Local adaptation and range boundary formation in response to complex environmental gradients across the geographical range of Clarkia xantiana ssp. xantiana

Theory predicts that lack of heritable trait variation and/or maladaptive gene flow can promote the formation of species geographical range boundaries even in the absence of barriers to dispersal. Little is known, however, about the patterns and drivers of differentiation across species' ranges and whether they influence boundary formation in the field. Using field measurements, two common garden studies, and Q_ST-F_ST analyses, we examined environmental and genetic influences on plant phenotype across the geographical range of Clarkia xantiana ssp. xantiana. This annual plant endemic to California has an eastern range border that lacks obvious physical barriers to dispersal. We find that across opposing environmental gradients that span the species core range, populations are likely to be locally adapted. Populations are phenotypically differentiated from each other in the field as well as common gardens. Traits are correlated with field environmental variables in both settings, and Q_ST-F_ST comparisons indicate variation in flowering time, degree of branching, and herkogamy may be adaptive. The strength and direction of environment-trait correlations at the range edge are similar to the range centre, and quantitative genetic variation is not reduced. Genetic correlations between traits are generally weak, also suggesting little constraint on trait evolution at the range edge. Synthesis. For traits measured here, it is unlikely that either low heritable variation at the range edge or maladaptive gene flow strongly inhibits eastward range expansion. Our results suggest that across environmentally complex landscapes, patterns of diversity generated by locally adaptive selection do not necessarily contribute broadly to the formation of range boundaries as predicted.