Sympatric speciation
Encyclopedia
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 they occur together at least in some places. If these organisms are closely related (e.g. sister species), such a distribution may be the result of sympatric speciation
. Etymologically, sympatry is derived from the Greek roots συν (together, with) and πατρίς (homeland or fatherland). The term was invented by Poulton in 1904, who explains the derivation.
Sympatric speciation is one of three traditional geographic categories for the phenomenon of speciation. Allopatric speciation
is the evolution of geographically isolated populations into distinct species. In this case, divergence is facilitated by the absence of gene flow, which tends to keep populations genetically similar. Parapatric speciation
is the evolution of geographically adjacent populations into distinct species. In this case, divergence occurs despite limited interbreeding where the two diverging groups come into contact. In sympatric speciation, there is no geographic constraint to interbreeding. It has been pointed out that these categories are special cases of a continuum from zero (sympatric) to complete (allopatric) spatial segregation of diverging groups.
In multicellular eukaryotic organisms, sympatric speciation is thought to be an uncommon but plausible process by which genetic divergence (through reproductive isolation
) of various populations from a single parent species and inhabiting the same geographic region leads to the creation of new species.
In bacteria, however, the analogous process (defined as "the origin of new bacterial species that occupy definable ecological niche
s") might be more common because bacteria are less constrained by the homogenizing effects of sexual reproduction and prone to comparatively dramatic and rapid genetic change through horizontal gene transfer
.
A number of models have been proposed to account for this mode of speciation. The most popular, which invokes the disruptive selection
model, was first put forward by John Maynard Smith
in 1966. Maynard Smith suggested that homozygous individuals may, under particular environmental conditions, have a greater fitness than those with allele
s heterozygous for a certain trait. Under the mechanism of natural selection
, therefore, homozygosity would be favoured over heterozygosity, eventually leading to speciation. Sympatric divergence could also result from the sexual conflict
.
Disruption may also occur in multiple-gene traits. The Medium Ground Finch
(Geospiza fortis) is showing gene pool divergence in a population on Santa Cruz Island
. Beak morphology conforms to two different size ideals, while intermediate individuals are selected against. Some characteristics (termed magic traits) such as beak morphology may drive speciation because they also affect mating signals. In this case, different beak phenotype
s may result in different bird calls, providing a barrier to exchange between the gene pools.
A well studied circumstance of sympatric speciation is when insects feed on more than one species of host plant
. In this case insects become specialized as they struggle to overcome the various plants' defense mechanisms. (Drès and Mallet, 2002)
Rhagoletis pomonella, the apple maggot
, may be currently undergoing sympatric or, more precisely, heteropatric (see heteropatry) speciation. The apple feeding race of this species appears to have spontaneously emerged from the hawthorn
feeding race in the 1800 - 1850 AD time frame, after apples were first introduced into North America
. The apple feeding race does not now normally feed on hawthorn
s, and the hawthorn feeding race does not now normally feed on apples. This may be an early step towards the emergence of a new species.
Isolated and relatively homogeneous habitats such as crater lakes and islands are among the best geographical settings in which to demonstrate sympatric speciation. For example, Nicaragua crater lake cichlid fishes include at least one species that has evolved by sympatric speciation
Allochrony
offers some empirical evidence that sympatric speciation has taken place, as many examples exist of recently diverged (sister taxa) allochronic species.
Sympatric speciation events are vastly more common in plants, as they are prone to developing multiple homologous sets of chromosomes, resulting in a condition called polyploidy
. The polyploidal offspring occupy the same environment as the parent plants (hence sympatry), but are reproductively isolated.
A rare example of sympatric speciation in animals is the divergence of "resident" and "transient" Orca
forms in the northeast Pacific. Resident and transient orcas inhabit the same waters, but avoid each other and do not interbreed. The two forms hunt different prey species and have different diets, vocal behaviour, and social structures. Some divergences between species could also result from contrasts in microhabitats.
The European Polecat
Mustela putorius exhibited a rare dark phenotype similar to the European mink
Mustela lutreola phenotype which is directly influenced by peculiarities of forest brooks.
