The evolution of thermal performance in native and invasive populations of Mimulus guttatus

Abstract

1. The rise of globalization has spread organisms beyond their natural range, allowing further opportunity for species to adapt to novel environments and potentially become invaders. Yet, the role of climatic niche evolution in promoting the success of invasive species remains poorly understood. Here, we use thermal performance curves (TPCs) to test hypotheses about thermal adaptation during the invasion process. First, if novel temperature regimes impose strong selection in the introduced range, invasive populations may evolve distinct TPCs relative to native populations. Second, invasive species may not exhibit specialist-generalist tradeoffs and instead may be "masters-of-all" with high maximum performance and broad TPCs. Third, with sufficient time, standing genetic variation, and temperature-mediated selection, TPCs of native and invasive populations may show parallel evolution in response to thermal gradients. 2. To test these hypotheses, we built TPCs for 18 native (United States) and 13 invasive (United Kingdom) populations of the yellow monkeyflower, Mimulus guttatus. We grew clones of multiple genotypes per population across six temperature regimes in growth chambers. 3. Invasive populations have not evolved different thermal optima or performance breadths, providing evidence for evolutionary stasis of thermal performance between the native and invasive ranges after over 200 years post-introduction. Further, both native and invasive populations experienced similar specialist-generalist tradeoffs whereby broad TPCS come at the cost of lower peak performance. Inconsistent with the idea that the degree of thermal specialization varies across spatial or temperature gradients, native and invasive populations did not exhibit adaptive clines in thermal performance breadth with latitude or temperature seasonality. However, thermal optimum increased with mean annual temperature in the native range, indicating some adaptive differentiation among native populations 4. Synthesis: These findings suggest that thermal niches were static during the invasion process, and that general-purpose genotypes, rather than rapid evolution in the introduced range, may promote invasion.