The evolution of sexes.

It is very likely that sexual differentiation into two morphologically indistinguishable mating types has preceded the evolution of anisogamy. Therefore, the study of the evolution of mating types in an isogamous population is more informative for understanding the forces responsible for the evolution of different sexes than the study of the evolution of anisogamy; the latter represents the secondary problem of how, after the establishment of two sexes, an increasing degree of gamete dimorphism may evolve. Mating type evolution has been analyzed theoretically in population genetic models. These explorations show that mating types may evolve as a consequence of selection for more efficient gamete recognition, and also as a result of intragenomic conflict between nuclear and cytoplasmic DNA. However, in both cases the selection forces have to be very strong, which makes these possible explanations less convincing. Nearly all theories proposed for the evolution of anisogamy assume two conflicting selection forces to be relevant: selection for greater gamete productivity, and selection for greater zygote size. Although the explanation is intuitively plausible, the comparative evidence is a bit disappointing. Alternatively, anisogamy can be explained as a side-effect of selection for a greater efficiency in finding a mating partner by using sexual pheromones. Firm empirical evidence is lacking, however. In both problem areas--mating type evolution and anisogamy evolution--experimental work is badly needed.