Clinal colour variation within a panmictic population of tree squirrels,Tamiasciurus douglasii (Rodentia: Sciuridae), across an ecological gradient

Local adaptation occurs when a population in a heterogeneous environment experiences divergent ecological selection but only if selection is stronger than the homogenizing effects of gene flow. The forest environments of Oregon vary along a physical and biotic gradient from a wet, closed‐canopy forest near the coast to a drier open‐canopy forest eastward across the Cascade Mountains. The present study explores patterns of local adaptation in Douglas squirrels (Tamiasciurus douglasii) in relation to these transitions in forest structure and ecology. We test for the presence of morphological clines in relation to gene flow and, more specifically, whether any such character clines correspond with environmental clines. We sampled animals at six locations (10 specimens each) and evaluated environmental parameters across a 240‐km west‐to‐east transect. Population structure analysis of 18 microsatellite loci indicates a single, panmictic squirrel population across the entire transect. Coalescent‐based estimates show bidirectional gene flow at similar west–east intensities between squirrels in coastal and interior forests. Of the four skull traits examined, none shows a significant clinal transition. By contrast, ventral fur colour shows a strong clinal transition, from deep‐orange in coastal forest to whitish–yellow in the interior forest. This pattern of phenotypic divergence coincides with the gradient in tree‐canopy cover. Ventral fur colour of T. douglasii exemplifies a gradation of continuous phenotypic variation maintained despite ongoing gene flow in a panmictic population. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 536–546.