Variability in fitness effects and the limitations of fitness optimization

Evolutionary biologists commonly assess the evolutionary advantage of an allele based on its effects on the lifetime survival and reproduction of individuals. However, alleles affecting traits like sex, evolvability, and cooperation can cause fitness effects that depend heavily on differences in the environmental, genetic, and social context of individuals carrying the allele. This variability makes it difficult to summarize the evolutionary fate of an allele based solely on its effects on any one individual. In this review we show how attempts to average over variability in the fitness effects of an allele can sometimes cause misleading results. We then describe a number of intriguing new evolutionary phenomena that have emerged in studies that explicitly model the fate of alleles that influence long-term lineage dynamics. We conclude with prospects for generalizations of population genetics theory and discuss how this theory might be applied to the evolution of infectious diseases.