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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). 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 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.
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.
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 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 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.    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.
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. While still debatable, well documented empirical evidence now exists, and the development of sophisticated theories incorporating multilocus genetics has followed.
- Polymorphism (biology)
- Adaptive radiation
- Hybrid speciation
- Wallace effect
- Poulton, E. B. 1904. What is a species? Proceedings of the Entomological Society of London 1903:lxxvii-cxvi.
- Futuyma, D. J. 2009. Evolution (2nd edition). Sinauer Associates, Inc.
- Fitzpatrick, B. M., J. A. Fordyce, and S. Gavrilets. 2008. What, if anything, is sympatric speciation? Journal of Evolutionary Biology 21: 1452-1459.
- Bolnick, D. I. and B. M. Fitzpatrick. 2007. Sympatric speciation: Models and empirical evidence. Annual Review of Ecology, Evolution and Systematics 38: 459-487.
- King, Stansfield, Mulligan (2006). Dictionary of Genetics, 7th, Oxford University Press.
- John Maynard Smith (1966). Sympatric Speciation. American Naturalist 100 (916): 637–650. 
- Thierry Lodé "La guerre des sexes chez les animaux" Eds O Jacob, Paris, 2006
- * (2007). Reproductive isolation of sympatric morphs in a population of Darwin's finches. Proc. Biol. Sci. 274 (1619): 1709–14.
- Begon, Townsend, Harper: Ecology - From individuals to ecosystems, 4th ed., p.10
- McPheron et al. 1988. Nature 336:64-66
- Smith, D.C. 1988. Nature 336:66-67
- 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.
- Hoetzel et al. 1998, Low genetic variation among killer whales (Orcinus orca) in the eastern North Pacific and genetic differentiation between foraging specialists, J Hered
- Thierry Lodé "Genetic divergence without spatial isolation in polecat Mustela putorius populations". J Evol Biol 14:228-236, 2001
|Modes of speciation:||
Allopatric • Peripatric • Parapatric • Sympatric • Polyploidy • Paleopolyploidy
|Basic topics in evolutionary biology||(edit)|
|Processes of evolution: evidence - macroevolution - microevolution - speciation|
|Mechanisms: selection - genetic drift - gene flow - mutation - phenotypic plasticity|
|Modes: anagenesis - catagenesis - cladogenesis|
|History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis|
|Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics - evo-devo|
|List of evolutionary biology topics | Timeline of evolution | Timeline of human evolution|
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