 'others' toward whom concern should be directed can vary among cultures and religions. Altruism is the opposite of...
, and 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...
.
In The Selfish Gene
The Selfish Gene
The Selfish Gene is a book on evolution by Richard Dawkins, published in 1976. It builds upon the principal theory of George C. Williams's first book Adaptation and Natural Selection. Dawkins coined the term "selfish gene" as a way of expressing the gene-centred view of evolution as opposed to the...
, author Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
asserts the gene is the only true unit of selection
Unit of selection
A unit of selection is a biological entity within the hierarchy of biological organisation that is subject to natural selection...
. (Dawkins also attempts to apply evolutionary theory to non-biological entities, such as cultural "memes
Meme
A meme is "an idea, behaviour or style that spreads from person to person within a culture."A meme acts as a unit for carrying cultural ideas, symbols or practices, which can be transmitted from one mind to another through writing, speech, gestures, rituals or other imitable phenomena...
", imagined to be subject to selective forces analogous to those affecting biological entities.)
Others, such as Stephen J. Gould, reject the notion that genetic entities are subject to anything other than genetic or chemical forces, (as well as the idea evolution acts on "populations" per se), reasserting the centrality of the individual organism as the true unit of selection, whose specific phenotype is directly subject to evolutionary pressures.
In 1972, the notion of gradualism in evolution was challenged by a theory of "punctuated equilibrium
Punctuated equilibrium
Punctuated 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...
" put forward by Gould and Niles Eldredge
Niles Eldredge
Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...
, proposing evolutionary changes could occur in relatively rapid spurts, when selective pressures were heightened, punctuating long periods of morphological stability, as well-adapted organisms coped successfully in their respective environments.
Discovery in the 1980's of Hox genes and regulators conserved across multiple phyletic divisions began the process of addressing basic theoretical problems relating to gradualism, incremental change, and sources of novelty in evolution. Suddenly, evolutionary theorists could answer the charge that spontaneous random mutations should result overwhelmingly in deleterious changes to a fragile, monolithic genome: Mutations in homeobox
Homeobox
A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of anatomical development in animals, fungi and plants.- Discovery :...
regulation could safely--yet dramatically--alter morphology at a high level, without damaging coding for specific organs or tissues.
This, in turn, provided the means to model hypothetical genomic changes expressed in the phenotypes of long-extinct species, like the recently discovered "fish with hands"' 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...
.
As these recent discoveries suggest, the synthesis continues to undergo regular review, drawing on insights offered by both new biotechnologies and new paleontological discoveries. (See also Current research in evolutionary biology
Current research in evolutionary biology
Current research in evolutionary biology covers diverse topics, as should be expected given the centrality of evolution to understanding biology...
).
After the synthesis
There are a number of discoveries in earth sciences and biology which have arisen since the synthesis. Listed here are some of those topics which are relevant to the evolutionary synthesis, and which seem soundly based.Understanding of Earth history
The Earth is the stage on which the evolutionary play is performed. Darwin studied evolution in the context of Charles LyellCharles Lyell
Sir Charles Lyell, 1st Baronet, Kt FRS was a British lawyer and the foremost geologist of his day. He is best known as the author of Principles of Geology, which popularised James Hutton's concepts of uniformitarianism – the idea that the earth was shaped by slow-moving forces still in operation...
's geology, but our present understanding of Earth history includes some critical advances made during the last half-century.
- The age of the EarthAge of the EarthThe age of the Earth is 4.54 billion years This age is based on evidence from radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples...
has been revised upwards. It is now estimated at 4.56 billion years, about one-third of the age of the universeAge of the universeThe age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...
. It is worth noting that the PhanerozoicPhanerozoicThe Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
only occupies the last 1/9 of this period of time.
- The triumph of Alfred WegenerAlfred WegenerAlfred Lothar Wegener was a German scientist, geophysicist, and meteorologist.He is most notable for his theory of continental drift , proposed in 1912, which hypothesized that the continents were slowly drifting around the Earth...
's idea of continental driftContinental driftContinental 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...
came around 1960. The key principle of plate tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphereAsthenosphereThe asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
. This discovery provides a unifying theory for geology, linking phenomena such as volcanos, earthquakes, orogenyOrogenyOrogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
, and providing data for many paleogeographical questions. One major question is still unclear: when did plate tectonics begin?
