Natural Killer Cells: Remembrances of Things Past

therefore, sperm size appears to have evolved as a secondary adaptation to sperm competition that is independent of the initial increase in sperm velocity. The reasons for this macroevolutionary pattern remain unclear. One possibility is that an increment in speed may initially arise by morphological rearrangements of the sperm cell leading to higher thrust and reduced drag, for example through relative increase in the size of the midpiece, the mitochondrial engine of the sperm, and an optimization of the midpiece:flagellum ratio. These initial changes may have then set the scene for further selection on overall sperm length. Unfortunately, these sperm traits are difficult to measure in cichlid sperm and Fitzpatrick et al. [9] were unable to test these ideas. It is likely that resolving the mechanisms underpinning such evolutionary responses would require more information on the evolution of sperm morphology (for example, midpiece mass, flagellum length), and on the life-history trade-offs associated with sperm investment that male cichlids must have faced in their evolutionary past. But while much still needs to be done to understand the evolutionary operation of sperm competition, this study represents a promising step ahead. Darwin did not intuit sperm competition, so in a way this field represents a particular acid test of his evolutionary theory. He would have taken studies such as that by Fitzpatrick et al. [9] as a reassuring bicentenary present, confirming that his evolutionary theory is proving just as successful in explaining patterns of variation in sperm and ejaculate traits as it has been in explaining the very biological phenomena that inspired it.