The evolution of anisogamy.

Anisogamy is the occurrence within a population of two gamete types of different size, a very common condition both in plants and in animals. This paper shows conditions that anisogamy without disassortative fusion (pseudoanisogamety) should be favoured by individual natural selection; the results obtained analytically below are in basic agreement with those obtained through the use of numerical techniques by Parker, Baker & Smith (1972). Major results are as follows. First, a necessary condition that gametes of intermediate size should be least fit is that zygote survival should increase more steeply than linearly with zygote size, over at least part of the range of zygote size. Second, stable genetic equilibria involving two alleles may be established, whether these alleles determine gamete size in the haploid or in the diploid phase. Third, if the difference in size between the two gamete types persisting at equilibrium is very great, the two types of gamete-producers will be nearly equally frequent at equilibrium. These results are interpreted to mean that frequency-dependent natural selection may maintain a genetic equilibrium involving two gamete types, provided that the frequencyindependent criterion that zygote survival should increase more steeply than linearly with zygote size is satisfied. The importance of zygote size in protists and in multicellular organisms is briefly discussed, but satisfactory quantitative data are lacking. The anisogamy generated in this way is always associated with sexual bipolarity, and an explanation is offered. These arguments lead to the prediction that increasing gamete dimorphism will be associated with increasing vegetative complexity, and a number of phyletic series among the algae, fungi and protozoa were reviewed with this in mind. The Volvocales provide an excellent example of the expected correlation, but other series are less satisfactory. On the whole, the comparative evidence is held to support the predictions of Parker et al., but exceptions to the rule are so numerous that a more detailed examination of the aberrant cases is very desirable.