- Our understanding of the evolution of the Earth's atmosphereEarth's atmosphereThe 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...
has progressed. The substitution of oxygenOxygenOxygen 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...
for carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
in the atmosphere, which occurred in the ProterozoicProterozoicThe Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
, caused probably by cyanobacteria in the form of stromatolites, caused changes leading to the evolution of aerobic organismAerobic organismAn aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment.Faculitative anaerobes grow and survive in an oxygenated environment and so do aerotolerant anaerobes.-Glucose:...
s.
- The identification of the first generally accepted fossils of microbial life was made by geologists. These rocks have been dated as about 3.465 billion years ago. WalcottCharles Doolittle WalcottCharles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
was the first geologist to identify pre-Cambrian fossil bacteria from microscopic examination of thin rock slices. He also thought stromatolites were organic in origin. His ideas were not accepted at the time, but may now be appreciated as great discoveries.
- Information about paleoclimates is increasingly available, and being used in paleontology. One example: the discovery of massive ice ages in the Proterozoic, following the great reduction of CO2 in the atmosphere. These ice ages were immensely long, and led to a crash in microflora. See also CryogenianCryogenianThe Cryogenian is a geologic period that lasted from . It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran...
period and Snowball EarthSnowball EarthThe Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
.
- CatastrophismCatastrophismCatastrophism is the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope. The dominant paradigm of modern geology is uniformitarianism , in which slow incremental changes, such as erosion, create the Earth's appearance...
and mass extinctions. A partial reintegration of catastrophism has occurred, and the importance of mass extinctions in large-scale evolution is now apparent. Extinction events disturb relationships between many forms of life and may remove dominant forms and release a flow of adaptive radiationAdaptive radiationIn 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...
amongst groups that remain. Causes include meteorite strikesImpact eventAn impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...
(K–TK–T boundaryThe K–T boundary is a geological signature, usually a thin band, dated to 65.5 ± 0.3 Ma ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period...
junction; Upper DevonianLate Devonian extinctionThe Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...
); flood basaltFlood basaltA flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
provinces (Deccan trapsDeccan TrapsThe Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than thick and cover an area of and a volume of...
at K/T junction; Siberian trapsSiberian TrapsThe Siberian Traps form a large region of volcanic rock, known as a large igneous province, in the Russian region of Siberia. The massive eruptive event which formed the traps, one of the largest known volcanic events of the last 500 million years of Earth's geological history, continued for...
at P–T junction); and other less dramatic processes.
Conclusion: Our present knowledge of earth history strongly suggests that large-scale geophysical events influenced macroevolution
Macroevolution
Macroevolution is evolution on a scale of separated gene pools. Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes within a species or population.The process of speciation may fall...
and megaevolution. These terms refer to evolution above the species level, including such events as mass extinctions, 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...
, and the major transitions in evolution.
Symbiotic origin of eukaryotic cell structures
Once symbiosisSymbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
was discovered in lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
and in plant roots (rhizobia
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...
in root nodules) in the 19th century, the idea arose that the process might have occurred more widely, and might be important in evolution. Anton de Bary
Anton de Bary
Heinrich Anton de Bary was a German surgeon, botanist, microbiologist, and mycologist ....
invented the concept of symbiosis; several Russian biologists promoted the idea; Edwin Wilson
Edwin Wilson
Edwin Wilson may refer to:* Edwin Bidwell Wilson , American mathematician* Edwin C. Wilson , U.S. Minister to Uruguay, 1939–1941; U.S. Ambassador to Panama, 1941–1943, and Turkey, 1945–1948...
mentioned it in his text The Cell; as did Ivan Emmanuel Wallin in his Symbionticism and the origin of species; and there was a brief mention by Julian Huxley
Julian Huxley
Sir Julian Sorell Huxley FRS was an English evolutionary biologist, humanist and internationalist. He was a proponent of natural selection, and a leading figure in the mid-twentieth century evolutionary synthesis...
in 1930; all in vain because sufficient evidence was lacking. Symbiosis as a major evolutionary force was not discussed at all in the evolutionary synthesis.
The role of symbiosis in cell evolution was revived partly by Joshua Lederberg
Joshua Lederberg
Joshua Lederberg ForMemRS was an American molecular biologist known for his work in microbial genetics, artificial intelligence, and the United States space program. He was just 33 years old when he won the 1958 Nobel Prize in Physiology or Medicine for discovering that bacteria can mate and...
