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Bottleneck

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In genetics, a population bottleneck is an event in which a population's size is greatly reduced. Gene frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population's genetic variation. The situation is even more extreme for the genes that are targeted for artificial selection. Humans only allow individuals carrying desirable gene versions to reproduce. This is a bit like going to the gumball machine when you only like cherry-flavored gum: you might reject many pieces of gum based on their color and will wind up with a handful of gumballs with very little variety.

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