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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.

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).

Example of inbred depression

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.

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 Zimbabwemarker, many of whom carry the trait of having only two toes due to a small gene pool.

Species not subject to inbreeding depression

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



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