Investigating Contact Zone Dynamics of Clarkia xantiana Sister Taxa
In the Sierra Nevada mountains in California, a pair of sister taxa of annual plant Clarkia xantiana exist in a unique zone of moment and circumstance, combining recent species divergence and incomplete reproductive isolation. The two taxa generally partition into different microhabitats and have different mating systems, but exist together in zones of secondary contact near the edges of their ranges, where they hybridize. I examined the distribution of phenotypic and genetic variation to ask whether two or three distinct phenotypic and genomic groups are present at one of these zones of secondary contact. Integrating spatial data, I asked whether the spatial distribution of phenotypes across the contact zone reflects the distribution of genetic admixture across the site. I found that a one-cluster model best represents the site population, reflective of early-generation hybrid formation, displayed in both intermediate phenotypic traits and genetic ancestry. A plant’s proximity to a line of inter-taxa contact did not reliably predict more intermediate phenotypes or increased genetic admixture. This contact zone provides a model for predicting the genetic, spatial, and phenotypic dynamics of hybridization and speciation in an increasingly variable climate. Understanding the evolutionary trajectories of closely related species, especially those particularly vulnerable to environmental pressures, will be important in predicting changes in the tree of life.