Replicate altitudinal clines reveal that evolutionary flexibility underlies adaptation to drought stress in annual Mimulus guttatus.

Examining how morphology, life history and physiology vary along environmental clines can reveal functional insight into adaptations to climate and thus inform predictions about evolutionary responses to global change. Widespread species occurring over latitudinal and altitudinal gradients in seasonal water availability are excellent systems for investigating multivariate adaptation to drought stress. Under common garden conditions, we characterized variation in 27 traits for 52 annual populations of Mimulus guttatus sampled from 10 altitudinal transects. We also assessed variation in the critical photoperiod for flowering and surveyed neutral genetic markers to control for demography when analyzing clinal patterns. Many drought escape (e.g. flowering time) and drought avoidance (e.g. specific leaf area, succulence) traits exhibited geographic or climatic clines, which often remained significant after accounting for population structure. Critical photoperiod and flowering time in glasshouse conditions followed distinct clinal patterns, indicating different aspects of seasonal phenology confer adaptation to unique agents of selection. Although escape and avoidance traits were negatively correlated range-wide, populations from sites with short growing seasons produced both early flowering and dehydration avoidance phenotypes. Our results highlight how abundant genetic variation in the component traits that build multivariate adaptations to drought stress provides flexibility for intraspecific adaptation to diverse climates.