1. The concept of the opportunity for selection (I), measured as the variance in relative fitness, is over 60 years old, yet remains poorly understood by, or even unknown to, many ecologists and evolutionary biologists. This essay aims to clarify key conceptual and practical issues concerning use and estimation of I, which represents a theoretical upper limit on the rate of evolutionary adaptation. 2. The component of I caused by linear selection on a single trait is equal to the square of its standardised selection differential i. Even if no phenotypic selection occurs (i=0), however, realised I will typically be nonzero, owing to environmental or stochastic variation in fitness. 3. The opportunity for viability selection depends only on the mean survival rate, but selective mortality will typically account for only a small fraction of total mortality. 4. Fecundity selection is accompanied by expected overdispersion in reproductive success (variance > mean), but overdispersion can also occur for reasons unrelated to phenotype, or by chance. The estimated opportunity for fecundity selection can also vary independently of i if estimated mean reproductive success is affected by study design, e.g. offspring are counted at different life stages or a variable fraction are missed. 5. For these reasons, I alone should not be used to make inferences about selection in progress and its drivers. Nevertheless, some empirical studies have documented weak to moderate correlations between I and phenotypic selection, so variation in I or related metrics across ecological contexts might provide clues that unmeasured traits are under variable selection. 20-Oct-2022 --

Data collected and collated by staff at the Netherlands Institute of Ecology over many years at the National Park de Hoge Veluwe.