Disease threatens many host populations. Hosts are expected to evolve resistance to infection in order to combat disease. The amount of resistance hosts evolve depends on epidemic size and the costs of resistance. Recent modeling work suggests hosts may evolve less resistance when epidemics are large, a counter-intuitive result lacking empirical support. We model this pattern of evolution and its implications for host populations. We test these model predictions with a mesocosm experiment. In addition, we conduct field surveys of natural, evolving populations to look for signal of such an eco-evolutionary pattern.
Results/Conclusions
Models show that hosts may evolve decreased resistance when epidemics are large because their fitness benefits more from maximizing fecundity while infected. While this strategy maximizes fitness, it harms the host population as a whole by decreasing density and increasing the proportion infected. Mesocosm experiments also show hosts evolving less resistance while epidemics are larger, leading to increased prevalence. The distribution of epidemic sizes in natural populations supports these results, showing a higher number of large epidemics than ecological modeling would predict. These large epidemics are consistent with our eco-evolutionary modeling in which moderately large epidemics drive evolution of decreased resistance, leading to larger epidemics. This work suggests that evolving host populations may have thresholds of disease above which eco-evolutionary feedbacks lead to very large epidemics.