Short-term fluctuating and long-term divergent selection on sympatric Monkeyflowers: insights from decade-spanning reciprocal transplants

Sympatric species are often locally adapted to distinct microhabitats. However, temporal variation may cause local maladaptation and species boundary breakdown, especially during extreme climatic events. Repeated reciprocal transplants can reveal the interplay between spatially and temporally varying patterns of natural selection. To examine long-term patterns of selection between sympatric Monkeyflowers occupying dramatically different niches, Mimulus guttatus and M. laciniatus, we performed three replicated transplants and combined them with previous experiments to leverage a dataset of five single-year transplants spanning a decade. We performed phenotypic selection analyses on parents and hybrids in each species habitat in Yosemite NP, CA during years of drastically differing snowpack. If there is ecological isolation, then we predicted divergent phenotypic selection between habitats in line with species differences and local adaptation. We found interannual fluctuations in phenotypic selection, sometimes in unpredicted directions. However, a combined-year analysis detected longer-term differences in the magnitude of selection between habitats on flowering time, a key temporally isolating and adaptative trait, suggesting that selection may reinforce species boundaries despite short-term fluctuations. Finally, we found temporal variation in local adaptation with M. laciniatus locally adapted in low snowpack years, while an extreme snowfall event contributed to overall local maladaptation of M. guttatus.