Slender goldenweed (Xanthisma gracile; synonym Haplopappus gracilis) (2n = 4) is an annual, highly polymorphic, and taxonomically controversial species occurring in the southwestern United States and northern Mexico. It is very similar in morphology to Haplopappus ravenii, but differs in chromosome number, with H. ravenii having 2n = 8. Some botanists have considered these to be separate species in the past, but H. ravenii has not been recognized in the most recent taxonomic treatments of this complex. Numerous researchers have questioned the morphological distinctiveness of these two taxa, but no study has quantified differences in morphology across geographically diverse populations or tested for genetic differentiation between these taxa. The difference in chromosome number suggests that these taxa could be reproductively isolated, which would support recognition of these as unique taxa under the biological species concept. Here, we applied an alternative species concept, the genotypic cluster criterion to test the hypothesis that X. gracile and H. ravenii are distinct species and warrant recognition at this level. Flower and leaf characters were measured on herbarium specimens and genetic diversity was quantified using AFLP's. Little evidence of morphological differences in samples assigned to each species was found as no characters exhibited significantly different means. Genetic differentiation between the two groups of samples was significant in an analysis of molecular variance, but the level of divergence was quite low (φST = 0.015, P < 0.05) in comparison to values for other clearly demarcated species. Clustering analysis did not indicate cleanly separated groups of morphologically or genetically divergent samples. Although this study did not test for reproductive isolation, little evidence was found to indicate that these taxa are divergent, an expected outcome if these species are not exchanging genes. We suggest that X. gracile is a polymorphic species with a varied distribution of cytotypes and should continue to be recognized as a single species.
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Vol. 63 • No. 3