Allopatry (ancient Greek allos, "other" + Greek patrā, "fatherland") refers to organisms whose ranges are entirely separate, so 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.
Allopatric speciation, also known as geographic speciation, is the phenomenon whereby biological populations are physically isolated by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation, such that if the barrier should ever vanish, individuals of the populations can no longer interbreed. Allopatry is a common method by which new species arise.
Evolution of reproductive isolation is generally thought to be an incidental by-product of genetic divergence of other traits, particularly adaptive changes that evolve through natural selection in response to different environmental conditions in separate geographic areas. Ernst Mayr, an evolutionary biologist and famous proponent of allopatric speciation, hypothesized that adaptive genetic changes that accumulate between allopatric populations cause negative epistasis in hybrids, resulting in sterility or inviability.
Allopatric speciation may occur when a species is subdivided into two large populations for example by plate tectonic or other geological events, or when a small number of individuals colonize a novel habitat on the periphery of a species' geographic range. Because natural selection is a powerful evolutionary force in large populations, adaptive evolution likely causes the genetic changes that results in reproductive isolation in vicariant speciation. In peripatric speciation, however, the genetic changes that are thought to occur within the peripheral isolate are more controversial.
Proponents of peripatric speciation contend that small population size in the peripheral isolate (sometimes referred to as a "splinter population") allows genetic drift, which can be a more powerful force than natural selection in small populations, to deconstruct complex genotypes, allowing the creation of novel gene combinations. Both forms need not be mutually exclusive; in practice, passive isolation or fragmentation as well as active dispersal seem to play a role in many cases of speciation.
A famous example of allopatric speciation is that of Charles Darwin's Galápagos Finches.