ABSTRACT: Geographic isolation promotes local adaptation but may also prevent the movement of advantageous gene complexes throughout a species distribution. We identify a geographic barrier that inhibits gene flow and adaptation across the disjunct species range of the Rocky Mountain endemic sunflower Helianthus pumilus. Habitat suitability modeling and least-cost path analysis were used to characterize environmental preferences across the species range and to identify barriers to gene flow. Simple sequence repeat data were used to assess genetic structure and estimate recent levels of gene flow across the distributional disjunction. A common garden was used to study phenotypic differentiation and local adaptation. Controlled crosses tested for intrinsic barriers to reproduction.Pivotal results. Despite significant differences in available habitat, individuals in northern and southern regions occupy similar sites, suggesting limited adaptive differentiation in environmental preference. A region of low-suitability habitat separates the two regions. Northern and southern plants are genetically distinct. There has been little recent gene flow. Southern plants had higher fitness (measured as seed set) in the northern region than northern plants. A suite of morphological characters, present in half of southern plants, conveyed a twofold fitness advantage in the north, but this adaptive phenotype is virtually nonexistent in the northern range. Between-region crosses were equal to or more productive than within-region crosses.Conclusions. The Pikes Peak massif, a high-altitude eastward projection of the Rocky Mountains, forms a geographic barrier to gene flow in H. pumilus, a lowland foothills species. No intrinsic barriers to gene exchange between regions have yet evolved. Therefore, the geographic barrier appears to impede the movement of the adaptive phenotype found in the southern region, resulting in a northern group that is less fit than it might be, contrary to the expectation of local adaptation.
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