Systematic relationship of parapatric tree squirrel species (Tamiasciurus) in the Pacific Northwest

Phenetic analysis of 20 cranial characters was used in an evaluation of the systematics of two Pacific Northwest tree squirrels, Tamiasciurus douglasii and T. hudsonicus. Although these species are predominantly allopatric, there exist at least three zones of sympatry along a parapatric border in British Columbia, Washington, and Oregon. Two populations of supposedly hybrid squirrels have been described, and these were evaluated in relation to allopatric populations of both species from throughout the area. Principal components analysis, clustering of taxonomic distance coefficients, and discriminant analysis showed a strong morphologic distinctness between the two species. These analyses also consistently placed "hybrid" specimens with one species or the other and gave no indication of intergradation. The Pleistocene glacial history of the Pacific Northwest is used to explain the divergence of the two species as well as the formation of the parapatric species border. In addition, postglacial character convergence in pelage coloration and squirrel size in forest areas allowing noncompetitive coexistence is used to explain the apparent hybrids and to account for the disjunct Vancouver Island distribution of T. douglasii-like T. hudsonicus. It is concluded that with the lack of hybridization and the presence of character convergence in zones of sympatry, reproductive isolation between T. douglasii and T. hudsonicus is complete.