In population genetics, the bottleneck effect consists of a sharp reduction in size of a population due to environmental stochastic events. The effect is characterized when a large number of individuals die and the population is restored from a gene pool smaller than before. That is, this effect occurs when some natural event exterminates the majority of a population, thereby reducing the variation present in the gene pool of the species. A stochastic event can reduce the population size of a species to a number considerably smaller than its typical size. However, there is a limit to how small a population may be, since species with small populations are at risk of facing extinction. A consequence of the bottleneck effect is that a single period of small population size, may result in a significant loss of heterozygosity.
A classic example of a genetic bottleneck developed in northern elephant seals, prior found in large numbers along the California coast, whose population fell to around 20 individuals in 1884 on a remote beach of Isla de Guadalupe, California. Hunting restrictions established by the American and Mexican governments have caused that the population could be restored to the point of reaching over 30,000 individuals. However, the level of genetic similarity has become high.
The European bison (Bison bonasus), was threatened with extinction in the early twentieth century. The animals living today are all descended from 12 individuals and they have extremely low genetic variation. Only 2 variants of the Y chromosome are known in this species.
The population of American bison (Bison bison) had declined drastically due to overhunting, leading almost to extinction around the year 1890, although in the year 2000 its population has been restored to around 360000 individuals.
One possible case of bottleneck occurring in humans occurred among Neanderthal people, possibly caused by glaciation. A clear bottleneck event among humans was the great flood, reducing the human population to only eight individuals (Genesis 7:7 ).
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