How to quantify (the response to) sexual selection on traits

Natural selection operates via fitness components like mating success, fecundity and longevity, which can be understood as intermediaries in the causal process linking traits to fitness. Sexual selection occurs when traits influence mating or fertilisation success, which, in turn, influences fitness. We show how to quantify both these steps in a single path analysis, leading to better estimates of the strength of sexual selection. Our model controls for confounding variables, such as body size or condition, when estimating the relationship between mating and reproductive success. Correspondingly, we define the Bateman gradient and the Jones index using partial rather than simple regressions, which better captures how they are commonly interpreted. The model can be applied both to purely phenotypic data and to quantitative genetic parameters estimated using information on relatedness. The phenotypic approach breaks down selection differentials into a sexually selected and a ‘remainder’ component. The quantitative genetic approach decomposes the estimated evolutionary response to selection analogously. We apply our method to analyse sexual selection in male dusky pipefish, Syngnathus floridae, and in two simulated datasets. We highlight conceptual and statistical limitations of previous path-based approaches, which can lead to substantial misestimation of sexual selection.