Evolutionary implications of host–pathogen specificity: fitness consequences of pathogen virulence traits

Pathogens and parasites can be strong agents of selection and often exhibit genetic specificity for individual host strains. Here we use a simple host–pathogen model to explore the consequences of host–pathogen specificity for selection on pathogen infectiousness and lethality. Pathogens that can transmit themselves more readily from host to host should have a clear reproductive advantage, as should pathogens that are less lethal to their hosts (and thus can survive and reproduce longer). One would therefore expect that evolution should favour pathogens that are more infectious or less lethal, everything else being equal. These expectations are borne out when the genetic specificity of pathogen infection is low: selection favours pathogen strains with higher intrinsic fitness (reproduction rate times longevity in the host). However, at higher degrees of host–pathogen specificity, selection for these traits can be diminished, nullified or even reversed by host–pathogen feedback. This feedback produces stabilizing selection that can maintain polymorphism in pathogen traits, despite large intrinsic fitness differences among pathogen strains. When host–pathogen specificity is high enough that host–pathogen feedback regulates pathogen trait frequencies, greater infectiousness will be disadvantageous for pathogens, because it puts their preferred hosts at a competitive disadvantage with other host strains. Thus differences in specificity can reverse the direction of selection for pathogen virulence.