An estimator of the Opportunity for Selection that is independent of mean fitness

Variation among individuals in number of offspring (fitness, k ) sets an upper limit to the evolutionary response to selection. This constraint is quantified by Crow’s Opportunity for Selection ( I ), which is the variance in relative fitness . Crow’s I has been widely used but remains controversial because it depends on mean offspring number in a sample . Here I used a generalized Wright-Fisher model that allows for unequal probabilities of producing offspring to evaluate behavior of Crow’s I and related indices under a wide range of sampling scenarios. Analytical and numerical results are congruent and show that rescaling the sample variance to its expected value at a fixed removes dependence of I on mean offspring number, but the result still depends on choice of . A new index is introduced, , which makes Î independent of sample without the need for variance rescaling. Δ I has a straightforward interpretation as the component of variance in relative fitness that exceeds that expected under a null model of random reproductive success. Δ I can be used to directly compare estimates of the Opportunity for Selection for samples from different studies, different sexes, and different life stages.