Competition and the origins of novelty: experimental evolution of niche-width expansion in a virus.

Competition for resources has long been viewed as a key agent of divergent selection. Theory holds that populations facing severe intraspecific competition will tend to use a wider range of resources, possibly even using entirely novel resources that are less in demand. Yet, there have been few experimental tests of these ideas. Using the bacterial virus (bacteriophage) 6 as a model system, we examined whether competition for host resources promotes the evolution of novel resource use. In the laboratory, 6 exhibits a narrow host range but readily produces mutants capable of infecting novel bacterial hosts. Here, we show that when 6 populations were subjected to intense intraspecific competition for their standard laboratory host, they rapidly evolved new generalist morphs that infect novel hosts. Our results therefore suggest that competition for host resources may drive the evolution of host range expansion in viruses. More generally, our findings demonstrate that intraspecific resource competition can indeed promote the evolution of novel resource-use phenotypes.