- Sympatric speciation
Sympatric speciation is the genetic divergence of various populations (from a single parent species) inhabiting the same geographic region, such that those populations become different species.
Etymologically, sympatry is derived from the roots sym- (meaning same, alike, similar, or fellow) and -patry (meaning homeland or fatherland).
Sympatry is one of four theoretical models for the phenomenon of
speciation. In contrast to allopatry, populations undergoing sympatric speciation are not geographically isolated by, for example, a mountain or a river.
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 Mayr, one of the foremost thinkers on evolution, completely rejected sympatry outright, ushering in a climate of hostility towards the theory. Since the 1980s, a more progressive ideology has been adopted. While still debatable, well documented empirical evidence now exists, and the development of sophisticated theories incorporating multilocus genetics have followed.
At least one recent study, though, provides evidence that sympatric speciation has occurred in Tennessee cave salamanders. [cite journal|title= Recent divergence with gene flow in Tennessee cave salamanders (Plethodontidae: Gyrinophilus) inferred from gene genealogies |author= MATTHEW L. NIEMILLER, BENJAMIN M. FITZPATRICK, BRIAN T. MILLER |journal= Molecular Ecology |volume=17 (9)|year=2008|pages=2258–2275 [http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-294X.2008.03750.x available online] ]
A number of models have been proposed to account for this mode of speciation. The most popular, which invokes the
disruptive selectionmodel, was first put forward by John Maynard Smithin 1962. 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. [*Harvard reference
Surname2= De León
Title= Reproductive isolation of sympatric morphs in a population of
Journal= Proc. Biol. Sci.
ID = PMID 17504742]
"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. [ [http://www.nature.com/nature/journal/v336/n6194/abs/336064a0.html McPheron et al. 1988. "Nature" 336:64-66] ] [ [http://www.nature.com/nature/journal/v336/n6194/abs/336066a0.html Smith, D.C. 1988. "Nature" 336:66-67] ] [ [http://www.nature.com/nature/journal/v336/n6194/abs/336061a0.html Feder et al. 1988. "Nature" 336:61-64] ] [Sympatric speciation in Nicaraguan crater lake cichlid fish. By: Barluenga, Marta; Stölting, Kai N.; Salzburger, Walter; Muschick, Moritz; Meyer, Axel. Nature, 2/9/2006, Vol. 439 Issue 7077, p719-723.] Allochronyoffers some empirical evidence that sympatric speciation has taken place, as many examples exist of recently diverged (sister taxa) allochronic species.
speciationevents are vastly more common in plants, as they are prone to developing multiple homologoussets 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"
Orcaforms in the northeast Pacific [ [http://web.mac.com/erich.hoyt/iWeb/www.erichhoyt.com/More%20Info_files/SC56SM15.pdf Burden et al.: Resident And Transient-Type Killer Wales SC/56/SM15.] ] . 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 polecat"Mustela putorius" exhibited a rare dark phenotype similar to the European mink"Mustela lutreola" phenotype which is directely influenced by peculiarities of forest brooks [ Thierry Lodé"Genetic divergence without spatial isolation in polecat Mustelaputorius populations". J Evol Biol 14:228-236, 2001] .
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