Demographic determinants of the phenotypic mother–offspring correlation

Phenotypic traits partly determine expected survival and reproduction, and so have been used as the basis for demographic models of population dynamics. Within a population, the distribution of phenotypic traits depends upon their transmission from parents to offspring, yet we still have a limited understanding of the factors shaping phenotypic transmission in wild populations. Phenotypic transmission can be measured using the phenotypic parent–offspring correlation (C), defined as the slope of the regression of offspring phenotypic trait on parental phenotypic trait, both traits measured at the same age, often at birth. This correlation reflects phenotypic variation due to both additive genetic effects and parental effects. Researchers seldom account for the possible influence of selection on estimates of the phenotypic parent–offspring correlation. However, because individuals must grow, survive, and reproduce before giving birth to offspring, these, aphic processes might influence the phenotypic parent–offspring correlation in addition to the inheritance process, the latter being the direct relationship between parental and offspring phenotypic traits when the parental trait is measured at age of reproduction and the offspring trait is measured at birth. Here we used a female-based population model to study the relative effects of fertility and viability selections, trait ontogeny and inheritance on C. The relative influence of each demographic process is estimated by deriving the exact formulas for the proportional changes in C to changes in the parameters of integral projection models structured by age and phenotypic traits. We illustrate our method for two long-lived species. We find that C can be strongly affected by both viability and fertility selections, mediated by growth and inheritance. Generally, demographic processes that result in mothers reproducing at similar phenotypic traits regardless of their birth traits, such as high fertility selection or converging developmental trajectories, lead to a decreased C. More generally, our models show how the age and phenotypic dependence of fertility and viability selections can influence phenotypic mother–offspring correlation to a much larger extent than ontogeny and inheritance. Our results suggest that accounting for such dependence is needed to model the distribution of offspring phenotypic traits and the ecoevolutionary dynamics of phenotypic traits reliably.