Data from: Sperm as a speciation phenotype in promiscuous songbirds

The paper compares 20 population pairs of Passerida songbirds and shows
that their rate of divergence in sperm length was positively associated
with the level of female promiscuity (expressed by the proportion of
offspring sired by extrapair males). The effect was largely caused by a
reduced population variance in sperm length in more promiscuous species.
The paper suggests that rapid sperm length divergence can promote
postcopulatory prezygotic isolation and hence rapid speciation in
promiscuous songbirds.