, one of the foremost thinkers on evolution, completely rejected sympatry outright, ushering in a climate of hostility towards the theory. While still debatable, well documented empirical evidence now exists, and the development of sophisticated theories incorporating multilocus genetics has followed.
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
, sympatric and sympatry
Sympatry
In biology, two species or populations are considered sympatric when they exist in the same geographic area and thus regularly encounter one another. An initially-interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation...
are terms referring to organisms whose ranges
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...
overlap or are even identical, so that they occur together at least in some places. If these organisms are closely related (e.g. sister species), such a distribution may be the result of sympatric 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...
. Etymologically, sympatry is derived from the Greek roots συν (together, with) and πατρίς (homeland or fatherland). The term was invented by Poulton in 1904, who explains the derivation.
Sympatric speciation is one of three traditional geographic categories for the phenomenon of speciation. 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...
is the evolution of geographically isolated populations into distinct species. In this case, divergence is facilitated by the absence of gene flow, which tends to keep populations genetically similar. Parapatric speciation
Parapatric speciation
Parapatry is a term from biogeography, referring to organisms whose ranges do not significantly overlap but are immediately adjacent to each other; they only occur together in the narrow contact zone, if at all. This geographical distribution is opposed to sympatry & allopatry or peripatry...
is the evolution of geographically adjacent populations into distinct species. In this case, divergence occurs despite limited interbreeding where the two diverging groups come into contact. In sympatric speciation, there is no geographic constraint to interbreeding. It has been pointed out that these categories are special cases of a continuum from zero (sympatric) to complete (allopatric) spatial segregation of diverging groups.
In multicellular eukaryotic organisms, sympatric speciation is thought to be an uncommon but plausible process by which genetic divergence (through 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...
) of various populations from a single parent species and inhabiting the same geographic region leads to the creation of new species.
In bacteria, however, the analogous process (defined as "the origin of new bacterial species that occupy definable ecological niche
Ecological niche
In 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...
s") might be more common because bacteria are less constrained by the homogenizing effects of sexual reproduction and prone to comparatively dramatic and rapid genetic change through 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...
.
Evidence
Disruptive selection
Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups...
model, was first put forward by 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....
in 1966. Maynard Smith suggested that homozygous individuals may, under particular environmental conditions, have a greater fitness than those with allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s heterozygous for a certain trait. Under the mechanism of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
, therefore, homozygosity would be favoured over heterozygosity, eventually leading to speciation. Sympatric divergence could also result from the sexual conflict
Sexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...
.
Disruption may also occur in multiple-gene traits. The Medium Ground Finch
Darwin's finches
Darwin's finches are a group of 14 or 15 species of passerine birds. It is still not clear which bird family they belong to, but they are not related to the true finches. They were first collected by Charles Darwin on the Galápagos Islands during the second voyage of the Beagle...
(Geospiza fortis) is showing gene pool divergence in a population on Santa Cruz Island
Santa Cruz Island (Galápagos)
Santa Cruz Island is one of the Galápagos Islands with an area of and a maximum altitude of .Situated in the center of the archipelago, Santa Cruz is the second largest island after Isabela. Its capital is Puerto Ayora, the most populated urban centre in the islands. On Santa Cruz there are some...
. Beak morphology conforms to two different size ideals, while intermediate individuals are selected against. Some characteristics (termed magic traits) such as beak morphology may drive speciation because they also affect mating signals. In this case, different beak phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
s may result in different bird calls, providing a barrier to exchange between the gene pools.
A well studied circumstance of sympatric speciation is when insects feed on more than one species of host plant
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
. In this case insects become specialized as they struggle to overcome the various plants' defense mechanisms. (Drès and Mallet, 2002)
Rhagoletis pomonella, the apple maggot
Apple maggot
The apple maggot , also known as railroad worm, is a pest of several fruits, mainly apples. The adult form of this insect is about 5 mm long, slightly smaller than a house fly, with a white dot on its thorax and a characteristic black banding shaped like an "F" on its wings...