, and finally brought to light by Lynn Margulis
Lynn Margulis
Lynn Margulis was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted...
in a series of papers and books. It turns out that some cell organelles are of microbial origin: mitochondria and chloroplasts definitely, cilia, flagella and centrioles possibly, and perhaps the nuclear membrane and much of the chromosome structure as well. What is now clear is that the evolution of eukaryote
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...
cells is either caused by, or at least profoundly influenced by, symbiosis with bacterial and archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
n cells in the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
.
The origin of the eukaryote cell by symbiosis in several stages was not part of the evolutionary synthesis. It is, at least on first sight, an example of megaevolution by big jumps. However, what symbiosis provided was a copious supply of heritable variation from microorganisms, which was fine-tuned over a long period to produce the cell structure we see today. This part of the process is consistent with evolution by natural selection.
Trees of life
The ability to analyse sequence in macromolecules (protein, 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...
, RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
) provides evidence of descent, and permits us to work out genealogical trees covering the whole of life, since now there are data on every major group of living organisms. This project, begun in a tentative way in the 1960s, has become a search for the universal tree or the universal ancestor, a phrase of 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,...
. The tree that results has some unusual features, especially in its roots. There are two domains of prokaryotes: 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 archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, both of which contributed genetic material to the eukaryotes, mainly by means of symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
. Also, since bacteria can pass genetic material to other bacteria, their relationships look more like a web than a tree. Once eukaryotes were established, their sexual reproduction produced the traditional branching tree-like pattern, the only diagram Darwin put in the Origin. The last universal ancestor (LUA
Last universal ancestor
The last universal ancestor , also called the last universal common ancestor , or the cenancestor, is the most recent organism from which all organisms now living on Earth descend. Thus it is the most recent common ancestor of all current life on Earth...
) would be a prokaryotic cell before the split between the bacteria and archaea. LUA is defined as most recent organism from which all organisms now living on Earth descend (some 3.5 to 3.8 billion years ago, in the Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
era).
This technique may be used to clarify relationships within any group of related organisms. It is now a standard procedure, and examples are published regularly. April 2009 sees the publication of a tree covering all the animal phyla, derived from sequences from 150 genes in 77 taxa.
Early attempts to identify relationships between major groups were made in the 19th century by Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...
, and by comparative anatomists such as Thomas Henry Huxley and E. Ray Lankester. Enthusiasm waned: it was often difficult to find evidence to adjudicate between different opinions. Perhaps for that reason, the evolutionary synthesis paid surprisingly little attention to this activity. It is certainly a lively field of research today.
Evo-devo
What once was called embryologyEmbryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
played a modest role in the evolutionary synthesis, mostly about evolution by changes in developmental timing (allometry and heterochrony
Heterochrony
In biology, heterochrony is defined as a developmental change in the timing of events, leading to changes in size and shape. There are two main components, namely the onset and offset of a particular process, and the rate at which the process operates...
). Man himself was, according to Bolk
Louis Bolk
Lodewijk 'Louis' Bolk was a Dutch anatomist who created the fetalization theory about the human body. It states that when a human being is born, it is still a fetus, as can be seen if one pays attention to its big head, to its uncoordinated motility or to its absolute helplessness, for instance...
, a typical case of evolution by retention of juvenile characteristics (neoteny
Neoteny
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...
). He listed many characters where "Man, in his bodily development, is a primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
foetus that has become sexually mature". Unfortunately, his interpretation of these ideas was non-Darwinian, but his list of characters is both interesting and convincing.
Modern interest in Evo-devo springs from clear proof that development is closely controlled by special genetic systems, and the hope that comparison of these systems will tell us much about the evolutionary history of different groups. In a series of experiments with the fruit-fly 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...
, Edward B. Lewis
Edward B. Lewis
- External links :* *...
was able to identify a complex of genes whose proteins bind to the cis-regulatory regions of target genes. The latter then activate or repress systems of cellular processes that accomplish the final development of the organism. Furthermore, the sequence of these control genes show co-linearity: the order of the loci in the chromosome parallels the order in which the loci are expressed along the anterior-posterior axis of the body. Not only that, but this cluster of master control genes programs the development of all higher organisms. Each of the genes contains a homeobox
Homeobox
A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of anatomical development in animals, fungi and plants.- Discovery :...
, a remarkably conserved DNA sequence. This suggests the complex itself arose by gene duplication. In his Nobel
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
lecture, Lewis said "Ultimately, comparisons of the [control complexes] throughout the animal kingdom should provide a picture of how the organisms, as well as the [control genes] have evolved".
