This paper examines the effect of natural selection on the composition of a population 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.