Parapatry

Parapatry is a type of speciation in the theory of natural selection. It occurs when the zones of two species with a similar ecological niche abut but do not overlap. The two species may have contact at the common boundary of their zones. The concept of parapatry is necessary to satisfy the postulate of natural selection that two species cannot co-exist in the same location over the longer term with the same ecological niche. It suggests that there may be some factor apart from distance or ecological role which keeps the two species apart.

It contrasts with allopatry where different species are geographically isolated from each other, and sympatry where different species evolve in the same area by finding different ecological roles. If there is a zone where hybridisation occurs providing gene flow between the two base populations, then it is questionable whether they are indeed distinct species. Parapatry is relatively uncommon in temperate areas, but can be seen more often in tropical areas. Examples of the boundaries between two species include a change in elevation or a crossable river.