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

Abstract

Sympatric species are often locally adapted to distinct microhabitats, yet temporal variation may cause local maladaptation and weaken species boundaries, particularly during extreme climatic events. To examine the interplay between spatially and temporally varying selection, we studied sympatric Monkeyflowers occupying dramatically different niches, Mimulus guttatus and M. laciniatus. We conducted three replicated reciprocal transplants and combined them with previous experiments to leverage a dataset of five single-year transplants spanning a decade. We estimated the strength of phenotypic selection on experimental hybrids in each species habitat across years of drastically differing snowpack. If ecological isolation maintains species differences, we predicted divergent phenotypic selection between habitats consistent with species differences and local adaptation. We found spatial and interannual fluctuations in phenotypic selection, often in unpredicted directions, with temporal variation associated with snowpack. In a combined-year analysis, we detected a significant difference in the strength of long-term selection on flowering time, a key temporally isolating and adaptative trait, between species habitats, suggesting that selection may reinforce species boundaries despite short-term fluctuations. Finally, we found parental local adaptation varied among years where M. laciniatus was locally adapted during low snowpack years, while an extreme snowfall event contributed to overall local maladaptation of M. guttatus.