The uncertain evolution of the sexes

James P. Randerson Laurence D. Hurst* Dept of Biology & Biochemistry, Bath University, Claverton Down, Bath, UK BA2 7AY. *e-mail: [email protected] There are many differences between males and females (e.g. ornaments, size and fighting ability), but only one is universal, namely the size difference between sperm and eggs. Assuming ISOGAMY (see Glossary) to be ancestral, we can ask: why did the gametes change in relative size? This question defines one of the oldest debates in evolutionary biology (see references in Refs 1,2), but has, for many3–5, been satisfactorily answered by the seminal work of Parker et al.6 and its subsequent developments7–13. But are the assumptions of the model reasonable? More importantly, do the assumptions and the predictions of the theory command empirical support? We argue that the empirical base for the established model is weak. It makes an unusual and untested assumption, and the limited support for its comparative predictions could have a simple alternative explanation14. Therefore, it is helpful to ask what other advantages there might be to small sperm and large eggs.