Present distributions of flora and fauna in Alaska are
intimately tied to the past. That past includes the most
recent glacial advance that occurred during the Late
Pleistocene approximately 22,000-14,000 BP. Much of Alaska
remained ice free, creating a large refugium and a link with Asia
known as Beringia. Southern Alaska and much of Canada,
however, were overrun by massive ice sheets which acted as a
barrier isolating plants and animals either to the north or
south. Following the recession of ice sheets, these areas were
again colonized from Beringia, the southern refugia, and possible
northwest coastal refugia.
The Alexander Archipelago of southeast Alaska
derived its faunal composition from multiple sources of
colonization. Although there is controversy as to whether some
mammalian species (e.g., brown bears) persisted in the
archipelago during maximum glaciation, most of the archipelago
was probably ice covered. One can envision a tabula
rasa, or "clean slate," scenario with rapid postglacial colonization
of the archipelago by flora and fauna. The history of
colonization can be traced using molecular markers such as
mitochondrial and nuclear DNA sequences as powerful tools
for addressing questions of historical biogeography,
evolution, and conservation biology.
In this study, three mammals are being examined, including the old-growth-dependent northern flying squirrel (Glaucomys sabrinus), the masked shrew (Sorex cinereus), and the dusky shrew (S. monticolus). My results indicate similarities and differences between biogeographical histories for these species. All three species share similar distributions throughout most of Alaska, except in southeast Alaska. The fragmented and complex landscape of the Alexander Archipelago has differentially filtered species as they colo- nized the region postglacially.
Minimal-sequence variation is seen among populations of flying squirrels throughout Alaska. Even within this limited variation, populations from the Prince of Wales Island group are distinct (genetically and morphologically) from other mainland and island populations, suggesting limited gene flow across Clarence Strait.
Sequence variation among masked shrews in the archi- pelago is also minimal. Significant genetic and morphological differentiation in masked shrews occurs further north in Alaska. Masked shrews belong to the S. cinereus species complex which consists of 15 species, including 5 tundra forms found in Far East Russia, Alaska, and Canada. The geographic isolation of ancestral populations of S. cinereus during the Pleistocene has been postulated to have led to a major split in this complex. Populations isolated in Beringia gave rise to the morphologically distinct Panarctic tundra forms. The woodland form (S. cinereus) found in southeast Alaska subsequently colonized northward from a southern refugium. A postglacial contact zone between the tundra and woodland forms on the Seward Peninsula, Alaska, has been identified and is being characterized.
The dusky shrew has a more dynamic history in the archipelago than the flying squirrel or masked shrew. Molecular analysis has uncovered two previously undetected lineages that converge in southeast Alaska. One lineage inhabits most islands (excluding Baranof and Chichagof Islands) in the archipelago, as well as most of the mainland. The other lineage occurs only on the mainland of northern southeast Alaska (Haines, Yakutat). Since the dusky shrew belongs to a complex that includes 4 additional species dis- tributed from Mexico to Alaska, the origins and affinities of the southeast Alaska lineages have also been investigated. Molecular analyses of dusky shrews from Alaska, Arizona, British Columbia, Colorado, Montana, New Mexico, Utah, and Washington, and one of its sister species (S. pacificus), has uncovered "coastal" and "interior" lineages (Fig. 1). These lineages probably diverged due to separation (either side of the Cascade Range) south of the ice sheets during the most recent glacial period. Subsequent colonization of south- east Alaska by the dusky shrew, therefore, originated from two distinct sources.
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