The Leading Edge Matters Too: Fitness and the Expression of Adaptive Differentiation Are Greatest at the High-Elevation Edge of a Species' Range.

Understanding adaptation at species' climate limits is key for predicting evolutionary and ecological responses to climate change. In montane environments, warming may drive species distribution shifts, yet the adaptive potential of populations at leading and rear edges remains unclear. Few studies have simultaneously tested both fitness and local adaptation across elevational range limits. We conducted common garden experiments across the range of Erythranthe laciniata, an annual plant endemic to the Sierra Nevada, to test the disequilibrium hypothesis (higher fitness at leading edges, signalling range shifts) and the rear-leading edge hypothesis (lower adaptive differentiation at the leading edges due to founder effects and limited genetic variation). Fitness was highest at the high-elevation garden, supporting potential range expansion and revealing strong high-elevation climate adaptation, challenging assumptions of low adaptive potential at leading edges. This study provides a rare empirical test of both hypotheses and highlights the conservation importance of high-elevation edge populations.