Quantifiable geographic variation in DNA or morphology is often used to gauge past and present levels of population interchange and has thus helped define taxonomic boundaries, resolve evolutionary histories, and develop effective conservation strategies to preserve evolutionary diversity. We examined rangewide patterns of genetic (mtDNA and microsatellites) and morphological diversity within the Lark Sparrow (Chondestes grammacus), focusing on diversity patterns across potentially independent Western, Central, and Eastern USA regional breeding zones. Phylogenetic analysis of mtDNA sequences did not support phylogeographic patterns associated with previously characterized subspecies, though the presence of regionally specific clades suggests incipient evolutionary diversification. Regional differentiation was evidenced by significant differences in morphological traits, significant levels of genetic differentiation, reduced estimates of migration among regions, and the characterization of 2 distinct populations through Bayesian clustering. Morphometric and genetic variation distinguished Western Lark Sparrow populations historically characterized as subspecies C. g. actitus from conspecifics across a secondary cline. By contrast, regional variation between Central and Eastern populations, encompassing subspecies C. g. strigatus and C. g. grammacus, was less pronounced and was consistent with a primary cline across the American Great Plains. Our results indicate that clinal variation among populations of long-distance migratory birds may reflect incipient evolutionary divergence, secondary contact zones, and local adaptation of populations to continuously variable environments.
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Vol. 131 • No. 3