Age-Specific Selection. II. The Interaction between r and K during Population Growth

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The interplay between density-independent (r) and density-dependent (K) components of selection is investigated using models of Mendelian populations with breeding at discrete intervals and overlapping generations. Selection within each genotype is age-specific, while the contribution of each genotype to population growth is logistic. The interaction between the r- and K-characteristics of the genotypes, in conjunction with the initial population structure, determines the patterns of population growth and gene frequency change. Moreover, these patterns may be strikingly different from the sigmoid curves obtained with simpler models. In a constant environment, K-characteristics alone determine the ultimate outcome of selection. Heterozygote advantage with respect to K is necessary for a balanced genetic polymorphism. The situation is different if the environment varies. We have investigated the effects of two kinds of environmental variation on the interaction between r- and K-selection. In the first of these, population size is reduced at the end of each environmental cycle. As the number of breeding intervals in each cycle is reduced, the r-characteristics become progressively more important and ultimately determine both equilibrium gene frequencies and population size. In the second, the genotypic K's are treated as seasonal variables. The relative amplitude of oscillations in K defines a genotype's ability to buffer the effects of environmental variation. In turn, the patterns of oscillation strongly influence changes in population size and gene frequency. Examples are presented to illustrate these points. Finally, the meaning of fitness under this model of age-specific, density-regulated selection is discussed.