Volume 79, Issue 1, 1 January 2025

Sperm competition often imposes strong positive selective pressures on reproductive traits and genes, but do these selective pressures correlate with the intensity of sperm competition across many species? We identified several sperm traits that correlated with proxies of sperm competition intensity, consistent with previous studies. Surprisingly, we also found that spermatogenesis genes tended to evolve more rapidly in smaller testes species rather than large testes species, due to relaxed purifying selection. Our work demonstrates that reduced sperm competition may drive more rapid evolution of spermatogenesis through relaxation of purifying selection, an underappreciated pattern in the molecular evolution of reproduction.

Extrapolations from classic models have left many in the sexual selection community with the impression that direct selection on mating preferences will always be stronger than indirect selection. A recent paper has challenged this view using quantitative genetic models, but direct and indirect selection cannot be compared in quantitative genetic models without making assumptions about the magnitude of genetic variances and covariances. Here, population genetic models, in which the magnitude of indirect selection emerges through the evolution of linkage disequilibrium as a variable in the model, are used to assess the claim that indirect selection can be stronger than direct selection.

To understand how new species are formed we need to know more about which traits cause reproductive isolation and how fast such traits evolve. Here we study a key reproductive trait, the sperm cell, and how fast sperm cell length diverges between populations of songbirds. We found that sperm length diverges faster in species with more promiscuous females. Sperm length is also under stronger selection at higher promiscuity levels, which reduces the variation in sperm lengths among males. Our study suggests that sperm length divergence can rapidly evolve, possibly leading to prezygotic reproductive isolation in promiscuous songbirds.