Genetic Bottleneck

A genetic bottleneck (sometimes population bottleneck) is an event, such as a natural disaster, that causes the death of (or sometimes the loss of reproductive ability in) a significant number of organisms in a population, thereby drastically reducing the total population size.

Genetic drift increases in bottlenecks, since drift is inversely proportional to population size. This is because in a smaller population, mutated alleles have less competition when it comes to replacing the original alleles (i.e. fixation is easier). Consequently the population can undergo rapid evolutionary change. However, there is also a higher incidence of inbreeding in genetic bottlenecks and in a few generations, there may be a lot of genetic homogeneity among the individuals. This makes the smaller population susceptible to extinction as a result of, for example, a disease pathogen to which none of the remaining organisms are genetically resistant.

In the case of a founder effect, where a small group of organisms is isolated from its parent population and forms a 'sub population', only some of the alleles from the parent population may be represented, determining a different fate for the generations to follow and perhaps leading to the evolution of a new species. A founder effect is not specifically a genetic bottleneck, however, since a smaller population has simply been isolated; the larger population is not dead.