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From an evolutionary perspective, populations change over time. The size and composition of a population's gene pool is key to maintaining genetic diversity. Gene pool change in a small population because of chance is known as genetic drift. The founder effect is a case of genetic drift in which a small population of a limited number of individuals breaks off from a larger population.
The effect on the genetic makeup of a population can be quite profound, as the prevalence of disease can increase. The lower the number of individuals involved, the more that the breakaway population may be impacted. This effect continues until the population size is large enough for errors from generation to generation to become minimal. If the population continues to be isolated, the effects can persist.
- Change in a small population's gene pool due to chance is known as genetic drift.
- The founder effect is a case of genetic drift caused by a small population with limited numbers of individuals breaking away from a parent population.
- The occurrence of retinitis pigmentosa in the British colony on the Tristan da Cunha islands is an example of the founder effect.
- The prevalence of Ellis-van Creveld syndrome in the Amish in Eastern Pennsylvania is another example of the founder effect.
Founder Effect Examples
If a small population breaks off from a larger population to colonize an island, for example, the founder effect can occur. If some of the colonizers are carriers or homozygous recessive, the prevalence of the recessive allele can be quite dramatic in the small population versus the larger parent population.
When a new generation has alleles distributed at random, with a large enough sample size, we can expect that the gene pool of the new generation will roughly represent the gene pool of the prior generation. We'd expect a certain distribution of traits in a given population, when that population is sufficiently large. When a population is small, the gene pool from generation to generation might not be accurately represented. This is due to sampling error because of the small size of the population. Sampling error refers to the disproportion of results in a small population or sample.
Retinitis Pigmentosa Example
Not all genes have a simple dominant recessive occurrence. Others are polygenic traits and can be due to changes in a number of genes. For example, in the early 1800s a number of individuals migrated to the Tristan da Cunha islands to form a British colony. At least one of the colonists appears to have been a carrier and had a recessive allele for retinitis pigmentosa. Retinitis pigmentosa is a relatively rare disorder where the cells in the retina are lost or break down resulting in loss of sight. Individuals who are homozygous for the allele have the disease.
By some estimates, in the 1960s, of the 240 residents in the colony, four had the disorder and at least nine others were carriers. This is much more prevalent than would be expected based on the rarity of retinitis pigmentosa in larger populations.
Eastern Pennsylvania is home to the Amish, who provide a striking example of the founder effect. It is estimated that about 200 individuals that immigrated from Germany founded their community. The Amish typically marry from within their own community and are isolated, so genetic mutations tend to persist.
For example, polydactyly, possessing extra fingers or toes, is a common symptom of Ellis-van Creveld syndrome. The syndrome is a rare disorder that also is characterized by dwarfism and sometimes congenital heart defects. Due to the founder effect, Ellis-van Creveld syndrome is much more prevalent among the Amish.
Founder effect in animals and plants
While the movement of human populations can provide examples of the founder effect, the effect is not limited to humans. It can occur in animals or plants as well, whenever small populations break off from larger ones.
The founder effect can have a profound impact on small populations due to genetic drift. The impact can persist when the population remains isolated so that genetic variation is minimal. Inherited diseases like retinitis pigmentosa and Ellis-van Creveld syndrome are examples of the consequences of the founder effect.
- “Genetic Drift and the Founder Effect.” PBS, Public Broadcasting Service, www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html.
- Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.