Competitive extinctions between asexual and sexual populations: simulations define criteria for sexual survival

Abstract

ABSTRACT To investigate the reasons for the rarity of asexuality, despite the twofold advantage in growth rate of the latter, computer-simulated populations of asexual and sexual individuals of defined genetic composition were allowed to compete at a fixed carrying capacity. Variables included population size, mutation rate, selection coefficient, dominance coefficient, and percentages of mutations which were advantageous. Critical was to allow the two groups to evolve separately for a variable number of generations before being mixed in a single competing population, when one or other population would inevitably go extinct. Multiple reiterations under standard sets of conditions enabled accurate measures of extinction rates, loci carrying mutant alleles, fixed mutant alleles, and fitness values. For all of the above variables, values could readily be found which enabled the sexual population to survive in more than 50% of trials. The diploid state enables dominance to assist in the survival of sexuality. A general implication is that provided the two groups are not constrained geographically, or that niches are available which delay or avoid mixing, then the asexual competitors, under realistic values for mutation rate, population size and other parameters, will not drive the sexual population to extinction.