The opportunity for selection: A slippery concept in ecology and evolution

Natural selection can only occur if individuals differ in fitness. For this reason, the variance in relative fitness has been equated with the ‘opportunity for selection’ ((Formula presented.)), which has a long, albeit somewhat controversial, history. In this paper we discuss the use/misuse of (Formula presented.) and related metrics in evolutionary ecology. The opportunity is only realised if some fraction of (Formula presented.) is caused by trait variation. Thus, (Formula presented.) does not imply that selection is occurring, as sometimes erroneously assumed, because all fitness variation could be independent of phenotype. The selection intensity on any given trait cannot exceed (Formula presented.), but this upper limit will never be reached because (a) stochastic factors always affect fitness, and (b) there might be multiple traits under selection. The expected magnitude of the stochastic component of (Formula presented.) is negatively correlated with mean fitness. Uncertainty in realised (Formula presented.) is also larger when mean fitness or population/sample size are low. Variation in (Formula presented.) across time or space thus can be dominated (or solely driven) by variation in the strength of demographic stochasticity. We illustrate these points using simulations and empirical data from a population study on great tits Parus major. Our analysis shows that the scope for fecundity selection in the great tits is substantially higher when using annual number of recruits as the fitness measure, rather than fledglings or eggs, even after adjusting for the dependence of (Formula presented.) on mean fitness. This suggests nonrandom survival of juveniles across families between life stages. Indeed, previous work on this population has shown that offspring recruitment is often nonrandom with respect to clutch size and laying date of parents, for example. We conclude that one cannot make direct inferences about selection based on fitness data alone. However, examining variation in (Formula presented.) (the opportunity for fecundity selection adjusted for mean fitness) across life stages, years or environments can offer clues as to when/where fecundity selection might be strongest, which can be useful for research planning and experimental design.