Abiotic and biotic environments vary across the landscape. Over evolutionary time, divergent selection regimes can favor the evolution of local adaptation, wherein ecotypes have elevated fitness in their home environment and depressed fitness in contrasting environments. Human activities are simultaneously modifying multiple abiotic and biotic agents of selection, likely leading to growing discrepancies between current and optimal phenotypes. We hypothesize that novel selection imposed by climate change shifts fitness landscapes, disrupting local adaptation. As a consequence, local populations could experience diminished growth rates.
Climatic variation across mountains affords the opportunity to test hypotheses about the evolution local adaptation to continuous environmental variation. We predict that in future climates, local ecotypes will have reduced fitness in their home sites relative to low elevation families in that same site, whose modern climates are similar to projections for higher elevations. We test this prediction in the mustard Boechera stricta, which is native to the Rocky Mountains and inhabits elevations as low as 1500m and as high as 3500m. In this region, warming winter temperatures reduce snowpack and warming spring temperatures cause the remaining snow to melt early. We simulated these climate change dynamics via snow removal experiments in five common gardens (elevations: 2530m, 2710m, 2890m, 3133m, 3340m) near the Rocky Mountain Biological Lab including transplants of 28,490 seeds and 17,832 juveniles (N= 270 families from 68 populations).
We have found clear evidence for adaptation to local environments over short geographic distances in Boechera stricta. We have also found consistent genetic clines in heritable life history traits: high elevation families flower early, at a small size, and for a short duration, likely representing an evolutionary response to short growing seasons. Foliar herbivory increases with provenance elevation in all gardens, revealing limited herbivore resistance of high elevation families. Short cool summers could limit herbivore numbers and reduce selection for defense at high elevations. The rapid and favorable responses of herbivores to climate change could disproportionately diminish the future fitness of high elevation plants.
Poor performance of high elevation families at lower sites foreshadows future maladaptation as temperatures warm. Across sites in 2014, fecundity declined by 15% for every 100m difference between source and garden elevations (p<0.0001) and snow removal depressed fecundity by 10% relative to ambient snowmelt (p=0.0026), as expected. Furthermore, lower elevation families appear to outperform local families even under control conditions, suggesting that climate warming may have already disrupted local adaptation.