This is a picture of beads in a bottle being poured out. The beads in the bottle represent a population an its gene pool. The beads remaining in the bottle represent the population and its gene pool remaining after a sudden event in the environment, such as a flood. This drastic reduction in population size due to an environmental event can cause the bottleneck effect. By chance, some alleles may be overrepresented, underrepresented, or even completely absent in the remaining population gene pool. In other words, a bottleneck can significantly reduce a populations genetic variation. Reduced genetic variation means that the population may not be able to adapt to new selection pressures, because the genetic variation that selection would act on may be out of the population. After a bottleneck effect, genetic drift will most likely have these effects on the gene pool until the population becomes large enough so that chance events have less of an impact.
