Several species of the North American shrub genus Ceanothus have been proposed as exemplars of diploid hybrid species. Based on observations of natural hybrids, synthesis of garden hybrids, cytology, distribution, and perceived morphological intermediacy, Nobs (1963) concluded that C. masonii and C. sonomensis (among others) are diploid hybrid species, derived from separate hybridization events between C. cuneatus and C. gloriosus. We tested Nob's proposal for the origins of C. masonii and C. sonomensis using molecular (i.e., nuclear ribosomal and chloroplast DNA sequences and allozyme diversity) and morphological (i.e., foliar characteristics) data. Results of our molecular analyses provided no unequivocal support for the proposed hybrid origin of either C. masonii or C. sonomensis. Additionally, statistical and multivariate analyses of foliar characteristics indicated that the proposed hybrid species possess character states more extreme than either parent. Recent studies of character inheritance and expression in hybrids suggest that the assumption of morphological intermediacy in hybrids is unwarranted, therefore, our morphological results are not inconsistent with possible hybrid origins. However, our results do call into question at least one of Nobs' original premises. As an alternative to the hybrid origin scenario, we propose that C. masonii and C. sonomensis are the products of divergent speciation, driven by recent climatic and geological changes effecting the western edge of the North American continent.

Key words: allozymes, Ceanothus, cpDNA, diploid hybrid speciation, morphometric, rDNA