Population Genetics of Island Endemics: Neutral and Major Histocompatibility Loci

Island archipelagoes are ideal for the study of microevolutionary forces due to their multiple, closely related but geographically disjunct populations. I used both neutral and major histocompatibility complex (MHC) loci to determine the population genetic structures of bird species endemic to the Galapagos Islands. MHC molecules recognize foreign pathogens in the body, and these loci are known for their high degree of genetic variability maintained by natural selection. Small island populations are predicted to have reduced genetic variability due to the effects of genetic drift; however, selection may be strong enough to prevent the loss of variability at MHC loci. First, I characterized neutral genetic structure in the Galapagos hawk (Buteo galapagoensis). Analyses of both neutral nuclear VNTR (Chapter 1) and mitochondrial (Chapter 2) loci showed low within-population variability but high between-population differentiation. In Chapter 3, we found that smaller, more inbred populations had birds with higher louse loads and, in general, lower and less variable natural antibody titres than the larger, more genetically variable hawk populations. Chapter 4 presents MHC work done on the Galapagos penguin (Spheniscus mendiculus), a seabird whose breeding colonies experience population bottlenecks associated with El Niqo events. Galapagos penguins had low variability, having only three MHC alleles which differed by only a few base pairs. MHC work on the Galapagos hawks (Chapter 5) revealed similarly low variability. Galapagos hawks had fewer and less divergent alleles than the Swainson?s hawk, their closest mainland relative. The MHC diversity in both the Galapagos species was lower than in the mainland species, indicating that genetic drift has had an overwhelming effect. Lastly, in Chapter 6, I characterized the neutral population genetic structure of six Galapagos mockingbird (Mimus spp.) populations. Genetic variability increased with island area and we found a pattern of isolation by distance, both indicating the influence of genetic drift. Significant levels of genetic and morphological differentiation existed among all six populations, though morphological distances were smaller between islands of similar area suggesting the influence of natural selection