Phylogeography And Population Genetic Structure Of Beringian Landbirds

Abstract

Molecular genetic approaches can be used to evaluate the historic and current relationships among populations. Mitochondrial DNA sequences and nuclear microsatellite loci were used to examine questions in avian community ecology, biogeography, and population genetics, including: (1) how have simple, high latitude bird communities been historically assembled; (2) how have past climate changes affected a species that has its entire distribution in an area that has experienced many glacial cycles; and (3) what are the genetic effects of sequential peripheral isolation in a natural vertebrate system? Landbird communities in the Aleutian Islands are simple and replicated, having only eight members. Traditional community assembly theory would describe this co-distribution as being due to nonrandom, likely contemporary ecological factors. However, I found that many species had unique colonization and persistence patterns. Results suggest that these communities were assembled randomly, and that simple ecological assembly rules could not adequately describe this complex process. Species endemic to Beringia, such as the rock sandpiper (Calidris ptilocnemis), would likely have been strongly affected by Pleistocene glacial cycles. MtDNA data suggest that these past climate changes have shaped current distribution and geographic variation in this species. Multiple instances of isolation and differentiation in glacial refugia and subsequent post-glacial population expansions are apparent. This study shows the complex biological responses of endemic Beringian species to climate change and isolation in glacial refugia. Populations that are increasingly isolated from a species' main distribution should provide a useful model for examining the genetic effects of sequential peripheral isolation. Song sparrows (Melospiza melodia) in Alaska have such a distribution, and the most distant populations have morphological and behavioral differences concordant with trends in peripheral or island populations in other species. I examined mtDNA sequences and nuclear microsatellite loci that evolve at different rates, and found that the combined processes of genetic drift, isolation, and divergent selection likely caused rapid morphological and behavioral changes in the most peripheral populations. Results of this study suggest that the examination of molecular markers that evolve at different rates can provide insight into the processes that lead to subspecies differentiation or the first steps in speciation.