Population studies in predominately self-pollinated species, IX. Frequency-dependent selection in Phaseolus lunatus.

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Abstract
Relative fitness values associated with linkage block heterozygotes marked by the S/s locus were estimated in populations of lima beans in which the frequency of heterozygotes and/or degree of selection of the genetic background had been modified. The relative fitnesses of homozygotes and heterozygotes were independent of the generation from which they were derived, but there was a significant correlation between the frequency of heterozygotes and their selective value. The fitness of heterozygotes increased as their frequency in populations increased until, when they were at very low frequencies, heterozygotes produced about 3 times as many progeny as homozygotes. This frequency dependency acts to maintain a stable nontrivial equilibrium at the S/s locus and hence promotes the retention of segregational variability in these inbreeding populations.