Abstract
The structure of parasite populations can have important consequences for virulence evolution. Both the scale of competition, whether it is more intense in the within versus between host environment, and the mode of transmission, whether parasites transmit alone or in groups between hosts, influence population structure and can select for changes in virulence.In a selection experiment, we evolved metapopulations of the spider mite parasite Tetranychus urticae, under local (fixed sampling per host patch) or global (sampling determined by host patch productivity) competition, with either co- (with a female parasite from the same host patch) or random (with parasites from different host patches) transmission. After 33 generations of evolution, the fecundity and virulence of each replicate population was measured.Females from the ‘local-random’ regime were more virulent with higher fecundity than females from the ‘global-random’ regime. There was no difference in virulence or fecundity between females from the ‘global-random’ or ‘global co-transmission’ selection regimes.These results show that the scale of competition, and not the type of transmission, impacted virulence evolution for T. urticae. Local competition may maintain higher levels of genetic diversity at the level of the metapopulation. As such, intense local, within-host, competition selected for more productive individuals better able to exploit their environment and be the one to transmit to a new host.