Allopatric speciation

Allopatric speciation is a mode of speciation in which populations of a species become geographically separated to a degree that prevents or interferes with gene flow between them. Once a physical barrier divides a population, the two groups accumulate different mutations and face different selective pressures, and over time this isolation leads to reproductive isolation, so that even if the groups later come back into contact they can no longer interbreed and are recognized as separate species. The snapping shrimp genus Alpheus is a well documented example: phylogenetic studies support fifteen pairs of sister species split across the Isthmus of Panama, whose closure roughly 2.7 to 3.5 million years ago separated Pacific and Caribbean populations that then evolved independently into distinct species.