Migration, virulence, and the geographic mosaic of adaptation by parasites

The geographic mosaic theory of coevolution is predicated on structured populations of interacting species where gene flow and the force of selection can vary among populations, leading to a mosaic of traits in space. Here, I briefly review some recent studies of adaptation by a sterilizing parasite to structured populations of a freshwater snail. The results show geographic structure as expected under the geographic mosaic model. I then consider the effects of virulence and migration on local adaptation by parasites using a computer simulation. The results suggest that high virulence and low migration contribute to the strength of local adaptation by parasites. Highly virulent parasites showed adaptation to local hosts for migration rates of up to 10% of the population per generation. In addition, because of the dynamic nature of host-parasite coevolution, the magnitude of local adaptation fluctuates over time. During some points in the cycle, parasites may be no more effective at infecting individuals from local host populations, even though they would be shown to be locally adapted if examined over enough generations. Contrary to expectation, parasite local adaptation was not affected by giving the parasite a longer generation time than the host, but differences in local selection intensities had a dramatic effect.