How phenotypic variation and life history trait correlation enhance mean fitness in prey populations

The relative effects of phenotypic variation and trait correlations on mean population fitness are poorly understood. Moreover, when fitness is evaluated in the context of ecological processes such as predation, theoretical development is lacking. To address this deficiency, we simulate a predator-prey system, where the prey population is composed of three phenotypes that differ in fecundity and predator evasiveness. We use equilibrium population size and allele frequencies to examine the relative effects of trait variation and correlation on mean population fitness. When no correlation is present, increasing variance in either trait increases equilibrium population size. Negative correlations coupled with increasing variance in both traits increases equilibrium population size. Positive correlations are more likely to increase equilibrium population size when variances are inversely related. With respect to allele frequencies, as variance in both traits increases, populations with positive correlations experience greater loss of genetic diversity than those with negative correlations. Thus, selection as a dynamic ecological process (frequency-dependent predation) favors trade-offs, whether they are between trait means (negative correlations) or variances (positive correlations).