HOST POPULATION-STRUCTURE AND THE EVOLUTION OF VIRULENCE - A LAW-OF-DIMINISHING-RETURNS

Structure in a population of host individuals, whether spatial or temp oral, can have important effects on the transmission and evolutionary dynamics of its pathogens. One of these is to limit dispersal of patho gens and thus increase the amount of contact between a given pair or w ithin a small group of host individuals. We introduce a ''law of dimin ishing returns'' that predicts an evolutionary decline of pathogen vir ulence whenever there are on average more possibilities of pathogen tr ansmission between the same pair of hosts. Thus, the effect of repeate d contact between hosts will be to shift the balance of any trade-off between virulence and transmissibility toward lower virulence.