Population bottleneck

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. It also changes the relationship of natural selection (see: inbreeding).

Humans

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. One theory about this bottleneck is the Toba catastrophe theory, positing that the human population was reduced to a few thousand individuals when the Toba supervolcano in Indonesia erupted and triggered a massive environmental change.

Examples in the animal world

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.

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. Also, 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.

Sometimes further deductions can be inferred from an observed population bottleneck. Among the Galapagos archipelago's giant tortoises (themselves a prime example of a founder effect), the comparatively large population on the slopes of Alcedo volcano is significantly less diverse than four other tortoise populations on the same island. Researchers' DNA analysis dates the bottleneck around 88,000 YBP, according to a notice in Science, October 3, 2003. About 100,000 YBP the volcano erupted violently, burying much of the tortoise habitat deep in pumice and ash. The coincidence is suggestive.

See also

• Small population size
• effective population size
• Founder effect
• Overpopulation
• Ice age
• Black Death
• AIDS
• Toba catastrophe theory

External links

• http://www.sciencedaily.com/releases/2000/01/000110142554.htm
• Northern elephant seals
• http://essp.csumb.edu/eseal/kristi_west/history.html