Defining fitness in an uncertain world

Résumé

Fitness maximisation is a theoretically controversial but empirically much used deduction about natural selection. In the presence of environmental and demographic uncertainty, current popula- tion models suggest that selection does not simply maximise mean fitness, but also reduces the variance. We show that by returning to the conceptual foundations of reproductive value, we can define fitness relative to the current state of a fluctuating structured population and, for the first time, recover a sim- ple fitness-maximisation result in a realistically general model. Fitness will depend on the reproductive output of the individual, the current environment, and the state of the structured population. We show that a parent is selected to maximise the probability-weighted arithmetic average over environmental and demographic uncertainty of the sum of (her genetic share of) the reproductive values of her offspring. This radically simplifies our conceptual understanding of natural selection under uncertainty.