The evolution of plasticity in drought-related traits in central and edge populations of Mimulus cardinalis

Phenotypic plasticity can influence population persistence, range dynamics, and evolutionary potential in heterogeneous landscapes. If adaptive, plasticity may buffer populations against environmental variability, climate change, and extreme events. Yet, few studies have examined how plasticity and its evolution vary across species' ranges. This is especially important for edge populations, which are often most vulnerable to novel conditions. The Climate Variability Hypothesis postulates that greater environmental variability selects for genotypes that exhibit greater phenotypic plasticity. However, field tests of this hypothesis across species' ranges remain rare. Using scarlet monkeyflower (Mimulus cardinalis), we combined a resurrection approach with common gardens to test whether (1) central and edge populations differ in plasticity in drought-associated traits, (2) there were evolutionary shifts in plasticity following an extreme drought, and (3) plasticity was adaptive. We measured plasticity in first flower date, specific leaf area, and leaf dry matter content across two leading-edge, two range-center, and two trailing-edge populations, comparing pre-drought ancestors and post-drought descendants grown in three gardens spanning the latitudinal range following a historic drought event. We found significant plasticity in all traits, and plasticity in the date of first flower was greatest in trailing-edge populations that have experienced greater interannual variation in precipitation. Evolutionary changes in plasticity occurred in some populations but did not consistently increase first-year fitness. The adaptive value of plasticity was trait-specific. These results show that plasticity is shaped by both spatial and temporal environmental variation and highlight the need to examine trait- and population-level responses to understand selection on plasticity at range edges.