Allopatric speciation is a type of speciation that is caused due to geographical isolation that results in the separation of the population of the same species that once inhabited the same geographical area.
Due to this geographic separation, the species become so isolated that no more gene flow occurs between the two species and therefore, with time new species get formed.
The various geographic changes that can contribute to the separation of the two species can be - formation of mountains, islands, water bodies, glaciers, etc.
The populations that get separated due to the geographic barriers are subjected to different selective pressures and hence, speciation occurs.
