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.^[1]

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 effect.

Population bottlenecks reduce the genetic variation and, therefore, the population's ability to adapt to new selective pressures, such as climatic change or shift in available resources. Genetic drift can eliminate alleles that could have been positively selected on by the environment if they had not already drifted out of the population.^[2]

Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the population size. The reduction in a population's dispersal leads, over time, to increased genetic homogeneity. If severe, population bottlenecks can also markedly increase inbreeding due to the reduced pool of possible mates (see small population size).

Examples

Humans

Evolutionary biologist Richard Dawkins has postulated that human mitochondrial DNA (inherited only from one's mother) and Y chromosome DNA (from one's father) show coalescence at around 140,000 and 60,000 years ago, respectively. In other words, all living humans' female line ancestry can be traced back to a single female (Mitochondrial Eve) at around 140,000 years ago. Via the male line, all humans can trace their ancestry back to a single male (Y-chromosomal Adam) at around 60,000 to 90,000 years ago.^[3]

This is consistent with the Toba catastrophe theory that suggests that a bottleneck of the human population occurred c. 70,000 years ago, proposing that the human population was reduced to perhaps 15,000 individuals^[4] when the Toba supervolcano in Indonesia erupted and triggered a major environmental change. The theory is based on geological evidences of sudden climate change and on coalescence evidences of some genes (including mitochondrial DNA, Y-chromosome and some nuclear genes)^[5] and the relatively low level of genetic variation with humans.^[4]

However, such coalescence is genetically expected and does not, in itself, indicate a population bottleneck, because mitochondrial DNA and Y-chromosome DNA are only a small part of the entire genome, and are atypical in that they are inherited exclusively through the mother or through the father, respectively. Most genes in the genome are inherited from either father or mother, and thus can be traced back in time via either matrilineal or patrilineal ancestry.^[6] Research on many genes finds different coalescence points from 2 million years ago to 60,000 years ago when different genes are considered, thus disproving the existence of more recent extreme bottlenecks (i.e., a single breeding pair).^[4][7]

On the other hand, 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.^[8] This would be consistent with suggestions that in sub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the Late Stone Age.^[9]

Other animals

Year	American bison (est)
Before 1492	60,000,000
1890	750
2000	360,000

Wisent, also called European bison (Bison bonasus), faced extinction in the early 20th century. The animals living today are all descended from 12 individuals and they have extremely low genetic variation, which may be beginning to affect the reproductive ability of bulls (Luenser et al., 2005). The population of American bison (Bison bison) fell due to overhunting, nearly leading to extinction around the year 1890, though it has since begun to recover (see table).

A classic example of a population bottleneck is that of the northern elephant seal, whose population fell to about 30 in the 1890s. Although it now numbers in the hundreds of thousands, the potential for bottlenecks within colonies remains. Dominant bulls are able to mate with the largest number of females — sometimes as many as 100. With so much of a colony's offspring descended from just one dominant male, genetic diversity is limited making the species more vulnerable to diseases and genetic mutations. The golden hamster is a similarly bottlenecked species, with the vast majority descended from a single litter found in the Syrian desert around 1930. And cheetahs are sufficiently closely related to one another that transplanted skin grafts do not provoke immune responses,^[10] thus suggesting an extreme population bottleneck in the past.

The genome of the giant panda shows evidence of a severe bottleneck that took place about 43,000 years ago.^[11] There is also evidence of at least one primate species, the golden snub-nosed monkey, that also suffered from a bottleneck around this time.

Further deductions can sometimes be inferred from an observed population bottleneck. Among the Galápagos Islands giant tortoises — themselves a prime example of a bottleneck — the comparatively large population on the slopes of Alcedo volcano is significantly less diverse than four other tortoise populations on the same island. DNA analyses date the bottleneck to around 88,000 years before present (YBP).^[12] About 100,000 YBP the volcano erupted violently, burying much of the tortoise habitat deep in pumice and ash.

Bottlenecks also exist among pure-bred animals (e.g., dogs and cats: pugs, Persian) because breeders limit their gene pools by breeding with close relatives for their looks and behaviors. The extensive use of desirable individual animals at the exclusion of others can result in a popular sire effect.

Before Europeans arrived in North America, prairies served as habitats to greater prairie chickens. In Illinois alone their numbers plummeted from over 100 million in 1900 to about 50 in 1990. These declines in population were the result of hunting and habitat destruction, but the random consequences have been a great loss in species diversity. DNA analysis comparing the birds from 1990 and mid-century shows a steep genetic decline in recent decades. The greater prairie chicken is currently experiencing low reproductive success.^[13]

Plants

Research showed that there is no genetic variability in the genome of the Wollemi Pine (Wollemia nobilis), indicating that the species (of which there are only around 100 specimens in the wild and tens of thousands cultivated) went through a severe population bottleneck.

In evolutionary theory

As a population becomes smaller, genetic drift plays a bigger role 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.

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 usefulness of the MVP.

See also

• Population boom
• Baby boom
• Small population size
• Effective population size
• Founder effect

References

External links

• ScienceDaily: Big Bang Theory Of Human Evolution?
• Northern Elephant Seal History
• Masatoshi Nei (May 1, 2005). "Bottlenecks, Genetic Polymorphism and Speciation". Genetics (The Genetics Society of America) 170 (1): 1–4. PMC 1449701. PMID 15914771. http://www.genetics.org/cgi/content/full/170/1/1. Retrieved 2008-10-19.