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'Allopatric speciation', also known as geographic speciation, occurs when huge populations physically isolated by an extrinsic barrier evolve intrinsic (genetic) reproductive isolation such that if the barrier between the populations breaks down, individuals of the two populations can no longer interbreed. Although there is some debate about the frequency of other types of speciation (such as sympatric speciation, parapatric speciation, and heteropatric speciation), all evolutionary biologists agree that allopatry is a common way that new species arise.
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 seems to play a role in many cases of speciation.
Perhaps the most famous example of allopatric speciation is Charles Darwin's Galápagos Finches.