The term deep homology
Deep homology
In evolutionary developmental biology, the concept of deep homology is used to describe cases where growth and differentiation processes are governed by genetic mechanisms that are homologous and deeply conserved across a wide range of species....
was coined to describe the common origin of genetic regulatory apparatus used to build morphologically and phylogenetically disparate animal features. It applies when a complex genetic regulatory system is inherited from a common ancestor, as it is in the evolution of vertebrate and invertebrate eyes. The phenomenon is implicated in many cases of parallel evolution
Parallel evolution
Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...
.
A great deal of evolution may take place by changes in the control of development. This may be relevant to punctuated equilibrium
Punctuated equilibrium
Punctuated 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...
theory, for in development a few changes to the control system could make a significant difference to the adult organism. An example is the giant panda
Giant Panda
The giant panda, or panda is a bear native to central-western and south western China. It is easily recognized by its large, distinctive black patches around the eyes, over the ears, and across its round body. Though it belongs to the order Carnivora, the panda's diet is 99% bamboo...
, whose place in the 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...
was long uncertain. Apparently, the giant panda's evolution required the change of only a few genetic messages (5 or 6 perhaps), yet the phenotypic and lifestyle change from a standard bear is considerable. The transition could therefore be effected relatively swiftly.
Fossil discoveries
In the past thirty or so years there have been excavations in parts of the world which had scarcely been investigated before. Also, there is fresh appreciation of fossils discovered in the 19th century, but then denied or deprecated: the classic example is the Ediacaran biota from the immediate pre-Cambrian, after the CryogenianCryogenian
The Cryogenian is a geologic period that lasted from . It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran...
period. These soft-bodied fossils are the first record of multicellular life. The interpretation of this fauna is still in flux.
Many outstanding discoveries have been made, and some of these have implications for evolutionary theory. The discovery of feathered dinosaurs
Feathered dinosaurs
The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...
and early birds from the Lower Cretaceous of Liaoning, N.E. China have convinced most students that birds did evolve from coelurosaurian theropod dinosaurs. Less well known, but perhaps of equal evolutionary significance, are the studies on early insect flight, on stem tetrapods from the Upper Devonian, and the early stages of whale evolution.
Recent work has shed light on the evolution of flatfish
Flatfish
The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development...
(pleuronectiformes), such as plaice
Plaice
Plaice is the common name of four species of flatfishes.Plaice or PLAICE may also refer to:* USS Plaice , a Balao-class submarine* PLAICE, an open source hardware FLASH programmer, memory emulator, and logic analyzer...
, sole
Sole (fish)
Sole is a group of flatfish belonging to several families. Generally speaking, they are members of the family Soleidae, but, outside Europe, the name sole is also applied to various other similar flatfish, especially other members of the sole suborder Soleoidei as well as members of the flounder...
, turbot
Turbot
The turbot is a species of flatfish in the family Scophthalmidae. It is native to marine or brackish waters of the North Atlantic, Baltic Sea and the Mediterranean Sea.-Etymology:...
and halibut
Halibut
Halibut is a flatfish, genus Hippoglossus, from the family of the right-eye flounders . Other flatfish are also called halibut. The name is derived from haly and butt , for its popularity on Catholic holy days...
. Flatfish are interesting because they are one of the few vertebrate groups with external asymmetry. Their young are perfectly symmetrical, but the head is remodelled during a metamorphosis, which entails the migration of one eye to the other side, close to the other eye. Some species have both eyes on the left (turbot), some on the right (halibut, sole); all living and fossil flatfish to date show an 'eyed' side and a 'blind' side. The lack of an intermediate condition in living and fossil flatfish species had led to debate about the origin of such a striking adaptation. The case was considered by Lamark, who thought flatfish precursors would have lived in shallow water for a long period, and by 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...
, who predicted a gradual migration of the eye, mirroring the metamorphosis of the living forms. Darwin's long-time critic St. George Mivart thought that the intermediate stages could have no selective value, and in the 6th edition of the Origin, Darwin made a concession to the possibility of acquired traits. Many years later the geneticist Richard Goldschmidt
Richard Goldschmidt
Richard Benedict Goldschmidt was a German-born American geneticist. He is considered the first to integrate genetics, development, and evolution. He pioneered understanding of reaction norms, genetic assimilation, dynamical genetics, sex determination, and heterochrony...
put the case forward as an example of evolution by saltation
Saltation (biology)
In biology, ""saltation"" is a sudden change from one generation to the next, that is large, or very large, in comparison with the usual variation of an organism...
, bypassing intermediate forms.
