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Genetic drift is a change in gene frequency within a population due to chance instead of natural selection. If the population is small, then there can be considerable genetic drift, which can change the gene frequency quite drastically. Think of the genetic variation of a population as a bowl of jelly beans with all sorts of colors, representing healthy genetic variation.
Another important small population effect is known as the founder principle or founder effect. This occurs when a small amount of people have many descendants surviving after a number of generations. The result for a population is often high frequencies of specific genetic traits inherited from the few common ancestors who first had them.