This paper examines the effect of natural selection on the composition of a pop- ulation when the expected numbers of offspring of the different genotypes are decreasing functions of population size. It is shown that the eventual outcome of selection (whether there is a stable polymorphism or fixation of one or other allele) is determined by the genotypic K-values, but that the r-values influence the composition of equilibrium populations. With certain r-values there may be permanent oscillations in population size and gene frequency. A different model of selection, where selective differences are unrelated to differences in r and K in a regulated population, is also described and its relevance to the problem of genetic load is discussed.
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