Conspecific Sperm and Pollen Precedence and Speciation

The evolution of reproductive isolation is perhaps the most significant stage in the process of species formation, and the study of reproductive barriers currently dominates investigations of speciation. The discovery that conspecific sperm and pollen precedence play an important role in the reproductive isolation of some closely related animals and plants is one of the real surprises to emerge from this field in recent years. This review begins with a brief history of the study of reproductive isolation with the aim of understanding why conspecific sperm and pollen precedence were generally overlooked in early work on reproductive barriers. It then examines: case studies, the prevalence of conspecific sperm and pollen precedence, the isolating potential of this class of reproductive barriers, the mechanisms that account for the operation of these barriers, and potential explanations for the rapid divergence of populations in traits related to fertilization. Conspecific sperm and pollen precedence appear to be quite effective in limiting gene exchange; these barriers are widespread although not universal in animals and plants, and they operate through a number of different mechanisms. Much more work remains to be done on a number of fronts to elucidate the processes responsible for the evolution of these reproductive barriers.