Coevolutionary Epidemiology: Disease Spread, Local Adaptation, and Sex

How does evolution in parasite populations affect the rate of disease spread? In the present study, I derived the mean reproductive rate ([Formula: see text]) for a genetically diverse parasite population that is evolving with a similarly diverse host population. Assuming a matching-alleles model, I found that [Formula: see text] is a positive function of the covariance between the frequencies of "matching" host and parasite genotypes. Computer simulations further showed that evolution in the parasite population tends to increase the covariance, which can lead to epidemiological feedbacks. However, the covariances can also become negative during counteradaptation by the host, leading to oscillatory dynamics in host and parasite fitness. Nonetheless, when parasite-mediated selection is strong, the covariance is positive on average, which facilitates the spread of disease. Positive covariances may also underlie patterns of local adaptation in parasite populations and increase the selective advantage of cross-fertilization in host populations.