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Allopatric speciation among fruit flies.

Allopatric speciation, also known as geographic speciation, is the phenomenon where huge biological populations are physically isolated by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation, such that if the barrier breaks down, individuals of the populations can no longer interbreed. Evolutionary biologists agree that allopatry is a common way that new species arise. (The word is derived from Greek allos, "other" + Greek patrā, "fatherland".) By contrast, the frequency of other types of speciation, such as sympatric speciation, parapatric speciation, and heteropatric speciation), is debated.

Comparison of allopatric, peripatric, parapatric and sympatric speciation.

The 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 (dichopatric or vicariant speciation) or when a small number of individuals colonize a novel habitat on the periphery of a species' geographic range (peripatric speciation). 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 Charles Darwin's Galápagos Finches.