Evolution of pathogen virulence: the role of variation in host phenotype

Selection on pathogens tends to favour the evolution of growth and reproduc tive rates and a concomitant level of virulence (damage done to the host) t hat maximizes pathogen fitness. Yet, because hosts often pose varying selec tive environments to pathogens, one level of virulence may not be appropria te for all host types. Indeed, if a level of virulence confers high fitness to the pathogen in one host phenotype but low fitness in another host phen otype, alternative virulence strategies may be maintained in the pathogen p opulation. Such strategies can occur either as polymorphism, where differen t strains of pathogen evolve specialized virulence strategies in different host phenotypes or as polyphenism, where pathogens facultatively express al ternative virulence strategies depending on host phenotype. Polymorphism po tentially leads to specialist pathogens capable of infecting a limited rang e of host phenotypes, whereas polyphenism potentially leads to generalist p athogens capable of infecting a wider range of hosts. Evaluating how variat ion among hosts affects virulence evolution can provide insight into pathog en diversity and is critical in determining how host-pathogen interactions affect the phenotypic evolution of both hosts and pathogens.