Multiple clines within an ecotype of the yellow monkeyflower, Mimulus guttatus

Premise: A key goal of evolutionary biologists is to understand how and why genetic variation is partitioned within species. In the yellow monkeyflower, Mimulus guttatus (syn. Erythranthe guttata), coastal perennial populations collectively constitute a single genetically and morphologically differentiated ecotype compared to inland populations of M. guttatus. While the distinctiveness of the coastal ecotype has now been well documented, there is also variation in environmental factors across the range of the coastal ecotype that could drive differentiation among its component populations in a more continuous way. Methods: Based on previous observations of a potential cline within this ecotype, we quantified plant height across coastal perennial accessions from 74 total populations in a greenhouse common garden experiment. To evaluate possible environmental factors driving the relationship between trait variation and latitude, we regressed height against multiple climatic factors, including temperature, precipitation, and coastal wind speeds. Results: Multiple traits were correlated with latitude of origin, but none more than plant height. Plant height was negatively correlated with latitude and plants directly exposed to the open ocean were shorter than those that were more protected from onshore coastal winds. Further analyses revealed that height was correlated with climatic factors (precipitation, temperature, and windspeeds) that were autocorrelated with latitude. We hypothesize that one or more of these climatic factors drove the evolution of latitudinal clinal variation within the coastal ecotype. Conclusion: Overall, our study illustrates the complexity of how the distribution of environmental variation can simultaneously drive the evolution of distinct ecotypes as well as continuous clines within those ecotypes.