Investigating a hybrid mixed population leads to recognizing a new species of Arctostaphylos (Ericaceae)

While investigating the potential for <i>Arctostaphylos</i> species to hybridize in the mixed populations of Point Sal and Burton Mesa in Santa Barbara County, California, we discovered that <i>Arctostaphylos</i> from the Nipomo Mesa (San Luis Obispo County), formerly considered a northern population of <i>A.rudis</i>, are genetically and morphologically distinct. We name this new taxon <i>A.nipumu</i> after the ytt (Northern Chumash language) word for the Nipomo Mesa region. For morphological and molecular analyses, we sampled 54 plants, focusing on <i>A.purissima</i>, <i>A.rudis</i>, and <i>A.crustacea</i> from multiple species and comparative single species populations. Parametric and nonparametric clustering analyses (STRUCTURE and PCA) of ddRADseq data show that <i>Arctostaphylos</i> from the Nipomo Mesa segregate from all other samples in the dataset. In mixed populations <i>A.purissima</i> and <i>A.crustacea</i> samples cluster with samples from other unmixed populations of the same species but <i>A.rudis</i> samples form two distinct clusters. One is composed of the mixed populations in Santa Barbara County, and the other consists of the Nipomo Mesa population. Additionally, the Santa Barbara County <i>A.rudis</i> samples are admixed in STRUCTURE analysis unlike the samples from the Nipomo Mesa. A principal component analysis of eight morphological characters shows that <i>A.rudis</i> individuals from Santa Barbara County tend to be phenotypically variable, occurring in a wide morphological cluster that overlaps with the tight clusters formed by <i>A.purissima</i>, <i>A.crustacea</i>, and <i>Arctostaphylos</i> from the Nipomo Mesa. Based on this evidence we describe the Nipomo Mesapopulation as a new species of <i>Arctostaphylos</i>. Given its limited and fragmented distribution we believe that <i>A.nipumu</i> is of critical conservation concern.