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balancing selection
Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population ''at frequencies longer than for expected from genetic drift alone''. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value than the homozygote.〔King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics'', 7th ed. Oxford. p44〕 In this way genetic polymorphism is conserved.
Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is ubiquitous in panmictic populations. It is a genetic expression of the field experience of Darwin, Wallace and others, that natural populations in the wild are extraordinarily varied (though not all such variation is of genetic origin).
There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are ''heterozygote advantage'' and ''frequency-dependent selection''.
== Mechanisms of balancing selection ==
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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