Population bottleneck
In population genetics and evolutionary biology, 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, and the population is reduced by 50% or more, often by several orders of magnitude. A graph of this change resembles the neck of a bottle, from wide to narrow; hence the name.
Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the population size, which is reduced. It also changes the relationship of natural selection (see: inbreeding).
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DNA evidence suggests that humans today are a legacy of a population bottleneck which occurred 70,000 years ago. This would have had the result of limiting the overall level of genetic diversity in the human species, possibly by a large amount.
Wisent, also called European bison, faced extinction in the early 20th century. The 3600 animals living in 2000 are all descended from 12 individuals and only two distinct Y chromosomes are left in the species.
The population of American bison fell due to overhunting, nearly leading to extinction around the year 1890 and has since begun to recover.
All existing cheetahs are extremely close genetically suggesting an extreme population bottleneck in the past.
Another largely bottlenecked species is the Golden hamster, for which the vast majority are descended from a single litter found in the Syrian desert around 1930.
Humans
Examples in the animal world
Bison population size:
60,000,000 estimate for pre-1492
750 in 1890
350,000 in 2000
A classic example of a population bottleneck is that of the northern elephant seals, whose population fell to about 30 in the 1890's although it now numbers in the tens of thousands.See also
External links