When sexual selection meets genetic drift: the coevolution of male traits and female preferences in finite populations

Fishers mechanism is central to sexual selection theories, where mate choice generates a genetic correlation between male trait and female preference alleles, driving the coevolution of the trait and preference with positive feedback. However, how Fishers mechanism operates in finite populations remains unclear, as sexual selection can interact with genetic drift, influencing both trait-preference correlation and allele frequencies. By using population genetic models, this study addresses the gap in our understanding of interactions between fundamental evolutionary forces. We show that more frequent recombination increases trait-preference correlations in infinitely large populations, unless a positive correlation initially exists. In finite populations, interactions between sexual selection and drift elevate trait-preference correlation when the male trait is rare but reduce the correlation when the trait is common, potentially making it negative when recombination is rare or population size is small. Also, these interactions tend to slow the spread of the male trait while promoting the evolution of preferences. These results differ from the Hill-Robertson effect under natural selection due to two key factors: mate choice generates positive linkage disequilibrium, and the strength of indirect selection on preferences increases with linkage disequilibrium. The fixation of trait and preference alleles is positively correlated. This correlation peaks at intermediate recombination rates and is often stronger in small populations than in large ones, so large population sizes tend to reduce the likelihood that both trait and preference alleles fix. We discuss how the results provide insights into the progression of sexual selection in nature.