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.

See also: adaptive radiation