Genetic drift
Founder events: these occur when a small group buds off from a larger population. The small group then lives separately from the main population. The human species is often quoted as having been through such stages, for example, when groups left Africa to set up elsewhere (see human evolution). Apparently, we have less variation than would be expected from our worldwide distribution. Groups that arrive on islands far from the mainland are also good examples. These groups, by virtue of their small size, cannot carry the full range of alleles to be found in the parent population.