, may be currently undergoing sympatric or, more precisely, heteropatric (see heteropatry) speciation. The apple feeding race of this species appears to have spontaneously emerged from the hawthorn
Crataegus
Crataegus , commonly called hawthorn or thornapple, is a large genus of shrubs and trees in the rose family, Rosaceae, native to temperate regions of the Northern Hemisphere in Europe, Asia and North America. The name hawthorn was originally applied to the species native to northern Europe,...
feeding race in the 1800 - 1850 AD time frame, after apples were first introduced into North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. The apple feeding race does not now normally feed on hawthorn
Crataegus
Crataegus , commonly called hawthorn or thornapple, is a large genus of shrubs and trees in the rose family, Rosaceae, native to temperate regions of the Northern Hemisphere in Europe, Asia and North America. The name hawthorn was originally applied to the species native to northern Europe,...
s, and the hawthorn feeding race does not now normally feed on apples. This may be an early step towards the emergence of a new species.
Isolated and relatively homogeneous habitats such as crater lakes and islands are among the best geographical settings in which to demonstrate sympatric speciation. For example, Nicaragua crater lake cichlid fishes include at least one species that has evolved by sympatric speciation
Allochrony
Allochrony
The term allochrony is used in ecology to describe a situation where two biological entities occur in the same area, and are thus sympatric, but are never or rarely active simultaneously...
offers some empirical evidence that sympatric speciation has taken place, as many examples exist of recently diverged (sister taxa) allochronic species.
Sympatric speciation events are vastly more common in plants, as they are prone to developing multiple homologous sets of chromosomes, resulting in a condition called 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...
. The polyploidal offspring occupy the same environment as the parent plants (hence sympatry), but are reproductively isolated.
A rare example of sympatric speciation in animals is the divergence of "resident" and "transient" Orca
Orca
The killer whale , commonly referred to as the orca, and less commonly as the blackfish, is a toothed whale belonging to the oceanic dolphin family. Killer whales are found in all oceans, from the frigid Arctic and Antarctic regions to tropical seas...
forms in the northeast Pacific. Resident and transient orcas inhabit the same waters, but avoid each other and do not interbreed. The two forms hunt different prey species and have different diets, vocal behaviour, and social structures. Some divergences between species could also result from contrasts in microhabitats.
The European Polecat
European polecat
The European polecat , also known as the black or forest polecat , is a species of Mustelid native to western Eurasia and North Africa, which is classed by the IUCN as Least Concern due to its wide range and large numbers. It is of a generally dark brown colour, with a pale underbelly and a dark...
Mustela putorius exhibited a rare dark phenotype similar to the European mink
Mink
There are two living species referred to as "mink": the European Mink and the American Mink. The extinct Sea Mink is related to the American Mink, but was much larger. All three species are dark-colored, semi-aquatic, carnivorous mammals of the family Mustelidae, which also includes the weasels and...
Mustela lutreola phenotype which is directly influenced by peculiarities of forest brooks.
Controversy
Debated almost since the beginning of popular evolutionary thought, sympatric speciation is still a highly contentious issue. By 1980 the theory was largely unfavourable given the void of empirical evidence available, and more critically the conditions scientists expect to be required. Ernst MayrErnst 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...
, one of the foremost thinkers on evolution, completely rejected sympatry outright, ushering in a climate of hostility towards the theory. While still debatable, well documented empirical evidence now exists, and the development of sophisticated theories incorporating multilocus genetics has followed.
See also
- 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...
- EcotypeEcotypeIn evolutionary ecology, an ecotype,Greek: οίκος = home and τύπος = type, coined by Göte Turesson in 1922 sometimes called ecospecies, describes a genetically distinct geographic variety, population or race within species , which is adapted to specific environmental conditions.Typically, ecotypes...
- PolyploidyPolyploidyPolyploid 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...
- 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...
- Hybrid speciationHybrid speciationHybrid speciation is the process wherein hybridization between two different closely related species leads to a distinct phenotype. This phenotype in very rare cases can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if...
- CladisticsCladisticsCladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
- PhylogeneticsPhylogeneticsIn biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
- TaxonomyTaxonomyTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
- Wallace effectWallace effectThe Wallace Effect is a hypothesis developed by British naturalist Alfred Russel Wallace which posits that natural selection can contribute to the reproductive isolation of incipient species by encouraging varieties to develop barriers to hybridization....