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

Abstract

The rise of globalization has spread organisms beyond their natural range, allowing further opportunity for introduced species to adapt to novel environments and potentially become aggressive invaders. Yet, the role of niche evolution in promoting the success of invasive species remains poorly understood. Here, we use thermal performance curves (TPCs) to test the following hypotheses about thermal adaptation during the invasion process. First, in response to strong selection from novel temperature regimes, populations in the invasive range should evolve distinct TPCs relative to native populations. Second, by exhibiting a broad TPC with high maximum performance, invasive species may overcome specialist-generalist tradeoffs, whereby tolerance across a wide range of temperatures comes at the cost of lower peak performance. Third, with sufficient time, standing genetic variation, and temperature-mediated selection, native and invasive populations may exhibit parallel adaptation to thermal gradients.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 a range of temperatures in growth chambers.We found that invasive populations have not evolved different thermal optima or thermal performance breadths, and there were similar specialist-generalist tradeoffs in both native and invasive populations. Consistent with the hypothesis that they are thermal generalists, native and invasive populations did not exhibit adaptive clines in thermal performance breadth with latitude or temperature seasonality. Thermal optimum increased with mean annual temperature in the native and invasive ranges. However, this relationship was primarily driven by populations in the native range, with weak adaptive differentiation of thermal optimum across mean annual temperature in the invasive range. Because thermal breadth of native and invasive populations did not differ and the invasive range exhibits a narrow range of thermal conditions compared to the native range, there may not have been strong selection for thermal specialization in the invasive range.Synthesis: These findings suggest that broad thermal tolerance, rather than rapid adaptation in the novel range, may promote invasion.

Historical museum samples enable the examination of divergent and parallel evolution during invasion

“…Our results indicate that when assessing native and invasive genetic differences, supposed evolutionary divergence should not be attributed solely to novel invasive range selection pressures or the processes involved in invasion itself. Many studies utilize genetic comparisons between invasive and native ranges to examine evolutionary divergence (Leger & Rice, 2007 ; Liu et al, 2020 ; Querns et al, 2020 ). Research into evolution within invasive systems often focuses on the divergent evolution within the invasive range in response to invasion processes or new selection regimes (Lee, 2002 ).…”