Founder effect

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Simple illustration of founder effect. The original population is on the left with three possible founder populations on the right.

The founder effect was defined by Ernst Mayr in 1963 to be the effect of establishing a new population by a small number of individuals, carrying only a small fraction of the original population's genetic variation. As a result, 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. The founder effect is a feature that can also occur in memetic evolution.

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 can also cause a founder effect even though it isn't strictly a new population.

In addition to founder effects, the new population is often very small and shows random genetic drift, and an increase in inbreeding due to small population size. Furthermore, the new population is often characterized by low genetic variation, due to the population bottleneck. This can be observed in the limited gene pool of Easter Islanders and those native to Pitcairn Island.

[edit] Founder effects in island ecology

Founder populations are essential to the study of island biogeography and island ecology. A natural tabula rasa is not easily found, but a classic series of studies on founder population effects were done following the catastrophic 1883 eruption of Krakatau (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 m 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.

[edit] 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) [1]. 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 (Hey 2005) 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. 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. Similar religious and cultural founder effects have been demonstrated through the prevalence of otherwise rare diseases among South African Muslims (among whom cleidocranial dysostosis is more common, due to one founder who was a Chinese immigrant who converted to Islam) and the presence of fumarase deficiency among members of the breakaway Fundamentalist Church of Jesus Christ of Latter Day Saints. All of these cultures participate in the practice of endogamy.

[edit] See also

• Small population size

[edit] References

• Mayr, E. 1963. Animal Species and Evolution. Harvard University Press, Cambridge, Massachusetts.
• 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 [2]

[edit] External links

• Founder effect
• Quebec Founder Population Project