Conclusions
Our study on natural plant populations in the Rocky Mountains revealed significant genetic variation in circadian period on a relatively fine regional scope: populations separated by up to 100 km varied by up to 3 h, while maternal families within a population could vary by as much as 6 h – ranges of variation that are striking relative to the global range of natural variation reported among divergent accessions of the model system A. thaliana . Further, higher-elevation populations had shorter average circadian periods and narrower ranges of within-population variation than those sampled below 3000 m, suggesting that spatial environmental heterogeneity across elevations explains a proportion of the trait´s diversity. Overall, the observed magnitude of natural genetic variation in circadian period within the sampled area supports the idea that the trait might not be under constant selective pressure in the wild and that its diversity could be linked to adaptation to climatic factors rather than diel cycles of varying lengths (Hozer et al. 2020, Mah et al. 2020, Salmela and Weinig 2019). To determine what role the clock plays in local adaptation in variable environments, it would be important to simultaneously assess other fitness-related traits with common genetic bases, e.g., photoperiodic responses (Leinonen et al. 2020) and flowering time (Salmela et al. 2018).