Isolation

The Hardy-Weinberg principle assumes that there is no migration: no individuals, and so no alleles, move into or out of the population. The movement of alleles between populations is called gene flow. Immigration brings in new alleles and emigration removes alleles, and both change allele frequencies.Isolation is the separation of populations so that gene flow between them stops. Once two populations are isolated, their gene pools change independently. Different mutations arise in each, different selection pressures act, and genetic drift works differently, so their allele frequencies diverge.Isolating mechanisms include:Geographical isolation: a physical barrier such as a mountain range, river or sea separates the populations. Speciation after geographical isolation is called allopatric speciation.Reproductive isolation: populations living in the same area cannot interbreed successfully, which can lead to sympatric speciation. Causes include ecological isolation (different habitats), seasonal or temporal isolation (breeding at different times), behavioural isolation (different courtship displays), mechanical isolation (incompatible reproductive organs) and gametic isolation (gametes cannot fuse). Hybrids that are sterile, such as the mule, are a further barrier.If isolation lasts long enough, the populations may become so different that they can no longer interbreed to produce fertile offspring. They are then separate species.