A population bottleneck (or genetic bottleneck) is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing.
A slightly different sort of genetic bottleneck can occur if a small group becomes reproductively separated from the main population. This is called a founder effect.
Population bottlenecks reduce the genetic variation and, therefore, the population's ability to adapt to new selective pressures, such as climatic change or shift in available resources. Genetic drift can eliminate alleles that could have been positively selected on by the environment if they had not already drifted out of the population.
Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the population size. The reduction in a population's dispersal leads, over time, to increased genetic homogeneity. If severe, population bottlenecks can also markedly increase inbreeding due to the reduced pool of possible mates (see small population size).
Read more about Population Bottleneck: In Evolutionary Theory, Minimum Viable Population Size
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—Ralph Waldo Emerson (1803–1882)