Genetic variation and prevalence of blood parasites do not correlate amongbird species

Models of host-parasite co-evolution suggest that parasites can exert frequ ency-dependent selection on their hosts, favouring rare alleles that confer resistance against widespread parasites and thus contributing to the maint enance of genetic variation, at some loci at least. If parasites are import ant in maintaining variation at many loci, then host species incurring a hi gh prevalence of parasite infections should exhibit greater levels of genet ic variation than host species incurring a lower prevalence. Using data fro m electrophoretic studies and from field surveys of haematozoan infections, we constructed a dataset including 103 species of North American and Europ ean birds to test this prediction. After controlling for sampling effort an d phylogenetic influences, we found no relationship between parasite preval ence and either heterozygosity or polymorphism. These results do not suppor t a role for parasites in the overall maintenance of genetic variation via frequency-dependent selection.