The nature and stability of equilibria under optimizing selection.

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Abstract
Wright''s quadratic deviation model of optimizing selection was investigated to determine the joint effects of the position of the optimum, level of dominance, linkage, inbreeding, and heterozygote advantage on the nature and stability of gene frequency equilibria. Level of dominance on the primary scale governs the range of values of the optimum under which stable nontrivial equilibria involving 2 or more loci are possible. Restriction of recombination widens this range while inbreeding restricts it. Heterozygote advantage on the fitness scale, particularly in combination with tight linkage, leads to stable equilibrium under a very wide range of optimum values. This effect is relatively greater under inbreeding than under random mating. The values of the parameters of optimizing selection which permit stable nontrivial gene frequency equilibria often fall into ranges which are biologically reasonable. Optimizing selection therefore appears to be favorable for the long-time maintenance of genetic variability, even in undivided populations. As shown by Wright, prospects for long-term maintenance of genetic variability are expected to be even greater in populations subdivided into partial isolates.
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