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.