Allopatric speciation (from the ancient Greek allos, "other" + Greek patra, "fatherland") or geographic speciation is speciation that occurs when biological populations of the same species become vicariant — isolated from each other to an extent that prevents or interferes with genetic interchange. This can be the result of population dispersal leading to emigration, or by geographical changes such as mountain formation, island formation, or large scale human activities (for example agricultural and civil engineering developments). The vicariant populations then undergo genotypic or phenotypic divergence as: (a) they become subjected to different selective pressures, (b) they independently undergo genetic drift, and (c) different mutations arise in the populations' gene pools.
The separate populations over time may evolve distinctly different characteristics. If the geographical barriers are later removed, members of the two populations may be unable to successfully mate with each other, at which point, the genetically isolated groups have emerged as different species. Allopatric isolation is a key factor in speciation and a common process by which new species arise. Adaptive radiation, as observed by Charles Darwin in Galapagos finches, is a consequence of allopatric speciation among island populations.
Read more about Allopatric Speciation: Isolating Mechanisms, Allopatric Speciation in Peripheral Populations, Genesis of Reproductive Barriers, Alternative Modes of Speciation, Examples