Population bottleneck


Population bottleneck
Population bottleneck followed by recovery or extinction

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).

Contents

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).

Overhunting pushed the northern elephant seal to the brink of extinction by the late 19th century. Though they have made a comeback, the genetic variation within the population remains very low.

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

References

  1. ^ Population Bottleneck | Macmillan Genetics
  2. ^ "Evolution 101". University of California Museum of Paleontology. http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml. Retrieved 2 February 2011. 
  3. ^ Dawkins, Richard (2004). The Ancestor's Tale, A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company. ISBN 0297825038. ISBN. 
  4. ^ a b c Dawkins, Richard (2004). "The Grasshopper's Tale". The Ancestor's Tale, A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company. pp. 416. ISBN 0297825038. ISBN. 
  5. ^ Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans by Stanley H. Ambrose
  6. ^ See the chapter All Africa and her progenies in Dawkins, Richard (1995). River Out of Eden. New York: Basic Books. ISBN 0465016065. ISBN. 
  7. ^ 'Templeton tree' showing coalescence points of different genes
  8. ^ Population Bottlenecks and Pleistocene Human Evolution
  9. ^ BBC news : Human line 'nearly split in two'
  10. ^ HowStuffWorks "The Endangered Cheetah"
  11. ^ Zhang, Ya-ping, et al. (2002). "Genetic diversity and conservation of endangered animal species" (PDF). Pure Appl. Chem. 74 (Vol. 74, No. 4): 575. doi:10.1351/pac200274040575. http://www.iupac.org/publications/pac/2002/pdf/7404x0575.pdf. 
  12. ^ Luciano B. Beheregaray, Claudio Ciofi, Dennis Geist, James P. Gibbs, Adalgisa Caccone, and Jeffrey R. Powell (2003). "Genes Record a Prehistoric Volcano Eruption in the Galápagos" (PDF). Science 302 (5642): 75. doi:10.1126/science.1087486. http://www.sciencemag.org/content/302/5642/75.full.pdf?sid=f4e0c19b-a2c2-470a-b39f-a5bb82452ecd. 
  13. ^ "Brain & Ecology Deep Structure Lab". Brain & Ecology Comparative Group. Brain & Ecology Deepstruc. System Co., Ltd.. 2010. http://www.brainecology.net/info/show.asp?bh=73. Retrieved March 13, 2011. 
  • 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.
  • Luenser, K., J. Fickel1, A. Lehnen, S. Speck and A. Ludwig. 2005. Low level of genetic variability in European bisons (Bison bonasus) from the Bialowieza National Park in Poland. European Journal of Wildlife Research 51 (2): 84-87.
  • Soulé, M. (Ed.). (1987). Viable populations for conservation. Cambridge: Cambridge Univ. Press.

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