Data from: Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

Rapid diversification of sexual traits is frequently attributed to sexual
selection, though explicit tests of this hypothesis remain limited.
Spermatozoa exhibit remarkable variability in size and shape, and studies
report a correlation between sperm morphology (sperm length and shape) and
sperm competition risk or female reproductive tract morphology. However,
whether postcopulatory processes (e.g. sperm competition and cryptic
female choice) influence the speed of evolutionary diversification in
sperm form is unknown. Using passerine birds, we quantified evolutionary
rates of sperm length divergence among lineages (i.e. species pairs) and
determined whether these rates varied with the level of sperm competition
(estimated as relative testes mass). We found that relative testes mass
was significantly and positively associated with more rapid phenotypic
divergence in sperm midpiece and flagellum lengths, as well as total sperm
length. In contrast, there was no association between relative testes mass
and rates of evolutionary divergence in sperm head size, and models
suggested that head length is evolutionarily constrained. Our results are
the first to show an association between the strength of sperm competition
and the speed of sperm evolution, and suggest that postcopulatory sexual
selection promotes rapid evolutionary diversification of sperm morphology.