Allopatric Speciation

Allopatric speciation is speciation that happens when populations are separated by a geographical barrier. 'Allopatric' means 'different homeland'. It is thought to be the most common way that new animal species form.The stages are:1. Separation. A physical barrier divides a population, for example a rising sea level cutting off an island, a river changing course, a mountain range forming, or a few individuals colonising a remote island.2. No gene flow. The two groups cannot meet, so they cannot interbreed. Each has its own gene pool.3. Divergence. Different random mutations arise in each group. The environments on each side of the barrier differ (for example climate, food or predators), so there are different selection pressures. Natural selection favours different alleles, and genetic drift may change allele frequencies by chance, especially in small groups.4. Reproductive isolation. After many generations the groups differ so much in phenotype, behaviour or chromosomes that, if the barrier is removed, they cannot interbreed to produce fertile offspring. Two species now exist.Example. Squirrels on either side of the Grand Canyon in the USA were separated by the canyon. The Kaibab squirrel on the north rim and the Abert's squirrel on the south rim now differ clearly in fur colour. They are usually still classed as subspecies, so this shows speciation in progress.Islands are especially good at producing new species by allopatric speciation, because the sea is an effective barrier and each island has its own conditions. This is why islands often have species found nowhere else.