Sperm as a speciation phenotype in promiscuous songbirds

Area of Science:

• Evolutionary biology
• Speciation research
• Animal reproductive strategies

Background:

• Sperm traits can influence speciation, but their role in prezygotic isolation is understudied.
• Female promiscuity's impact on sperm trait divergence during early speciation is largely unknown.

Purpose of the Study:

• To investigate if female promiscuity promotes reduced overlap in sperm length distributions in songbird populations.
• To test the hypothesis that female promiscuity influences early stages of the speciation process.

Main Methods:

• Assembled a dataset of 20 songbird population pairs with data on sperm length, divergence time, and female promiscuity (extrapair paternity/testis size).
• Analyzed the relationship between female promiscuity and the divergence rate of sperm length distributions.

Main Results:

• Sperm length distributions diverged more rapidly in populations with higher female promiscuity.
• Faster divergence was driven by reduced variance in sperm length, not faster divergence of the mean.
• Reduced variance is likely due to stronger stabilizing selection on sperm length under sperm competition.

Conclusions:

• Female promiscuity may promote prezygotic isolation by influencing sperm length variation.
• This mechanism could contribute to rapid speciation in songbirds, contingent on empirical validation of conspecific sperm precedence.
• Sperm competition, modulated by female mating behavior, appears critical in early speciation events.