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
.