Evolutionary bet-hedging involves a tradeoff between the mean and variance of fitness, such that phenotypes with reduced mean btness may be at a selective advantage under certain conditions. The theory of bet-hedging was first formulated in the 1970~ and recent empirical studies suggest that the process may operate in a wide range of plant and animal species.

Some of the more interesting recent extensions of evolutionary theory concern exceptions to the rule that natural selection favours traits that maximize an individual’s expected number of surviving offspring. For example, an individual’s eventual inclusive fitness can sometimes be increased by tactics that entail the production of fewer than the maximum possible number of offspring, when the phenotypes in question concern sex allocation or interactions among kin. Reduced mean fitness can also evolve if the environment varies temporally, in which case phenotypes with low variances of fitness may be favoured over alternatives with higher variances and higher mean fitnesses. This trade-off between the mean and variance of fitness has been called ‘bet-hedging’ ever since Slatkin’wrote a commentary entitled ‘Hedging one’s evolutionary bets’, concerning a model by Gillespie* showing how selection could reduce the variance of offspring numbers. Real environ-ments always vary temporally, so it seems likely that many kinds of phenotype have attributes that serve at least in part to hedge bets. Here we briefly review the basic ideas and describe some recent empirical developments. When the fitness of a genotype varies over generations, the ap-