Two closely related but morphologically disparate new species of Physcia from western North America

Two new species belonging to the common and widespread lichen genus Physcia are described from western North America. One of these, P. occidentalis, is an obligately saxicolous species that reproduces by fragmentation, primarily through the production of largely terminal blastidia. Only a single very immature apothecium was observed among all collections. The species occurs from British Columbia to California, with outliers in Colorado. The other species, P. rhizinata, is a corticolous, typically fertile species lacking asexual propagules, and has often been confused with P. stellaris. It is presently known only from California. The close evolutionary relationship of these morphologically dissimilar taxa highlights the currently limited understanding of factors that lead to phenotypic divergence in lichen symbioses.