The rare-male effect: what is its evolutionary significance?

Negatively frequency-dependent male mating success, the rare-male effect (RME), has been reported from many laboratory experiments, particularly with Drosophila spp. Problems with observer bias, lack of repeatability, with experimental design and with the analysis of data may indicate that the RME is considerably less well documented than has been supposed, even in the laboratory. Male competition is unlikely to be a common cause of the RME, except where there are behavioural differences between competing strains that result in lower competition between them than within them. A mixture of fixed female preferences seems the most likely cause, and further behavioural studies are required to investigate this mechanism. There is no convincing evidence that the RME is a consequence of frequency-dependent female preferences. An RME in the absence of negative assortment is not in general expected to lead to the avoidance of inbreeding because matings between relatives will not be reduced. Nor is it likely to contribute to the high levels of genetic polymorphism found in nature, because females would be required to base their mating preferences on genotypes at all or most loci, to show individual variation in respect of their preferences and to sum the information into an index of genomic rarity. Given the levels of polymorphism involved, all males are likely to be rare by some criterion. A varying direction of female preference, required for a two-sided RME and for the maintenance of genetic polymorphism, has yet to be reported from wild populations. The RME is therefore probably of limited evolutionary significance. Disassortative mating with respect to self-incompatibility alleles in plants, and possibly major histocompatibility complex (MHC) alleles in vertebrates, results in an RME, inbreeding avoidance and high levels of genetic polymorphism at these loci.