The Role of Life Cycle and Migration in Selection for Offspring Variance

Abstract

For two genotypes that have the same mean number of offspring but differ in the variance in offspring number, natural selection will favor the genotype with lower variance. The concept of fitness becomes cloudy under these conditions because the outcome of evolution is not deterministic. However, the effect of variance in offspring number on the fixation probability of mutant strategies has been calculated under several scenarios with the general conclusion that variance in offspring number reduces fitness but only in proportion to the inverse of the population size (Gillespie 1974, Proulx 2000). This relationship becomes more complicated under a metapopulation scenario where the "effective" population size depends on migration rate, population structure, and life cycle. We show that under hard selection and weak migration fitness in a metapopulation composed of equal sized demes is determined by deme size. Conversely, for high migration rates and hard selection the effective fitness depends on the total size of the metapopulation. Interestingly, under soft selection there is no effect of migration or neighboring population structure on effective fitness, and fitness depends only on deme size. We use individual based simulations in developed in Shpak (2005) to validate our analytical approximations and investigate deviations of our assumption of equal deme size.