Kin selection and parasite evolution: Higher and lower virulence with hard and soft selection

Conventional models predict that low genetic relatedness among parasites that coinfect the sa,ne host leads to the evolution of high parasite virulence. Such models assume adaptive, responses to hard selection only. We show that if soft selection is allowed to operate, low relatedness leads instead to the evolution of low virulence. With both hard and soft selection, low relatedness increases the conflict among coinfecting parasites. Although parasites can only respond to hard selection by evolving higher virulence and overexploiting their host, they can respond to soft selection ip evolving other adaptations, such as interference, that prevent overexploitation. Be cause interference can entail a cost, the host may actually De underexploited, and virulence will decrease as a result of soft selection. Our analysis also shows that responses to soft selection can have a much stronger effect than, responses to hard selection. After hard selection has raised virulence to a level that is an evolutionarily stable strategy, the population, as expected, cannot De invaded by more virulent phenotypes that respond only to hard selection. The population remains susceptible to invasion by a less virulent phenotype that responds to soft selection, however.