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Founder effect

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For the concept in organizations, see Founder's syndrome.
Simple illustration of founder effect. The original population is on the left with three possible founder populations on the right.

In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1952,[1] using existing theoretical work by those such as Sewall Wright.[2] As a result of the loss of genetic variation, the new population may be distinctively different, both genetically and phenotypically, from the parent population from which it is derived. In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species.

In the figure shown, the original population has nearly equal numbers of blue and red individuals. The three smaller founder populations show that one or the other color may predominate (founder effect), due to random sampling of the original population. A population bottleneck may also cause a founder effect even though it is not strictly a new population.

The founder effect is a special case of genetic drift.[3] [4] In addition to founder effects, the new population is often a very small population and so shows increased sensitivity to genetic drift, an increase in inbreeding, and relatively low genetic variation. This can be observed in the limited gene pool of Easter Islanders and those native to Pitcairn Island. Another example is the legendarily high deaf population of Martha's Vineyard which resulted in the famous Martha's Vineyard Sign Language.

General

The founder effect is a special case of genetic drift, occurring when a small group in a population splinters off from the original population and forms a new one. The new colony may have less genetic variation than the original population, and through the random sampling of alleles during reproduction of subsequent generations, continue rapidly towards fixation. This consequence of inbreeding makes the colony more vulnerable to extinction.

When a newly formed colony is small, its founders can strongly affect the population's genetic make-up far into the future. In humans, which have a slow reproduction rate, the population will remain small for many generations, effectively amplifying the drift effect generation after generation until the population reaches a certain size. Alleles which were present but relatively rare in the original population can move to one of two extremes. The most common one is that the allele is soon lost altogether, but the other possibility is that the allele survives and within a few generations has become much more dispersed throughout the population. The new colony can experience an increase in the frequency of recessive alleles as well, and as a result, an increased number who are homozygous for certain recessive traits. A well documented example is found in the Amish migration to Pennsylvania in 1744. Two members of the new colony shared the recessive allele for Ellis-van Creveld syndrome. Members of the colony and their descendants tend to be religious isolates and remain relatively insular. As a result of many generations of inbreeding, Ellis-van Creveld syndrome is now much more prevalent among the Amish than in the general population.[5][6]

The variation in gene frequency between the original population and colony may also trigger the two groups to diverge significantly over the course of many generations. As the variance, or genetic distance, increases, the two separated populations may become distinctively different, both genetically and phenotypically, although not only genetic drift but also natural selection, gene flow and mutation will all contribute to this divergence. This potential for relatively rapid changes in the colony's gene frequency led most scientists to consider the founder effect (and by extension, genetic drift) a significant driving force in the evolution of new species. Sewall Wright was the first to attach this significance to random drift and small, newly isolated populations with his shifting balance theory of speciation.[7] Following behind Wright, Ernst Mayr created many persuasive models to show that the decline in genetic variation and small population size accompanying the founder effect were critically important for new species to develop.[8] However there is much less support for this view today since the hypothesis has been tested repeatedly through experimental research and the results have been equivocal at best.[9]

Founder effects in island ecology

Founder populations are essential to the study of island biogeography and island ecology. A natural "blank slate" is not easily found, but a classic series of studies on founder population effects were done following the catastrophic 1883 eruption of Krakatoa, which erased all life on the island remnant. Another continuing study has been following the biocolonization of Surtsey, Iceland, a new volcanic island that erupted offshore between 1963 and 1967. An earlier event, the Toba eruption in Sumatra of about 73,000 YBP, covered some parts of India with 3–6 metres (9.8–19.7 ft) of ash, and must have coated the Nicobar Islands and Andaman Islands, much nearer in the ash fallout cone, with life-smothering layers, restarting their biodiversity from effectively zero.

Founder effects in human populations

Due to various migrations throughout human history, founder effects are somewhat common among humans in different times and places. The effective founder population of Quebec was only 2,600. After twelve to sixteen generations, with an eighty-fold growth but minimal gene dilution from intermarriage, Quebec has what geneticists call optimal linkage disequilibrium (genetic sharing).[10] The result: far fewer genetic variations, including those that have been well studied because they are connected with inheritable diseases.

