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 in a dominant-recessive trait, heterozygotes will not be selected against.
When recessive deleterious alleles occur in the heterozygous state, where their potentially deleterious expression is masked by the corresponding wild-type allele, this masking phenomenon is referred to as complementation (see Complementation (genetics).
In general, sexual reproduction in eukaryotes has two fundamental aspects: recombination during meiosis, and outcrossing. It has been proposed that these two aspects have two natural selective advantages respectively. A proposed adaptive advantage of meiosis is that it facilitates recombinational repair of DNA damages that are otherwise difficult to repair (see Meiosis – section: Theory that DNA repair is the adaptive advantage of meiosis). A proposed adaptive advantage of outcrossing is complementation, which is the masking of deleterious recessive alleles (see hybrid vigor or heterosis). The selective advantage of complementation may largely account for the general avoidance of inbreeding (see Kin recognition).
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