Inequality of opportunity: measuring the potential for sexual selection

The opportunity for selection (I) is defined as the variance in relative fitness and sets the upper limit for the intensity of selection on any given trait. For Poisson or binomial fitness variables such as mating success or survival, I is mathematically dependent upon both mean fitness and sample size. Covariation with both mean mating success ((x) over bar) and the number of individuals competing for mates (n) is typical of most indices of inequality (i.e. indices of spatial dispersion, resource monopolization and reproductive skew) and confounds comparisons among populations. Several standardized indices of inequality have been developed that remove the confounding influence of (x) over bar and n. We assessed the utility of these standardized indices as measures of the potential for sexual selection on male mealworm beetles. Specifically, we compared the opportunity for sexual selection (I-s) with the index of resource monopolization (Q), Morisita's index (I-delta) and the standardized Morisita index (I-p). We estimated male mating success from repeated scan samples in two separate experiments. Experiment I comprised nine treatments with all combinations of 10, 20 or 30 males and females, replicated three times for a total of 27 populations. Experiment 2 consisted of five populations with 10 males and 10, 20, 25, 30 or 35 females respectively. As expected, we found I-s to be negatively correlated with mean mating success and positively correlated with both number of males and sex ratio. In contrast, the indices of inequality increased as the sex ratio moved away from 1:1 in either direction, and reached maximum values at strongly female-biased sex ratios. We hypothesize that this concave relationship reflects a shift in the relative importance of female choice and male-male competition at female-biased versus male-biased sex ratios. We compare the properties of all four indices and recommend Morisita's index as the best estimator of the potential for sexual selection.