The fate of standing variation and new mutation under climate change

Many species face existence threat under anthropogenic climate change, and standing genetic variation was proposed as a way for sessile species to adapt to novel environments. However, it is still unknown whether standing genetic variants, being adaptive to current environmental variability, are sufficient to guarantee future survival. Here we investigate the relative importance of standing variation versus new mutations from the past to infer their future fate in nature. In the wild banana species Musa itinerans where the Taiwanese populations were ancient immigrants from the Chinese populations, new mutations exert larger effect size in precipitation-related variables, where Taiwan contains extreme environments beyond the ancestral climatic range, and new alleles have stronger association with novel environments. For temperature-related variables where Taiwan is within the ancestral climatic range, standing variants are more important than new mutations. The effect sizes of adaptive variants therefore differ under distinct environmental pressures, supporting theoretical predictions that natural selection favors new mutations with larger effect sizes in novel environments where the population is far from the adaptive peak. Despite their importance, large-effect variants also have higher mismatch and may be more vulnerable to future environmental perturbation, leaving minor-effect variants the main source of adaptive response to rapid anthropogenic climate change. Our work provides a support in natural environment to the previous conclusions from theoretical modeling and microbial experiments in well-controlled lab conditions.