Population bottleneck

From Free net encyclopedia

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. They also increase inbreeding due to the reduced pool of possible mates (see small population size).

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 event.

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. The evidence that all living humans are descended from fewer than ten thousand people alive at that time comes both from mitochondrial DNA coalescence, and the relatively small variations in the human Y chromosome.

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.

In 2000, a Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change (see "Population Bottlenecks and Pleistocene Human Evolution").

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. Another example are Cheetahs, which are so closely related to each other that skin grafts from one cheetah to another do not provoke immune responses, thus suggesting an extreme population bottleneck in the past. Another largely bottlenecked species is the Golden Hamster, of which the vast majority are descended from a single litter found in the Syrian desert around 1930.

According to a paper published in 2002, the genome of the Giant Panda shows evidence of a severe bottleneck that took place about 43,000 years ago¹. There is also evidence of at least one primate species that suffered from a bottleneck around this time scale.

Sometimes further deductions can be inferred from an observed population bottleneck. Among the Galápagos Islands 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 years before present (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.

Population bottlenecks in evolutionary theory

As population bottlenecks increase the rate of genetic drift they may be implicated in speciation. A land animal like a Brown Bear might find itself locally reduced to a few dozen pairs on an arctic island. That likely happened as the last Ice Age came to an end, and the Bering land bridge receded into the sea. In that circumstance, a beneficial trait appearing in an alpha male or two may change the color, size, swimming ability, cold resistance, or aggressiveness of the group in just a few generations. This would be an example of punctuated equilibrium. The small population size of many animals can make population bottlenecks more common than might otherwise be thought.

Minimum viable population size

In conservation biology, minimum viable population size (MVP) helps to determine the effective population size when a population is at risk for extinction (Gilpin and Soulé, 1986 and Soulé, 1987). There is considerable debate about the value of the MVP.

See also

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

Population bottleneck in fiction

Kurt Vonnegut 1985. Galápagos.

Notes

• ¹ Paper: "Genetic diversity and conservation of endangered animal species"

References

Gilpin, M.E., & Soulé, M.E. (1986). Minimum viable populations: The processes of species extinctions. In M. Soulé (Ed.). Conservation biology: The science of scarcity and diversity, pp. 13-34. Sunderland Mass: Sinauer Associates.

Soulé, M. (Ed.). (1987). Viable populations for conservation. Cambridge: Cambridge Univ. Press.

External links

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

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