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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.
Bottleneck Effect. One specific cause of genetic drift is the bottleneck effect, or population bottleneck. The bottleneck effect occurs when a larger population shrinks significantly in size in a short amount of time. Usually, this decrease in population size is generally due to a random environmental affect like a natural disaster or spread of disease.