Both selection and drift drive the spatial pattern of adaptive genetic variation in a wild mammal

The major histocompatibility complex (MHC) has been intensively studied to
test for the relative effects of different evolutionary forces in recent
decades. Pathogen-mediated balancing selection is generally thought to
explain the high polymorphism observed in MHC genes, but it is still
unclear to what extent MHC diversity is shaped by selection relative to
neutral drift. In this study, we genotyped MHC class II DRB genes and 15
neutral microsatellite loci across 26 geographic populations of European
badgers (Meles meles) covering most of their geographic range. By
comparing the variation of microsatellites and the diversity of MHC at
different levels, we demonstrate that both balancing selection and drift
have shaped the evolution of MHC genes. When only MHC allelic identity was
investigated, the spatial pattern of MHC variation was similar to that of
microsatellites. By contrast, when functional aspects of the MHC diversity
(e.g. immunological supertypes) were considered, balancing selection
appears to decrease genetic structuring across populations. Our
comprehensive sampling and analytical approach enable us to conclude that
the likely mechanisms of selection are heterozygote advantage and/or
rare-allele advantage. This study is a clear demonstration of how both
balancing selection and genetic drift simultaneously affect the evolution
of MHC genes in a widely-distributed wild mammal.