A recent examination of two fossil species from the 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...
has provided the first clear picture of flatfish evolution. The discovery of stem flatfish with incomplete orbital migration refutes Goldschmidt's ideas, and demonstrates that "the assembly of the flatfish bodyplan occurred in a gradual, stepwise fashion". There are no grounds for thinking that incomplete orbital migration was maladaptive, because stem forms with this condition ranged over two geological stages, and are found in localities which also yield flatfish with the full cranial asymmetry. The evolution of flatfish falls squarely within the evolutionary synthesis.
Horizontal gene transfer
Horizontal gene transferHorizontal 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...
(HGT) (or Lateral gene transfer) is any process in which an organism gets genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
material from another organism without being the offspring of that organism.
Most thinking in genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
has focused on vertical transfer, but there is a growing awareness that horizontal gene transfer is a significant phenomenon. Amongst single-celled organisms it may be the dominant form of genetic transfer. Artificial horizontal gene transfer is a form of genetic engineering
Genetic engineering
Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...
.
Richardson and Palmer (2007) state: "Horizontal gene transfer (HGT) has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryote
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...
s. However, the prevalence and importance of HGT in the evolution of multicellular eukaryotes remain unclear".
The bacterial means of HGT are:
- TransformationTransformation (genetics)In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
, the genetic alteration of a cellCell (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....
resulting from the introduction, uptake and expressionGene expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
of foreign genetic material (DNADNADeoxyribonucleic 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 RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
). - TransductionTransduction (genetics)Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...
, the process in which bacterial DNA is moved from one bacterium to another by a bacterial virus (a bacteriophageBacteriophageA bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
, or 'phage'). - Bacterial conjugationBacterial conjugationBacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...
, a process in which a bacterial cell transfers genetic material to another cell by cell-to-cell contact. - Gene transfer agentGene transfer agentA gene transfer agent or "GTA" is a virus-like element that contains random pieces of the host chromosome and are found in most members of the alphaproteobacteria order Rhodobacterales. They are encoded by the host genome...
or 'GTA' is a virus-like element which contains random pieces of the host chromosomeChromosomeA chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
. They are found in most members of the alphaproteobacteriaAlphaproteobacteriaAlphaproteobacteria is a class of Proteobacteria. Like all Proteobacteria, they are Gram-negative.-Characteristics:The Alphaproteobacteria comprise most phototrophic genera, but also several genera metabolising C1-compounds , symbionts of plants and animals, and a group of pathogens, the...
order RhodobacteralesRhodobacteralesIn taxonomy, the Rhodobacterales are an order of the Alphaproteobacteria.Gene transfer agents are viruslike elements produced by Rhodobacterales which transfer DNA and may be an important factor in their evolution.-External links:...
. They are encoded by the host genomeGenomeIn 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....
. GTAs transfer DNA so frequently that they may have an important role in evolution.
A 2010 report found that genes for antibiotic resistance could be transferred by engineering GTAs in the laboratory.
Some examples of HGT in metazoa are now known. Genes in bdelloid rotifers have been found which appear to have originated in 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...
, fungi, and plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s. This suggests they arrived by 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 capture and use of exogenous (~foreign) genes may represent an important force in bdelloid evolution. The team led by Matthew S. Meselson at Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
has also shown that, despite the lack 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...
, bdelloid rotifers do engage in genetic (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...
) transfer within a 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...
or clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
. The method used is not known at present.
See also
- Developmental systems theoryDevelopmental systems theoryIn developmental psychology, developmental systems theory is an overarching theoretical perspective on biological development, heredity, and evolution . It emphasizes the equal contributions of genes, environment, and epigenetic factors on developmental processes...
- Gene-centered view of evolutionGene-centered view of evolutionThe gene-centered view of evolution, gene selection theory or selfish gene theory holds that evolution occurs through the differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation, with gene defined as...
- History of evolutionary thoughtHistory of evolutionary thoughtEvolutionary thought, the conception that species change over time, has roots in antiquity, in the ideas of the ancient Greeks, Romans, and Chinese as well as in medieval Islamic science...
- Particulate inheritance theoryParticulate inheritance theoryParticulate inheritance is a pattern of inheritance discovered by Mendelian theorists showing that characteristics can be passed from generation to generation through "discrete particles"...
- Polymorphism (biology)Polymorphism (biology)Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
External links
- Rose MR, Oakley TH, The new biology: beyond the Modern Synthesis. Biology Direct 2007, 2:30. A review of biology in light of recent innovations since the initiation of modern synthesis.