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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").〔http://www.greek-language.gr/greekLang/index.html〕 The term was invented by Poulton in 1904, who explains the derivation.〔Poulton, E. B. 1904. "What is a species?" ''Proceedings of the Entomological Society of London'' 1903:lxxvii–cxvi.〕 Sympatric speciation is one of three traditional geographic categories for the phenomenon of speciation.〔Futuyma, D. J. 2009. ''Evolution'' (2nd edition). Sinauer Associates, Inc.〕 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. 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 niches") 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. ==Evidence== 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 alleles 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.〔Thierry Lodé ''La guerre des sexes chez les animaux'' Eds O Jacob, Paris, 2006〕 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 phenotypes may result in different bird calls, providing a barrier to exchange between the gene pools. A somewhat analogous system has been reported in horseshoe bats, in which echolocation call frequency appears to be a magic trait. In these bats, the constant frequency component of the call not only determines prey size but may also function in aspects of social communication. Work from one species, the large-eared horseshoe bat (''Rhinolophus philippinensis''), shows that abrupt changes in call frequency among sympatric morphs is correlated with reproductive isolation. A further 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)〔Begon, Townsend, Harper: ''Ecology – From individuals to ecosystems'', 4th ed., p.10〕 ''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 hawthorns, 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. Some parasitic ants may have evolved via sympatric speciation. 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 nine described species and dozens of undescribed species that have evolved by sympatric speciation. ''Monostroma latissimum'', a marine green algae, also shows sympatric speciation in southwest Japanese islands. Although panmictic, the molecular phylogenetics using nuclear introns revealed staggering diversification of population. 〔BAST, F., KUBOTA, S. AND OKUDA, K. 2014. Phylogeographic Assessment of Panmictic Monostroma Species from Kuroshio Coast, Japan Reveals Sympatric Speciation. Journal of Applied Phycology. (article ) 〕 African cichlids also offer some evidence for sympatric speciation. They show a large amount of diversity in the Great Lakes of Africa. Many studies point to sexual selection as a way of maintaining reproductive isolation. Female choice with regards to male coloration is one of the more studied modes of sexual selection in African cichlids. Female choice is present in cichlids because the female does much of the work in raising the offspring, while the male has little energy input in the offspring. She exerts sensory bias when picking males by choosing those that have colors similar to her or those that are the most colorful. This helps maintain sympatric speciation within the lakes. Cichlids also use acoustic reproductive communication. The male cichlid quivers as a ritualistic display for the female which produces a certain number of pulses and pulse period. Female choice for good genes and sensory bias is one of the deciding factors in this case, selecting for calls that are within her species and that give the best fitness advantage to increase the survivability of the offspring. Male-male competition is a form of intrasexual selection and also has an effect on speciation in African cichlids. Ritualistic fighting among males establishes which males are going to be more successful in mating. This is important in sympatric speciation because species with similar males may be competing for the same females. There may be a fitness advantage for one phenotype that could allow one species to invade another. Studies show this effect in species that are genetically similar, have the capability to interbreed, and show phenotypic color variation. Ecological character displacement is another means for sympatric speciation. Within each lake there are different niches that a species could occupy. For example, different diets and depth of the water could help to maintain isolation between species in the same lake. 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. A population bottleneck occurred around 200,000 years ago greatly reducing the population size at the time as well as the variance of genes which allowed several ecotypes to emerge afterwards. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sympatric speciation」の詳細全文を読む スポンサード リンク
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