Partitioning variance in reproductive success, within years and across lifetimes

Abstract

Variance in reproductive success ([Formula], with k=number of offspring) plays a large role in determining the rate of genetic drift and the scope within which selection acts. Various frameworks have been proposed to parse factors that contribute to [Formula], but none has focused on age-specific values of [Formula], which indicate the degree to which reproductive skew is overdispersed (compared to the random Poisson expectation) among individuals of the same age and sex. Here, an ANOVA sums-of-squares framework is used to partition variance in annual and lifetime reproductive success into between-group and within-group components. For annual reproduction, the between-age effect depends on age-specific fecundity (bx), but relatively few empirical data are available on the within-age effect, which depends on{phi} x. By defining groups by age-at-death rather than age, the same ANOVA framework can be used to partition variance in lifetime reproductive success into between-group, within-group, and longevity components. Analyses of simulated data and worked examples for black bears and great tits illustrate the methods and show that the largely-neglected within-age effect a) typically represents a substantial component of the overall variance (even under a null model of random reproductive success), and b) can dominate the overall variance when{phi} x>1.