Jump to content

Founder effect

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Ettrig (talk | contribs) at 08:10, 26 January 2011 (→Founder effects in human populations). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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 Iceland, Easter Islanders and those native to Pitcairn Island. Another example is the legendarily high deaf population of Martha's Vineyard, which resulted in the development of 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]

Serial founder effect

Serial founder effect have occurred when populations migrate over long distances. Such long distance migrations typically involve relatively rapid movements followed by periods of settlement. The populations in each migration carry only a subset of the genetic diversity carried from previous migrations. As a result, genetic differentiation tends to increase with geographic distance as described by the "Isolation by distance" model.[10] The migration of humans out of Africa is characterized by serial founder effects. Africa has the highest genetic diversity which is consistent with an African origin of modern humans. After the initial migration from Africa, the Indian subcontinent was the first major settling point for modern humans. Consequently, India has the second highest genetic diversity in the world. In general, the genetic diversity of the Indian continent is a subset of Africa, and the genetic diversity outside Africa is a subset of India.[11][12]

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 eightyfold growth but minimal gene dilution from intermarriage, Quebec has what geneticists call optimal linkage disequilibrium (genetic sharing).[13] 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[14] 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, phenomena such as polydactyly (extra fingers and toes, a symptom of Ellis-van Creveld syndrome) are more common in Amish communities than in the American population at large.[15] Similarly there is a high frequency 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 Jessop.[16]

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 [17][18] (by comparison, in the United States, only 0.003% of the population has complete achromatopsia[19]).

Also, in 1814, 15 British colonists founded a settlement on Tristan da Cunha, a group of small islands in the Atlantic Ocean midway between Africa and South America. Apparently, one of the colonists carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts homozygous individuals. Of the founding colonists' 240 descendants on the island in the late 1960's, 4 had retinitis pigmentosa. The frequency of the allele that causes this disease is ten times higher on Tristan da Cunha than in the populations from which the founders came.

More severe illnesses exist among certain Jewish groups. Ashkenazi Jews, for example, have a particularly high chance of suffering from Tay-Sachs disease, a fatal condition in young children (see Medical genetics of Ashkenazi Jews).

See also

References

  1. ^ Provine WB (2004). "Ernst Mayr: Genetics and speciation". Genetics. 167 (3): 1041–6. PMC 1470966. PMID 15280221. {{cite journal}}: Unknown parameter |month= ignored (help)
  2. ^ Templeton AR (1980). "The theory of speciation via the founder principle". Genetics. 94 (4): 1011–38. PMC 1214177. PMID 6777243. {{cite journal}}: Unknown parameter |month= ignored (help)
  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.{{cite book}}: CS1 maint: multiple names: authors list (link)
  9. ^ Howard, Daniel J.; Berlocher, Steward H. (1998). Endless Forms (Illustrated ed.). United States: Oxford University Press. p. 470. ISBN 9780195109016.
  10. ^ Ramachandran S, Deshpande O, Roseman CC, Rosenberg NA, Feldman MW, Cavalli-Sforza LL (2005). "Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa". Proc. Natl. Acad. Sci. U.S.A. 102 (44): 15942–7. doi:10.1073/pnas.0507611102. PMC 1276087. PMID 16243969. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  11. ^ Maji S, Krithika S, Vasulu TS (2009). "Phylogeographic distribution of mitochondrial DNA macrohaplogroup M in India" (PDF). J. Genet. 88 (1): 127–39. doi:10.1007/s12041-009-0020-3. PMID 19417557. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  12. ^ Thangaraj K, Chaubey G, Singh VK; et al. (2006). "In situ origin of deep rooting lineages of mitochondrial Macrohaplogroup 'M' in India". BMC Genomics. 7: 151. doi:10.1186/1471-2164-7-151. PMC 1534032. PMID 16776823. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  13. ^ genizon.com
  14. ^ Hey J (2005). "On the number of New World founders: a population genetic portrait of the peopling of the Americas". PLoS Biol. 3 (6): e193. doi:10.1371/journal.pbio.0030193. PMC 1131883. PMID 15898833. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: unflagged free DOI (link)
  15. ^ McKusick VA, Egeland JA, Eldridge R, Krusen DE (1964). "Dwarfism in the Amish I. The Ellis-van Creveld syndrome". Bull Johns Hopkins Hosp. 115: 306–36. PMID 14217223. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  16. ^ Forbidden Fruit:Inbreeding among polygamists along the Arizona-Utah border is producing a caste of severely retarded and deformed children, by John Dougherty, The Phoenix New Times News, December 29, 2005, page 2.
  17. ^ Hussels IE, Morton NE (1972). "Pingelap and Mokil Atolls: achromatopsia". Am. J. Hum. Genet. 24 (3): 304–9. PMC 1762260. PMID 4555088.
  18. ^ Sacks, Oliver (1997). The Island of the Colour-blind. Picador. ISBN 0-330-35887-1.
  19. ^ "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.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Founder_effect&oldid=410131580"