Coevolving parasites improve host evolutionary search on structured fitness landscapes

Evidence suggests that host-parasite coevolution can often result in host diversification. However, the host traits that coevolve often have primary functions affecting growth, creating the potential for conflicting selection pressures. For example, bacteriophage often infect bacteria by binding to nutrient uptake receptors, thus diversification of bacteria due to coevolution with phage may have an impact on resource competition. This paper uses a model of bacteria and phage in a chemostat to study the impact of coevolution with phage on the evolution of host growth rates, when infection and growth are affected by the same trait. Comparing (co)evolutionary outcomes on different growth rate fitness landscapes, with and without phage, shows that coevolutionary diversification allows hosts to cross fitness valleys and improve search efficiency on rugged landscapes, although it also prevents the whole community from reaching global optima. In effect, coevolution with parasites increases exploration but decreases exploitation in host evolutionary search.