Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

This is the peer-reviewed version of the following article: Rowe, M., Albrecht, T., Cramer, E. R., Johnsen, A., Laskemoen, T., Weir, J. T., & Lifjeld, J. T. (2015). Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology. Evolution, 69(4), 1044-1052, which has been published in final form at https://doi.org/10.1111/evo.12620. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.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.This work was supported by the Research Council of Norway grant 196554 (to JTL). In addition, JTW acknowledges a Discovery Grant from the National Sciences and Engineering Research Council of Canada, TA acknowledges a grant from the Czech Science Foundation (project P506/12/2472), and MR acknowledges a Young Research Talent grant from the Research Council of Norway (230434/F20)