The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced. Events like natural disasters can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors.
The allele frequencies in this group may be very different from those of the population prior to the event, and some alleles may be missing entirely. The smaller population will also be more susceptible to the effects of genetic drift for generations (until its numbers return to normal), potentially causing even more alleles to be lost.
