A small, isolated colony becomes geographically isolated from the widespread parent population, and diverges and acquires reproductive isolation. Often called the founder effect. Thought to occur by genetic drift as well as natural selection. Can also be an adaptive peak shift. Geographic populations differ genetically.
Differences are greatest when there is no gene flow.
Related species replace each other geographically, and are probably formerly allopatric species that are now parapatric. There is an epistatic interaction between alleles such that hybrids have reduced fitness. Selection may act directly on the trait that causes isolation, or there may be pleiotropic effects. The same model fits here, except that the change in allele frequency is due to drift rather than selection. The role of genetic drift Drift of neutral alleles is not likely to be a mechanism of speciation.