Phylogeny and Introgression of California Scrub White Oaks (Quercus section Quercus)

The taxonomy of oaks (Quercus) is always a challenge because many species exhibit variable phenotypes that overlap with other species. The scrub White Oaks of California are no exception. In California, Quercus section Quercus (i.e., White Oaks) includes six species of scrub oaks plus four tree oak species. Field identification utilizes leaf traits and acorns, when available, as well as geographic location, but often botanists – with the exception of specialists – are not confident of their assignments. Complicating our understanding of scrub oaks further is the historical and ongoing introgression among taxa. Fortunately, new research using nuclear microsatellite genetic markers and RADseq-based sequences are clarifying their evolutionary relationships. Based on these genetic markers, we describe the phylogenetic relationships among the California scrub and tree White Oaks. Given the impact of hybridization in oaks, we then present a specific example involving three Southern California oaks – one tree oak (Q. engelmannii) and two scrub oaks (Q. berberidifolia and Q. cornelius-mulleri) – to illustrate how the environment and geography play a role in promoting interspecific gene exchange. This paper provides an overview of a fascinating scrub oak complex in a topographically and climatically diverse region.