If synergistic epistasis occurs, each mutation added to a genome has a greater deleterious effect than preceding mutations. Without this effect it is difficult to explain how small populations can survive in the face of genetic drift, or how larger populations can survive a high mutation rate. In the 27 July Nature Peck and Waxman use a mathematical model to deduce that competition in small groups does, indeed, lead to synergistic epistasis (Nature 2000, 406:399-404). This competition also produces a large advantage for sexual populations, allowing them to resist invasion by asexual lineages.