ABSTRACT: Quaternary biogeography of Western North America: insights from MTDNA phylogeography of endemic vertebrates from Haida Gwaii

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Abstract

Population fragmentation and subsequent isolation in different refugia during the glacial advances of the Pleistocene are believed to have had a significant impact on current levels of genetic and morphological diversity. Our understanding of the distribution of glacial refugia on the northwestern coast of North America remains limited. As the most isolated group of islands in the Pacific Northwest, Haida Gwaii has been the subject of intense study. The ubiquitous presence of glacial features on this archipelago points to extensive ice cover during the late Wisconsin (Fraser glaciation). The large assemblage of unique mammalian and avian fauna found on Haida, Gwaii has led to suggestions that these divergent vertebrates actually evolved through long isolation by continuously inhabiting these islands or nearby regions throughout the last glacial maximum. To assess Haida Gwaii's role as a glacial refugium and the relictual status of its endemic black bear (Ursus americanus), marten (Martes amaericana), shorttailed weasel (Mustela erminea), caribou (Rangifer tarandus) and Saw-whet Owl (Aegolius acadicus), a broad phylogeographic study using sequence comparisons of the mitochondrial gene cytochrome b was undertaken. Phylogeographic structure was observed in the black bear (n = 33), marten (n = 18) and short-tailed weasel (n = 32). Based on parsimony, maximum likelihood, and neighbour-joining analyses of 719 bp of cytochrome b, two geographically structured black bear lineages were unambiguously identified: a continental lineage and a coastal lineage. Similarly, two geographically structured lineages, continental and coastal, were also identified in marten using the same types of analyses on 311 bp of cytochrome b. Phylogeographic structure was also observed in the short-tailed weasel using 148 to 673 bp of cytochrome b. Three major lineages were identified and named according to their putative refugial source areas: Beringia, a continental or southern source, and Haida Gwaii. Little or no phylogeographic structure was observed in the caribou and Saw-whet Owl. Of the 313 bp examined in two barren ground caribou (granti) and seven woodland caribou (four tarandus and three dawsoni), three tarandus and two dawsoni formed a lineage defined by one synapomorphy. Similarly, little phylogeographic structure was observed in the Saw-whet Owl. The phylogeographic patterns from these five species have two major implications with regard to the issue of glacial refugia and the relictual status of the Haida Gwaii endemics: (1) Close genetic affinity of these endemic subspecies with adjacent conspecifics suggest that population fragmentation caused by glaciers has had little effect on morphological differentiation; adaptation to local ecological environments has played a more influential role in their evolution. (2) Emerging data of a mid-Pleistocene split of many vertebrate taxa and the geographic distribution of these various genetic lineages, cumulatively suggests that a refugium existed on the continental shelf off the central coast of British Columbia. Given the broad assemblage of taxa which might have persisted here during the last glaciation, this refugium was probably ecologically productive and as such, was likely to have been an important alternate source area for the postglacial recolonization of northwestern North America.

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