Digest: Stabilizing selection drives sperm length divergence in promiscuous passerines

Divergence in gametic traits can play a key role in reproductive isolation. Lifjeld et al. (2025) examined the evolution of sperm length in pairs of songbird populations at various stages along the speciation continuum. Their analyses demonstrated that sperm length diverges more rapidly in species with higher levels of female promiscuity, likely due to stabilizing selection favoring sperm cells that fit within female sperm storage structures. This divergence in sperm length may kickstart speciation in promiscuous songbirds.