Inbreeding depression
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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 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. 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 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. Third, recessive deleterious alleles will be "masked" by heterozygosity, and so heterozygotes will not be selected against (assuming dominance).
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 (Heterosis), practiced by conservation managers and captive Zoo breeders to prevent homozygosity. However, intermixing two different populations may give rise to unfit polygenic traits, in outbreeding depression
See also
References
Genetic Effects of Straying of Non-Native Hatchery Fish into Natural Populations http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.html
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