Genetic drift is the variation in the relative frequency of different genotypes of a population, including the loss of particular genes if individuals die, do not reproduce, or they just don't get passed on. It occurs in all populations to an extent, but smaller ones are particularly vulnerable as they have less genetic diversity to start with. Over many generations, the loss of genetic variation can be harmful to the success of a species, as the potential genetic responses to environmental change is greatly depleted.
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Genetic drifts affects all populations, but small ones in particular because they have less genetic variety to begin with.