Random change in allele frequency through time is genetic drift, which is a state marked by the absence of gene flow. Drift is caused by chance alone and the result of very strong drift is either allele extinction or fixation. That is, alleles can be completely eliminated from the gene pool, while others can take hold in each and every individual. Drift is most commonly caused by a population bottleneck or by a founder effect and is most observable in small populations.
A population experiences a bottleneck when its size decreases relatively rapidly and then increases again, as the result, for instance, of famine, war, genocide, or disease. A founder effect is a type of bottleneck that occurs when a small subgroup is isolated from the population and begins a new population by mating with one another, like if, for instance, a small group of tourists got shipwrecked on an uncharted desert island. With drift, neutral traits like six fingers (polydactyly) or maladap-tive traits like Ellis-van Creveld Syndrome (a disorder that involves limb dwarfism, polydactyly, and heart and bone malformation and exists at higher than normal frequencies in the Old Order Amish of Pennsylvania) are perpetuated simply because of chance and small population sizes. Drift results in a decrease in the variation within a population or a gene pool.
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