LAUSR

PREMISE Changes in climate can impose selection on populations and may lead to rapid evolution. One such climatic stress is drought, which plant populations may respond to with escape (rapid growth and early flowering) or avoidance (slow growth and efficient water use). However, it is unclear if drought escape would be a viable strategy for populations that already flower early from prior selection. METHODS In an experimental evolution study, we subjected rapid-cycling Brassica rapa (RCBr), which was previously selected for early flowering, to four generations of experimental drought or watered conditions. We then grew ancestral and descendant populations concurrently under drought and watered conditions to assess evolution, plasticity, and adaptation. RESULTS RCBr evolved under drought had earlier flowering and lower water-use efficiency than RCBr evolved under watered conditions, indicating evolutionary divergence. The drought descendants also had a trend of earlier flowering compared to ancestors, indicating evolution. Evolution of earlier flowering under drought followed the direction of selection and increased fitness, and was consistent with studies in natural and experimental populations of this species, suggesting adaptive evolution. CONCLUSIONS We found evidence for rapid adaptive evolution of drought escape in RCBr and little evidence for constraints on flowering, even though RCBr already flowers extremely early. Our results suggest that some populations may harbor sufficient genetic variation for evolution even after strong selection has occurred. Our study also illustrates the utility of combining artificial selection, experimental evolution, and the resurrection approach to study the evolution of functional traits. This article is protected by copyright. All rights reserved.