Understanding the mechanisms that constrain or promote the evolution of species’ geographic ranges has emerged as a fundamental question at the intersection of evolution, ecology, and conservation. A long-held, empirically understudied idea is that asymmetrical gene flow from a more densely populated range center may restrict species’ geographic ranges by preventing local adaptation and the expansion of populations at the range edge. In this study, we use ecophysiology, population genetics, and niche modeling to evaluate whether asymmetrical gene flow from the range center toward the range edge swamps out physiological differentiation in peripheral populations preventing local adaptation. Our focal species, the plethodontid salamander Plethodon ouachitae, is isolated among six mountaintops (sky islands) in the Interior Highlands of the United States that differ in climate across mountaintop isolates and along elevational gradients within mountains. Within mountaintop isolates we found no genetic structure, uniformly high rates of gene flow, and no evidence of physiological divergence along elevational gradients. Although population density peaked at mid- to high-elevations, inferred gene flow was not consistently biased from high- to low-density sites. We found significant genetic differentiation among mountains and evidence for divergence in the environmental niche across mountains separated by low-elevation barriers to dispersal. Despite variation in climate among mountains and the potential for adaptive divergence, we found no evidence for differentiation in ecophysiology among populations isolated on different mountains. Our findings do not support asymmetric gene flow as a force restricting local physiological adaptation and expansion at the range edge, although uniformly high estimated gene flow within mountains could play such a role. However, a lack of physiological differentiation among sky-island populations that differ in climate provides additional support for the idea that gene flow does not limit local adaptation in this species.