Vicariant

Allopatric speciation (from the ancient Greek allos, meaning "other", and patris, meaning "fatherland") or geographic speciation is speciation that occurs when biological populations of the same species become vicariant, or 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 gene pools of the populations. Allopatric speciation is thought to be the dominant mode of speciation.

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.

Allopatric speciation may occur when a species is subdivided into geographically isolated populations. Such separation commonly is referred to as vicariance. Allopatric and allopatry are terms from biogeography, referring to organisms whose ranges are entirely separate such that they do not occur in any one place together. If these organisms are closely related (e.g. sister species), such a distribution is usually the result of allopatric speciation. Separation may be attributed to either geological processes or population dispersal.

...
Wikipedia