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background selection
Background selection describes the loss of genetic diversity at a non-deleterious locus due to negative selection against linked deleterious alleles.〔Charlesworth, B., M. T. Morgan, and D. Charlesworth. 1993. The effect of deleterious mutations on neutral molecular variation. Genetics. 134: 1289-1303.〕 It is one form of linked selection, where the maintenance or removal of an allele from a population is dependent upon the alleles in its linkage group. The name emphasizes the fact that the genetic background, or environment, of a neutral mutation has a significant impact on whether it will be preserved (genetic hitchhiking) or purged (background selection) from a population. In some cases, the term background selection is used broadly to refer to all forms of linked selection, but most often it is used only when neutral variation is reduced due to negative selection against deleterious mutations. Background selection and all forms of linked selection contradict the assumption of the Neutral theory of molecular evolution that the fixation or loss of neutral alleles is entirely stochastic, the result of genetic drift. Instead, these models predict that neutral variation is correlated with the selective pressures acting on linked non-neutral genes, that neutral traits are not necessarily oblivious to selection. Because they segregate together, non-neutral mutations linked to neutral polymorphisms result in decreased levels of genetic variation relative to predictions of neutral evolution.〔
==Background selection vs. the Neutral Model==
The reduction in neutral variation due to background selection can be modeled by an exponential function of the total mutation rate at the deleterious regions of the section of genome involved.〔 The overall effect of background selection on genetic diversity resembles a reduction in effective population size.〔Charlesworth, D., B. Charlesworth, and M. T. Morgan. 1995. The pattern of neutral molecular variation under the background selection model. Genetics. 141: 1619-1632.〕 As a consequence, background selection has been used to explain many of the inconsistencies between classical models of neutral variation and observed studies of genetic diversity. For instance, the observation that genetic diversity is weakly correlated with population size, or not correlated at all, is called the “paradox of variation”.〔Hahn, Matthew W. 2008. Towards a selection theory of molecular evolution. Evolution. 62(2): 255-265.〕〔Lewontin, R. C. 1974. The genetic basis for evolutionary change. Columbia Univ. Press, New York, NY.〕 Under the theory of background selection, the paradox is resolved, because neutral variation is predicted to be purged in conjunction with deleterious mutations.
Background selection also contributes to a selective explanation of the positive correlation between recombination and polymorphism across populations. In areas of high recombination, neutral loci are more likely to ‘escape' the effects of nearby selection and be retained in the population.〔 A version of this scenario has been observed in studies of ''Drosophila'', where regions of low recombination in the genome exhibit low levels of genetic variation. Hudson and Kaplan showed that the difference between the expected and observed levels of variation is explained by accounting for background selection.〔Hudson, Richard R. and Norman L. Kaplan. 1995. Deleterious background selection with recombination. Genetics. 141: 1605-1617.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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