Short-term rates of parasite evolution predict the evolution of host diversity.

Coevolution with parasites has been implicated as an important factor driving the evolution of host diversity. Studies to date have focussed on gross effects of parasites: how host diversity differs in the presence vs. absence of parasites. But parasite-imposed selection is likely to show rapid variation through time. It is unclear whether short-term fluctuations in the strength of parasite-imposed selection tend to affect host diversity, because increases in host diversity are likely to be constrained by both the supply of genetic variation and ecological processes. We followed replicate populations of coevolving, initially isogenic, bacteria and phages through time, measuring host diversity (with respect to bacterial colony morphologies), host density and rates of parasite evolution. Both host density and time-lagged rates of parasite evolution were good independent predictors of the magnitude of bacterial within- and between-population diversities. Rapid parasite evolution and low host density decreased host within-population diversity, but increased between-population diversity. This study demonstrates that short-term changes in the rate of parasite evolution can predictably drive patterns of host diversity.