Allopatric Speciation
Allopatric speciation is just a fancy name for speciation by geographic isolation (allo = other, patric = place). In this process, a geographic barrier divides a population and prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. Scientists think that geographic isolation is a common way for the process of speciation to begin: rivers change course (as in Fig. 2), mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into smaller populations. However the division happens, eventually the subpopulations evolve genetic differences (perhaps triggered by the different selective pressures in their separate environments) and become discrete species, unable to mate with one another and reunite their genes.
As an illustration of how this process might work, consider the following scenario from the Understanding Evolution website (illustrated in Fig. 3):
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The scene: a population of wild fruit flies is minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit, when...
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Disaster strikes and the population is divided: A hurricane washes the bananas (and the fruit fly larvae they contain) out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred: any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
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The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than does the mainland population. Body form, food preferences, and courtship displays change over many generations of natural selection.
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So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies because they’ve evolved different courtship behaviors. The few that do mate with the mainland flies produce inviable eggs because of other genetic differences. The lineage has split now that genes cannot flow between the populations.
A hypothetical example of allopatric speciation. Fruit flies from a mainland population are carried to an island in a hurricane. They evolve in isolation on the island and evolve genetic differences from the mainland population. Eventually, the island population evolves into a distinct species, which no longer mates with members of the mainland species
In the example above, the fruit flies alone were carried to an isolated location where speciation occurred. But, of course, when geographic isolation is caused by mountain ranges rising or by climate change that fragments a habitat into favorable and unfavorable patches, more than one species is likely to be affected. In fact, many different species living in the same area may simultaneously experience allopatric speciation if they are all similarly affected by large-scale physical disturbances.