Founder effects can also occur naturally as competing genetic lines die out. This means that an effective founder population consists only of those whose genetic print is identifiable in subsequent populations. Because in sexual reproduction, genetic recombination ensures that with each generation, only half the genetic material of a parent is represented in the offspring, some genetic lines may die out entirely, even though there are numerous progeny. A recent study[11] concluded that of the people migrating across the Bering land bridge at the close of the ice age, only 70 left their genetic print in modern descendants, a minute effective founder population— which is easily misread as though implying that only 70 people crossed to North America. The misinterpretations of "Mitochondrial Eve" are a case in point: it may be hard to explain that a "mitochondrial Eve" was not the only woman of her time.

In humans, founder effects can arise from cultural isolation, and inevitably, endogamy. For example, the Amish populations in the United States, which have grown from a very few founders, have not recruited newcomers, and tend to marry within the community, exhibit founder effects. Though still rare absolutely, phenomena such as polydactyly (extra fingers and toes, a symptom of Ellis-van Creveld syndrome) are more common in Amish communities than in the US population at large.[12] There is also the presence of high cases of fumarase deficiency among the 10,000 members of the Fundamentalist Church of Jesus Christ of Latter Day Saints community which practices both endogamy and polygyny, where it is estimated 75 to 80 percent of the community are blood relatives of just two men - founders John Y. Barlow and Joseph Smith.[13]

Another example is the frequency of total color-blindness among the inhabitants of Pingelap, an island in Micronesia. In approximately 1775, a typhoon reduced the population of the island to only 20. Among survivors, one of them was heterozygous for achromatopsia. After few generations, the prevalence of achromatopsia is 5% of population and 30% as carriers [14][15](by comparison, in the United States, only 0.003% of population have complete achromatopsia[16]).

See also

References

  1. ^ Provine, W.B. 2004. "Ernst Mayr: Genetics and Speciation" Genetics 167: 1041-1046.[1]
  2. ^ Templeton, A. R.(1979) "The theory of speciation via the founder theory". Genetics. 94:1011-38.
  3. ^ Hartwell, Hood, Goldberg, Reynolds, Silver, Veres, 2004, Genetics - from genes to genomes, page 688, McGraw Hill Higher Education
  4. ^ Raven, Evert, Eichhorn, 1999, Biology of plants, page 241, W H Freeman and Company
  5. ^ Cavalli-Sforza, L. L.; Menozzi, Paolo; Piazza, Alberto (1996), The history and geography of human genes, Princeton, N.J: Princeton University Press, p. 413, ISBN 0-691-02905-9
  6. ^ "Genetic Drift and the Founder Effect". Evolution. Public Broadcast System. Retrieved 2009-04-07.
  7. ^ Wade, Michael S.; Wolf, Jason; Brodie, Edmund D. (2000). Epistasis and the evolutionary process. Oxford [Oxfordshire]: Oxford University Press. p. 330. ISBN 0-19-512806-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. ^ Mayr, Ernst, Jody Hey, Walter M. Fitch, Francisco José Ayala (2005), Systematics and the Origin of Species: on Ernst Mayr's 100th anniversary (Illustrated ed.), National Academies Press, p. 367, ISBN 9780309095365{{citation}}: CS1 maint: multiple names: authors list (link)
  9. ^ Howard, Daniel J.; Berlocher, Steward H., Endless Forms (Illustrated ed.), United States: Oxford University Press, p. 470, ISBN 9780195109016
  10. ^ genizon.com
  11. ^ Hey, Jody, 2005. "On the Number of New World Founders: A Population Genetic Portrait of the Peopling of the Americas" in PLoS Biol 2005 May 24;3(6):e193 webpage
  12. ^ McKusick, V. A.; Egeland, J. A.; Eldridge, R.; Krusen, D. E.: Dwarfism in the Amish. I. The Ellis-van Creveld syndrome. Bull. Johns Hopkins Hosp. 115: 306-336, 1964. PMID 14217223
  13. ^ boston.com
  14. ^ Hussels IE, Morton NE (1972). "Pingelap and Mokil Atolls: achromatopsia". Am. J. Hum. Genet. 24 (3): 304–9. PMID 4555088.
  15. ^ Sacks, Oliver (1997). The Island of the Colour-blind. Picador. ISBN 0-330-35887-1.
  16. ^ "The Achromatopsia Group". Retrieved 2007-06-13.
  • Mayr, E. 1954. Change of genetic environment and evolution. In Huxley, J. (ed) Evolution as a Process, Allen and Unwin, London.
  • Mayr, E. 1963. Animal Species and Evolution. Harvard University Press, Cambridge, Massachusetts.
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