Explanation:

Allopatric speciation, also known as geographic speciation, is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration.

Over time, individuals in populations on each side of the barrier differentiate. This may be due to a founder effect, in which only part of the genetic variability originally present is represented in a new population; to chance loss of genetic variation through accidents of who happens to reproduce or not; or to adaptation to environmental differences on each side of the barrier. This differentiation between the divided populations is extensive enough to include the evolution of differences in reproductive biology, such that when the two populations are re-united by the removal of the barrier, they are no longer able to inter-breed and are viewed as separate species.