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

Abstract

SUMMARYSympatric species often adapt to distinct microhabitats, leading to reproductive isolation and phenotypic diversity. However, temporal variation in selection may cause local maladaptation and species boundary breakdown, especially in years with large climatic events leading to episodic selection. Repeated reciprocal transplants can reveal short and long-term patterns of natural selection.To examine evolutionary trajectories of sympatric Monkeyflowers adapted to different niches,Mimulus guttatusandM. laciniatus,we performed three replicated transplants and combined them with previous experiments to leverage a dataset of five transplants spanning 10 years. We performed phenotypic selection analyses on parents and hybrids in parental habitats in Yosemite NP, CA during years of differing snowpack. If there is ecological isolation, then we predicted local adaptation and divergent phenotypic selection between habitats in line with species’ differences.We found interannual fluctuations in selection, often not in predicted directions. Episodic selection due to extreme high snowpack caused a reversal of local adaptation and contributed to overall maladaptation ofM. guttatus. However, a combined-year analysis detected longer-term divergent selection on flowering time, a key temporally isolating and adaptative trait, in agreement with species’ differences.In conclusion, even with annual fluctuations, longer-term divergent selection may still promote species boundaries.