Propagule Interactions and the Evolution of Virulence

The evolution of parasite virulence is thought to involve a trade-off between parasite reproductive rate and the effect of increasing the number of propagules on host survivorship. Such a trade-off should lead to selection for an intermediate level of within-host reproduction (λ). Here I consider the effects of parasite propagule number on selection affecting λ when (i) the effect of each propagule is independent of propagule number, and (ii) when the effect of each propagule changes as a function of propagule number. Virulence evolves in these models as a correlated response to selection on l. If each propagule has the same effect (s) as all previous propagule, the survivorship of infected hosts is reduced by more than 60% at equilibrium, independent of the value of s. If, instead, each propagule has a more negative effect on host survivorship than previous propagules, host survivorship at equilibrium is expected to increase as the effect becomes more pronounced. These results are directly parallel to results derived for population mean fitness at mutation-selection balance; and they suggest that high virulence should be associated with parasites for which the effect of adding propagules either remains constant or diminishes with propagule number. 3 … if the deleterious effect of the mutant genes is proportional to the square of their number, the load under random mating becomes roughly half as large as in the case of no epistasis. Kimura & Maruyama (1966)