Inbreeding depression is reduced fitness in a given population as a result of breeding of related individuals. Breeding between closely related individuals, called inbreeding, results in more recessive deleterious traits manifesting themselves. The more closely related the breeding pair is, the more homozygous deleterious genes the offspring may have, resulting in very unfit individuals. Another mechanism responsible is overdominance of heterozygous alleles leading to a reduction in the fitness of a population with many homozygous genotypes, even if they are not deleterious. Currently it is not known which of the two mechanisms is more important. In general, populations with more genetic variation do not suffer from inbreeding depression. Inbreeding depression is often the result of a population bottleneck. Inbreeding depression seems to be present in most groups of organisms, but is perhaps most important in hermaphroditic species, most prominently in plants. The majority of plants are hermaphroditic and thus are capable of the most severe degree of inbreeding.

Contents 1 Inbreeding depression and natural selection 2 Managing inbreeding depression 3 In humans 4 Species not subject to inbreeding depression 5 See also 6 External links 7 References

[edit] Inbreeding depression and natural selection

Natural selection cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly through mutation within a population. Second, in a population where inbreeding occurs frequently, most offspring will have some deleterious traits, so few will be more fit for survival than the others. It should be noted, though, that different deleterious traits are extremely unlikely to equally affect reproduction. An especially disadvantageous recessive trait expressed in a homozygous recessive individual is likely to eliminate itself, naturally limiting the expression of its phenotype. Third, recessive deleterious alleles will be "masked" by heterozygosity, and so heterozygotes will not be selected against (assuming dominance).

[edit] Managing inbreeding depression

Introducing new genes from a different population can reverse inbreeding depression. Different populations have different deleterious traits, and therefore will not result in homozygosity in most loci in the offspring. This is known as outbreeding enhancement, practiced by conservation managers and zoo captive breeders to prevent homozygosity. However, intermixing two different populations may give rise to unfit polygenic traits in outbreeding depression.

[edit] In humans

Although severe inbreeding depression in humans seems to be highly uncommon and not widely known, there have been several cases of apparent forms of inbreeding depression in human populations. As with animals, this phenomenon tends to occur in isolated, rural populations that are cut off to some degree from other areas of civilization. A notable example is the Vadoma tribe of western Zimbabwe, many of whom carry the trait of having only two toes due to a small gene pool.^{[citation needed]}

[edit] Species not subject to inbreeding depression

Example taxa not subject to significant inbreeding depression despite extremely low effective population sizes:

Animals

• Chatham Islands Robin
• Laysan Duck (data equivocal; severe population fluctuations probably natural)
• Mauritius Kestrel
• Naked Mole Rat (mammal displaying eusocial reproductive structure and low genetic variation^[1][2])
• Stegodyphus dumicola and some other social spiders (live in highly inbred colonies)
• Thai Ridgeback, a dog breed

Plants

• Dandelion(reproduces asexually through apomixis)
• Nihoa Carnation
• Toromiro^{[verification needed]}

[edit] See also

• Inbreeding
• Hybrid vigour
• Minimum viable population
• Population genetics
• Backward evolution
• Vadoma
• Insular dwarfism
• Island gigantism

[edit] External links

• Genetic Effects of Straying of Non-Native Hatchery Fish into Natural Populations

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (February 2008)

[edit] References