Complex sperm evolution

Abstract

A world without sexual selection would be a somber place, lacking the rich medley of stimulating sights, sounds, and smells that help individuals win the evolutionary prize of reproductive success. Sexual selection was Darwin's second great idea (1), and we recognize it as a potent and widespread evolutionary force that probably interacts across the whole genome (2). We also recognize that sexual selection penetrates far beyond mating: females of most species mate with multiple males, generating additional selection from sperm competition (3) and the option for cryptic female influences on fertilization (4). With this greater depth and breadth of importance for sexual selection, modern evolutionary biologists have been facing questions similar to those that confronted Darwin and discovering an exciting diversity in reproductive form and function at the intimate level of the gamete (5). In PNAS (6), a comparative study of closely related water beetles reveals this diversity through a remarkable complexity of adaptations in sperm form and function, and analyzes their coevolution with an equally remarkable elaboration in female reproductive tract architecture.