Phylogeny, character evolution, and biogeography of Yucca L. (Agavaceae) as inferred from plant morphology and sequences of the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA

The genus Yucca consists of 46 perennial species distributed primarily in the warm deserts of Mexico and the United States. Although there are several taxonomic treatments of yuccas, the phylogeny of the group is poorly known. It is unclear which taxa retain primitive characters and how characters have evolved. In addition, the relationships of taxa within the genus are uncertain. The goals of this research were to discern the phylogeny and patterns of speciation and character evolution within Yucca.; Morphological and molecular analyses were undertaken to provide systematic data for phylogenetic analysis. The Internal Transcribed spacer (ITS) region of the nuclear ribosomal DNA repeat was sequenced to determine levels of genetic variation between Yucca species and outgroup genera. Morphological variation was examined using characteristics of plant habit, leaf, inflorescence, fruit and pollen morphology. Using parsimony analysis, the data sets were analyzed and compared. The sequence of character changes was mapped onto the tree topology to evaluate patterns of character evolution over time. Geographic distributions of closely aligned species were mapped using the phylogenetic relationships shown in the tree topology. From these, patterns of speciation through time and across the North American continent were evaluated.; The traditional taxonomic treatment of the genus was compared to a phylogeny generated by molecular data. In the past, the genus has been divided into four sections based upon shared fruit, pistil and leaf characters. Section Hesperoyucca, with a unique, capitate stigma and dry fruit, contains species of the Yucca whipplei complex. Section Clistocarpa, with spongy fruit, contains a monotypic species, Y. brevifolia. Section Chaenocarpa contains 22 dry-fruited species and sect. Sarcocarpa contains 23 fleshy-fruited species. Results indicated that the fleshy-fruited species are a monophyletic lineage, but the dry-fruited species are not. The species of sect. Hesperoyucca were shown to be more closely allied to Hesperaloe than to Yucca, providing support for the placement of sect. Hesperoyucca outside of Yucca. The dry fruit of the species of ser. Rupicolae arose from the lineage which gave rise first to spongy fruit (Y. brevifolia). The rest of the dry-fruited species are monophyletic and arose shortly after the lineage bearing Y. brevifolia. A taxonomic revision is warranted that will reflect the naturalness of the group.; Biogeographic analysis indicates that the Mohave and Sonoran deserts contain the species of Yucca most basal to the lineage. Most of the species with sympatric distributions are not closely related, suggesting a repeated pattern of dispersal or vicariance, then allopatric speciation within lineages over time.