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The Evolutionary Dynamics of Complex Polymorphisms

R. C. Lewontin and Ken-ichi Kojima
Evolution
Vol. 14, No. 4 (Dec., 1960), pp. 458-472
DOI: 10.2307/2405995
Stable URL: http://www.jstor.org/stable/2405995
Page Count: 15
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The Evolutionary Dynamics of Complex Polymorphisms
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Abstract

The joint effects of linkage and epistasis (interaction between non-allelic genes in determining fitness) have been examined for two-locus polymorphisms. The general results are of the following nature: (1) Gene frequencies change toward a stable equilibrium condition which corresponds to a local maximum in mean adaptive value. This change occurs in such a way that the rate of increase of mean adaptive value is not maximum. That is, the trajectory of gene frequency changes is not the "steepest path" on the adaptive surface. (2) In the absence of epistasis, linkage does not affect the final equilibrium of the population. (3) When epistasis is present, linkage must be fairly tight in order for there to be any effect on the final equilibrium. The amount of recombination allowed for such cases is, in general, a function of epistatic deviations. (4) If linkage is tighter than the value demanded by the magnitude of epistatic deviations, there may be permanent linkage disequilibrium of considerable magnitude, and the gene frequencies may also be affected. (5) There are some cases where a stable equilibrium is possible only with a tight linkage. Without such linkage complication, there will be no intermediate gene frequency stable equilibrium. (6) The approach to the equilibrium condition is affected by linkage irrespective of whether there is epistasis although the effect is greater when epistasis is present.

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