UC Riverside

\textit{Washingtonia} is an American genus of palms currently composed of two species, \textit{W. filifera} and \textit{W. robusta}. Poor understanding of their morphology and distribution led initially to the description of numerous new species and later, to a simplification resulting in the current understanding of the genus. This widely distributed group of palms native to the Baja California peninsula, southern California, Sonora, and Arizona has been a taxonomic challenge due to a lack in type specimens, incomplete protologues, highly variable vegetative morphology, human dispersal, limited fieldwork in native populations, and poor representation in herbaria. In the wild, both distinct forms occur in the extremes of a 1300-km gradient, linked by morphological intermediates, suggesting that there is continuous morphological variation of the two forms instead of the long-held idea of two species.

We addressed this hypothesis by taking measurements of morphological traits of stems and functional traits of leaves in 17 sites spanning the whole distribution range of the genus in Mexico and the US. We examined the relationship between the plants’ traits and latitude, individually, and between all traits combined (using a Principal Component Analysis) and latitude. We compared a linear model against a non-linear model to test whether traits of \textit{Washingtonia} change gradually along the latitudinal gradient or if there are clear geographical breaks among populations.

We also used Genotyping By Sequencing (GBS) data to understand population structure and delimit species. Using both a \textit{de novo} and reference based approaches, we analyzed 173 \textit{Washingtonia} individuals from 21 populations across its distribution range through multivariate and Bayesian methods.

We found strong statistical support for the hypothesis that there is a gradual change in the vegetative morphological traits of \textit{Washingtonia} along the latitudinal gradient of Peninsular California, instead of two clear morphologically distinguishable species with a hybrid zone. However, we also found that \textit{Washingtoni}a populations are highly structured with four major geographic regions having unique genotypes. Based on previous morphological results and the evidence herein we propose recognizing a single species of \textit{Washingtonia} with four subspecies. Our findings suggest \textit{Washingtonia} is composed of one highly variable species and that local differentiation of populations is related to environmental gradients. Our results provide a robust phylogenetic analysis of \textit{Washingtonia} settling a taxonomic debate that has lasted over a century.