Mutation and sex in a competitive world

How do deleterious mutations interact to affect fitness? The answer to this question has substantial implications for a variety of important problems in population biology, including the evolution of sex(1-3), the rate of ada ptation(4,5) and the conservation of small populations(3,6-8). Here we anal yse a mathematical model of competition for food in which deleterious mutat ions affect competitive ability. We show that, if individuals usually compe te in small groups, then competition can easily lead to a type of genetic i nteraction known as synergistic epistasis. This means that a deleterious mu tation is most damaging in a genome that already has many other deleterious mutations. We also show that competition in small groups can produce a lar ge advantage for sexual populations, both in mean fitness and in ability to resist invasion by asexual lineages. One implication of our findings is th at experimental efforts to demonstrate synergistic epistasis may not succee d unless the experiments are redesigned to make them much more naturalistic .