Sympatry is one of three theoretical models for the phenomenon of speciation. In complete contrast to allopatry, species undergoing sympatric speciation are not geographically isolated by, for example, a mountain or a river. The speciating populations share the same territory.
A number of models have been proposed to account for this mode of speciation. The most popular, disruptive speciation, was first put forward by John Maynard Smith in 1962. Smith suggested that heterozygous individuals may, under particular environmental conditions, have a greater fitness than those with alleles homozygous for a certain trait. Under the mechanism of natural selection, therefore, heterozygosity would be favoured over homozygosity, eventually leading to speciation.
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