Measuring and predicting the factors that determine adaptation and fitness patterns across species ranges is a vital step towards understanding how ranges may develop over time and how they may potentially respond to shifting climates. In particular, few studies have simultaneously determined how fitness varies among populations near warm and cold climate limits. Additionally, what constitutes or defines adaptation is an important, but complex consideration. We asked whether climate, spatial distance, or genetic distance (relatedness) better predicts fitness variation and adaptation across a species range, and also whether the adaptive picture changed depending on the definition of adaptation. We assessed lifetime fitness of an annual plant, cutleaf monkeyflower (Mimulus laciniatus) endemic to the California Sierra Nevada, among elevations at a warm-edge, cold-edge, and central garden among 23 populations from across the species range, including three populations local to each garden. We regressed lifetime fitness against climate, spatial, and genetic gradients at each garden and tested whether home versus away, local versus foreign, or sympatric (same climate edge) or allopatric (opposing climate edge) contexts of adaptation explained fitness variation.
Results/Conclusions
We found that climate environment of origin predicted fitness, but only at the cold-edge garden where fitness and fitness variance was highest among populations. Spatial or genetic distance was not predictive of fitness at any garden. A higher at-home fitness (versus away) advantage was only detected for the cold-limit garden population. The warm-limit home population performed better away at higher elevations. Local populations never performed best at their home garden; however, the top three performers in each range limit garden were populations originating from climates similar to those gardens. Cold-climate populations had superior fitness to warm-climate populations in the cold-limit garden, but not vice versa, driven in part by poor performance of the local warm-limit population. In general, evidence for fitness trade-offs between climate limits were unclear or weak. Genotype by environment interactions across the species range signal widespread adaptive genetic variation. Our findings indicate that adaptive genotypes are present at cold and warm climate limits, and that fitness variation leans uphill, consistent with a “leading-edge” range shift scenario, but challenge the notion that adaptive genetic variation is low at cold-climate limits. Our work underscores the importance of inter-population genetic variation maintaining climate niche breadth. Nevertheless, this genetic variation may become muted as climate change stress increases.