In classic evolutionary models, species remain distinct when isolated by one or more extrinsic or intrinsic barriers to hybridization. If these barriers reduce or prevent genetic exchange, the morphological discontinuities among reproductively isolated species will provide an effective mechanism for their classification. We used morphological and molecular analyses, and data on flowering phenology, breeding systems, and pollination, to explore the taxonomic relationships between two camas lilies, Camassia leichtlinii and C. quamash, which occur sympatrically in Oregon. Differing classifications persist for these North American taxa: Gould treated the species as distinct in his monograph, noting the absence of hybrids, whereas a recent California flora relegates C. leichtlinii to a subspecies of C. quamash Our surveys of single and mixed populations show extensive overlap in extrinsic factors such as flowering time and insect visitors. Morphological study confirms the presence of an intermediate form that differs significantly (P < 0.05) from one or more parental species in all but one of ten characters measured. Thus, pre-zygotic barriers appear weak, and interspecific hybridization may occur in Camassia. Preliminary allozyme analysis of ten loci also revealed additive banding patterns and higher than average heterozygosities in putative hybrid zones. Of ISSR primers analyzed to date, two diagnostic bands of the parental species C. leichtlinii and C. quamash appear in 78% and 9 % of the putative hybrids, respectively. Yet hybrid seeds produced from artificial pollen manipulations were inviable under glasshouse conditions, suggesting the presence of intrinsic, post-zygotic barriers to interbreeding. These data and multivariate numerical analyses imply that C. leichtlinii and C. quamash are best classified as separate species, but occasional hybridization and introgression may also play an important role in their evolution.

Key words: Camassia, hybridization, Liliaceae, reproductive isolation, speciation