Allopatric Speciation / Geographical Speciation

Allopatric speciation (geographical speciation) occurs when populations of a single species become physically separated by a geographic barrier — such as land separation, migration to a new area, or the formation of a mountain range — that blocks gene flow between them. Once separated, differing ecological conditions and environments on either side of the barrier drive each population to accumulate its own distinct adaptations; if a separated population fails to adapt to its new conditions it may simply fail to survive, but if it does adapt, the accumulating differences can eventually make the two populations reproductively incompatible, so that one ancestral species has effectively become two. Because the populations are already kept apart by geography, sexual isolation itself plays only a weak role in allopatric speciation compared with its central role in sympatric speciation. Darwin's finches are cited as one example of allopatric speciation, but the chapter's more detailed worked example is the apple maggot fly (Rhagoletis pomonella) in North America: this parasitic insect originally laid its eggs only in the fruit of wild hawthorn trees, but after apple trees (Malus domestica) were introduced to the same region, one subset of the fly population began laying eggs in apples instead. This subgroup came to specialis …