Mechanism explaining why heterogeneities in host metapopulations cause the evolution of elevated pathogen virulence

Figure 3: Heterogeneities in local environments create heterogeneity in the densities of local host populations. The left population is “resource-rich” for pathogens, as it offers a higher density of susceptible hosts, while the right population is “resource-scarce”, as it offers a lower density of susceptible hosts. For higher susceptible-host densities, more virulent pathogens are favored, while for lower susceptible-densities, milder pathogens are favored. Crucially, the net effect of these selection pressures across the metapopulation is not balanced, as the resource-rich population makes a more significant evolutionary contribution than the resource-scarce population, causing evolution to drive up pathogen virulence and infectiousness.