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

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. 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. 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. 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. These results are
discussed in the context of the classic criticisms of ecotypes being
intermediates in the process of speciation.