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LINDSAY, S. L. 1982. Systematic relationship of parapatric tree squirrel species (Tamiasciurus) in the Pacific Northwest.Can. J. Zool. 60: 2149-2156. 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.LINDSAY, S. L. 1982. Systematic relationship of parapatric tree squirrel species (Tamiasciurus) in the Pacific Northwest.Can. J. Zool. 60: 2149-2156. L'analyse phknetique de 20 caractkristiques criniennes a permis d'kvaluer la classification systCmatique de deux Ccureuils arboricoles du Nord-Ouest amkricain, Tamiasciurus douglasii et T . hudsonicus. Bien que ces espkces soient surtout allopatrides, il existe trois zones de cohabitation le long d'une frontikre parapatride, en Colombie Britannique ainsi que dans les ktats de Washington et d'oregon. Deux populations d'kcureuils supposCes hybrides ont Ct C dkcrites et elles ont kt6 CvaluCes en fonction de leur relation avec des populations allopatrides des deux espkces dans toute la region. Une analyse des composantes principales, un groupement en fonction des coefficients de distances taxonomiques et une analyse discriminante ont mis en lumikre l'existence d'une distinction morphologique importante entre les deux espkces. Ces analyses ont permis d'attribuer les spkcimens "hybrides" a l'une ou l'autre espkce et n'ont fourni aucun indice d'intergradation. L'histoire des glaciations du PlCistockne dans le Nord-Ouest amkricain est responsable de la divergence entre les deux espkces et explique l'existence d'une frontiere parapatride. En outre, la convergence post-glaciaire dans la taille et la coloration du pelage dans les for& qui permettent la coexistence non compktitive explique les spkcimens app...