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THE MANIFESTATION OF PHENOTYPIC SELECTION AT CONSTITUENT LOCI. I. STABILIZING SELECTION

There is general consensus that stabilizing selection on a quantitative trait will endow the governing loci with near neutrality (e.g., Lande, 1975; Kimura, 1981; Milkman, 1982). This is because stabilizing selection maintains the population near the optimal phenotype, and selection is weak when the population is near its optimum. However, if a population drifts away from its optimum, the magnitude of selection at individual loci will increase. Kimura (198 1) has addressed this problem with the sentence: "During this short period of directional selection, extensive shift of gene frequencies is expected to occur at many loci, but this process itself will seldom cause gene substitutions." In the present work we investigate how far the population mean drifts from the optimum under stabilizing selection and the extent of gene frequency shifts which may ensue from selection when the population mean is away from the optimum. Our conclusion is that perhaps Kimura overstated the importance of selection when the population mean deviates from the optimum. Rather, any significant selection at individual loci occurs when the population mean is at its optimum. We agree with the general consensus that selection at individual loci is often imperceptible. Assuming a particular form of stabilizing selection allows more specific analysis, for which purpose the following model is employed.