A cline within an ecotype of the yellow monkeyflower,Mimulus guttatus

ABSTRACT Premise A key goal of evolutionary biologists is to understand how and why adaptive 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. 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 69 total populations in two greenhouse common garden experiments. To evaluate possible environmental factors driving the relationship between plant height and latitude, we regressed height against multiple climatic factors, including temperature, precipitation, and coastal wind speeds. Results In both experiments, plant height was negatively correlated with latitude. Mimulus height correlated positively with annual precipitation and with mean wind speed, to a lesser degree, and negatively with annual mean temperature. We hypothesize that one or more of these factors drove clinal variation within the coastal ecotype. Conclusion Overall, our study illustrates the complexity of how the distribution of environmental variation can both simultaneously drive the evolution of distinct ecotypes as well as continuous clines within those ecotypes. These results are discussed in the context of the classic criticisms of ecotypes being intermediates in the process of speciation.