||This documentation needs attention from an expert in Genetics. (November 2009)|
A population bottleneck is a sharp reduction in size of a population due to environmental random events (such as earthquakes, floods, fires, or droughts) or human activities. Such events are able to reduce the variation in the gene pool of a population drastically, because when the population was close to extinction, the few left passed on those genes to their offspring, which became mostly dominant over the generations. If there are not enough individuals left, or the ones that survived are not able to reproduce sufficiently to re-form the species population, that population may go extinct. In consequence of such population size reductions and the loss of genetic variation, the ability of a population to adapt to prevalent environment or selection like climatic change or shift in available resources is reduced. However, depending upon the causes of the bottleneck, the survivors may have been the most fit individuals, hence improving the traits within the gene pool while shrinking it. This genetic drift can change the proportional distribution of an allele by chance and even lead to fixation or loss of alleles. Due to the smaller population size after a bottleneck event, the chance of inbreeding and genetic homogeneity increases and unfavoured alleles can accumulate. Thus, an ecological bottleneck can lead to a genetic bottleneck, in which the genetic variation within a population and the potential to adapt to a changing environment decrease.
A slightly different form of a bottleneck can occur, if a small group becomes reproductively (e.g. geographically) separated from the main population, for example through a founder event. Population bottlenecks play an important role in conservation biology (see minimum viable population size) and in the context of agriculture (biological/pest control).
The Toba catastrophe theory suggests that a bottleneck of the human population occurred c. 70,000 years ago, proposing that the human population was reduced to perhaps 10,000 individuals 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) and the relatively low level of genetic variation with humans.
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. 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).
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. 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.
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. 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. Cheetahs are sufficiently closely related to one another that transplanted skin grafts do not provoke immune responses, 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. 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). 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.
Research showed that there is no genetic variability in the genome of the Wollemi Pine (Wollemia nobilis). There are only around 100 specimens in the wild and tens of thousands cultivated. This indicates that the species went through a severe population bottleneck.
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. There is considerable debate about the usefulness of the MVP.
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- Northern Elephant Seal History
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