MAINTENANCE OF GENETIC-VARIABILITY UNDER STRONG STABILIZING SELECTION- A 2-LOCUS MODEL

We study a two locus model with additive contributions to the phenotyp e to explore the relationship between stabilizing selection and recomb ination. We show that if the double heterozygote has the optimum pheno type and the contributions of the loci to the trait are different, the n any symmetric stabilizing selection fitness function can maintain ge netic variability provided selection is sufficiently strong relative t o linkage. We present results of a detailed analysis of the quadratic fitness function which show that selection need not be extremely stron g relative to recombination for the polymorphic equilibria to be stabl e. At these polymorphic equilibria the mean value of the trait, in gen eral, is not equal to the optimum phenotype, there exists a large leve l of negative linkage disequilibrium which ''hides'' additive genetic variance, and different equilibria can be stable simultaneously. We an alyze dependence of different characteristics of these equilibria on t he location of optimum phenotype, on the difference in allelic effect, and on the strength of selection relative to recombination. Our overa ll result that stabilizing selection does not necessarily eliminate ge netic variability is compatible with some experimental results where t he lines subject to strong stabilizing selection did not have signific ant reductions in genetic variability.