c. Fluctuating selection
The parasite communities of stickleback differ significantly among the different sites on Vancouver Island (Bolnick et al., 2020; Stutz & Bolnick, 2017; Stutz, Schmerer, Coates, & Bolnick, 2015). This heterogeneity could select for different MHC genotypes at different sites. If it is true, we expect that the distributions of MHC genotypes and parasite species would be correlated across the stickleback metapopulation. The populations that have similar parasite communities should have similar MHC genotype compositions, and vice versa . To test this hypothesis, we first constructed a Euclidian distance matrix across all the sites for MHC alleles and for parasite species, respectively. Then we used Mantel test to examine if the two distance matrices were correlated. A previous analysis (Bolnick et al., 2019) found no significant effect of as-the-crow-flies (i.e., Euclidean distance) or as-the-fish-swims (i.e., the shortest in-water path) geographic distance on parasite community structure, so we omit that as a covariate in this distance analysis.