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 thermal 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, we tested the hypothesis that if species largely conserve their thermal niche in the introduced range, invasive populations may not evolve distinct TPCs relative to native populations, against the alternative hypothesis that thermal niche and therefore TPC evolution has occurred in the invasive range. Second, we tested the hypothesis that clines of TPC parameters are shallower or absent in the invasive range, against the alternative hypothesis that with sufficient time, standing genetic variation, and temperature-mediated selection, invasive populations would re-establish clines found in the native range in response to temperature 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 at six temperature regimes in growth chambers. We found that 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. Thermal optimum increased with mean annual temperature in the native range, indicating some adaptive differentiation among native populations which was absent in the invasive range. Further, native and invasive populations did not exhibit adaptive clines in thermal performance breadth with latitude or temperature seasonality. These findings suggest that thermal performance curves remained unaltered post-invasion, and that invasion may proceed via broad thermal tolerance and establishment in already climatically suitable areas rather than rapid evolution upon introduction.

All datafiles and scripts associated with the final manuscript are available in the GitHub repo: https://github.com/akquerns/Evolution-Letters-2022-Guttatus-TPC

Mimulus guttatus raw thermal performance data

Rawdata-withrangecol.csv: This file contains data on primary stem and secondary branch growth of individual plant clones from 18 native (United States) and 13 invasive (United Kingdom) populations of the yellow monkeyflower, Mimulus guttatus, across six temperature regimes. Following a two week period in which clones of various genotypes were acclimated in growth chambers set to 20°C day/15°C night, clones were subject to a week long treatment of one of six day/night temperature regimes (°C): 10/0, 20/10, 25/15, 30/20, 40/30, and 45/35. Temperature regimes followed a pattern of 16-hour days and 8-hour nights. See Querns et al. 2022 publication in Evolution Letters, for further details.

Rawdata-withrangecol_METADATA.csv: This file contains descriptions of column names in Rawdata-withrangecol.csv.

***NOTE: The column “Population_NEW” does not appear in "Rawdata-withrangecol" in the Git Repo. Subsequent scripts in Git Repo add new population codes to data following creation of TPC curves.

Mimulus guttatus population locations and bioclimatic variables

guttatus_worldclim_seed_collections.csv: This file contains bioclimatic variables (obtained from WorldClim v.2; ~1-km resolution, 1970-2000; Fick & Hijmans, 2017), latitude, and longitude of all seed collections. These data were used to run a PCA for selection of focal populations (Figure S1).

guttatus_worldclim_seed_collections_METADATA.csv: This file contains descriptions of column names in guttatus_worldclim_seed_collections.csv.

1-17bioclim_pops.csv: This file contains bioclimatic variables (obtained from WorldClim v.2; ~1-km resolution, 1970-2000; Fick & Hijmans, 2017), latitude, and longitude of 18 native (United States) and 13 invasive (United Kingdom) focal populations of M. guttatus used in the previously described thermal performance experiments. These data were used for graphing clines of thermal performance parameters against relevant bioclimatic variables (Figure 4).

1-17bioclim_pops_METADATA.csv: This file contains descriptions of column names in 1-17bioclim_pops.csv.

Fick, S.E. & Hijmans, R.J. (2017) WorldClim 2: new 1-km spatial resolution climate surfaces for global land areas. International Journal of Climatology, 37, 4302–4315.