Clinal colour variation within a panmictic population of tree squirrels, Tamiasciurus douglasii (Rodentia: Sciuridae), across an ecological gradient ANDREAS S. CHAVEZ* and G. J. KENAGY* Burke Museum and Department of Biology, University of Washington, Seattle, WA 98195, USA Received 11 March 2014; revised 20 May 2014; accepted for publication 20 May 2014 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 adapta- tion 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. ADDITIONAL KEYWORDS: cline analysis – coloration – divergent selection – gene flow – local adaptation – microsatellites – phenotypic variation – skull morphology. INTRODUCTION Patterns of phenotypic variation within continuously distributed populations suggest that local adapta- tion is common in nature: as exemplified by the ecogeographical ‘rules’ of Gloger (1833), Bergmann (1847), and Allen (1877). Such patterns of environmen- tally driven phenotypic variation can be associated with spatial differences in the selective environments experienced by continuously distributed populations (Endler, 1977; Martínez et al., 2014; Souto-Lima & Millien, 2014). Local adaptation is the consequence of local selection pressures that favour different traits in populations that occupy different environments. When selection is sufficiently strong, divergence can occur even when gene flow is strong enough to homogenize the populations at neutral loci (Kawecki & Ebert, 2004). Ecological selection plays an important role in adaptation and also potentially in speciation, either because divergent selection favours the prevalence of locally beneficial alleles or because closely linked loci contribute to reproductive incompatibilities that become fixed in genetically divergent populations (Maynard Smith & Haigh, 1974; Felsenstein, 1981; Barton, 2000; Nosil, Funk & Ortiz-Barrientos, 2009; Lenormand, 2012). Huxley (1938) coined the term ‘cline’ to describe geographical gradations of phenotypes within or between species, and this term can also apply to changes in allele frequencies (Slatkin, 1985). One of the most direct ways to measure the strength of selection in nature is to measure the rate of change in frequencies of genes or phenotypes across a cline *Corresponding author. E-mail: aschavez@uw.edu Biological Journal of the Linnean Society, 2014, 113, 536–546. With 2 figures © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 536–546 536 Downloaded from https://academic.oup.com/biolinnean/article/113/2/536/2415985 by guest on 09 July 2021