LEONARD, J.L.; Univ. of California-Santa Cruz: Sex and the Balanced Portfolio

The �cost of meiosis� problem is that for sex to be adaptive, sexually produced offspring must be, on average, either half as expensive to produce or twice as successful in terms of their future fitness. Applied to the problem of reproduction, Modern Portfolio Theory (MPT) suggests that an optimal allocation of investment should involve a) diversification of offspring to reduce covariance in their expected reproductive success; and b) balance of the portfolio to bias the investment toward the assets with the highest expected mean fitness. As with the Tangled Bank model, MPT predicts that there will be an advantage to producing a diverse progeny as insurance against temporal and/or spatial heterogeneity in the environment. A fair meiosis will accomplish this. However, MPT further suggests that investment in diverse progeny should be biased to those that have been most profitable in current and past environments. I suggest here that syngamy serves to balance the reproductive portfolio by tying the progeny�s fitness to those of individuals of proven current and past performance. Under random fertilization, there will be a benefit to an individual in that its haploid gametes will be more likely to fuse with those from parents with above average fitness than with those from parents of below average fitness. Any process that increases selectivity in gamete fusion, to identify those from parents of exceptional fitness, such as mate choice, will be favored. by natural selection. This model suggests that while sex may not be adaptive for the best genotype, it is adaptive for most genotypes under most circumstances, explaining its ubiquity. Further benefits such as the chance to purge deleterious alleles in haploid gametes and Muller�s ratchet will probably play a minor role.