Local adaptation, evolutionary potential and host–parasite coevolution: interactions between migration, mutation, population size and generation time

Host–parasite interactions constitute one of the best settings in which adaptation in spatially and temporally heterogeneous environments can be studied. Indeed, the effects of parasitism are likely to be highly variable in time and among different host populations. The recognition of such a geographical mosaic view of coevolution (Thompson, 1994, 1999) has stimulated several empirical, experimental and theoretical studies of species interactions. In spatially heterogeneous environments evolution may lead to the adaptation of populations to their local environmental conditions. Local adaptation occurs when the mean fitness of a population is higher on its own habitat than on a remote habitat (but see Gandon et al., 1998 for other definitions of local adaptation). When the mean quality of the habitat of different populations is identical across time one may use an averaged measure of local adaptation in the metapopulation (average over the different measures of local adaptation of each population). In this case, local adaptation is a property of the metapopulation as a whole and, assuming that adaptation has a genetic basis, it measures the adequacy between spatial variability of the environment and the distribution of adaptive genetic variation. The use of the above definition is particularly appropriate to study local adaptation in coevolving host– parasite systems because, as stated above, such systems are often characterized by important variations in space and time. In coevolutionary host–parasite systems each species constitutes an ever changing environment to which its opponent has to adapt. In such a variable environment adaptation depends on both the strength of selection and the evolutionary potential which is the ability to incorporate genotypes able to overcome the weaponry put forward by the opponent. The evolutionary potential thus depends on three processes: (1) mutation, (2) migration and (3) recombination. Moreover, what really matters is not just the rates but the