The Origin of Isogamous Sexual Differentiation

Abstract

It is a general biological rule that sexual reproduction invariably involves the fusion of gametes of two different types. Most evolutionary biologists presume that the differentiation of gametes into mating types started within a primordial population consisting entirely of undifferentiated gametes. However, none of the population genetical models for the evolution of mating types investigated thus far yields a satisfactory explanation for the stable establishment of differentiated gametes. It is the aim of this paper to extend these models by incorporating more realistic assumptions concerning the mating kinetics and the effects of inbreeding. A general class of models for the competition between mating types is developed and analyzed both in dynamical and game—theoretical terms. The results obtained in the general framework are used to investigate several specific models which take account of the inhomogeneous spatial structure of the gamete pool. Surprisingly, the refined models do not qualitatively affect the rather stringent conditions for the stable establishment of differentiated mating types. We have to conclude that a plausible scheme for the evolution of sexual differentiation is still missing.