Within-generation bet hedging: a seductive explanation?

Bet hedging occurs when a single genotype shows a variety of phenotypes in the same environment, and each phenotype is successful only when the particular circumstances to which it is adapted occur. The time scale of between-generation bet hedging ensures that all individuals with a given phenotype suffer the same fate-circumstances such as drought exert homogenous pressure on all members of a population. Under within-generation bet hedging, however, individuals with the same phenotype are subject to heterogeneous selection pressure-predation, for example, will affect some individuals but not others. An important consequence of this difference is that conditions favoring the evolution of within-generation bet hedging are very restricted. While a single lineage may realize increased fitness via within-generation bet hedging, this fitness advantage varies inversely with population size and becomes vanishingly small at even modest population sizes. Although several reviews and analyses have highlighted the differences between these two types of bet hedging, confusion persists regarding their respective definitions and evolutionary justification. Bet hedging is a seductive explanation because most students of evolution are trained to focus on costs and benefits at the individual level, and tend to seek adaptive explanations for individual traits. Although this focus is often successful, it leads us astray in the case of within-generation bet hedging. Only by assessing the fitness effects of a trait in the context of whole populations can one accurately identify traits that can and cannot be favored by within-generation bet hedging.