The evolutionary impact of population size, mutation rate and virulence on pathogen niche width.

Understanding the evolution of pathogen niche width is important for predicting disease spread and the probability that pathogens can emerge in novel hosts. Findings from previous theoretical studies often suggest that pathogens will evolve to be specialists in specific host environments. However, several of these studies make unrealistic assumptions regarding demographic stochasticity and the ability of pathogens to select their hosts. Here, an individual-based model was used to predict how population size, virulence and pathogen mutation rate affects the evolution niche specialism in pathogens. Pathogen specialism evolved regardless of virulence or populations size; thus, the findings of this study are somewhat consistent with those of previous work. However, because specialist pathogens had only a weak selective advantage over generalist pathogens, high mutation rates caused random trait variation to accumulate, preventing the evolution of specialism. Mutation rate varies greatly across different species and strains of pathogen. By showing that high mutation rates may prevent pathogen specialism evolving, this study highlights an intrinsic pathogen trait that may influence the evolution of pathogen niche width.