Spermatozoa are the most morphologically diverse cell type, leading to the widespread assumption that they evolve rapidly. However, there is no direct evidence that sperm evolve faster than other male traits. Such a test requires comparing male traits that operate in the same selective environment, ideally produced from the same tissue, yet vary in function. Here, we examine rates of phenotypic evolution in sperm morphology using two insect groups where males produce fertile and non-fertile sperm types (Drosophila species from the obscura group and a subset of Lepidoptera species), where these constraints are solved. Moreover, in Drosophila we test the relationship between rates of sperm evolution and the link with the putative selective pressures of fertilization function and postcopulatory sexual selection exerted by female reproductive organs. We find repeated evolutionary patterns across these insect groups-lengths of fertile sperm evolve faster than non-fertile sperm. In Drosophila, fertile sperm length evolved faster than body size, but at the same rate as female reproductive organ length. We also compare rates of evolution of different sperm components, showing that head length evolves faster in fertile sperm while flagellum length evolves faster in non-fertile sperm. Our study provides direct evidence that sperm length evolves more rapidly in fertile sperm, probably because of their functional role in securing male fertility and in response to selection imposed by female reproductive organs.
Keywords: fertilization; postcopulatory sexual selection; sperm design; sperm heteromorphism.