| Molecular studies of Croton show that a small clade of less than 30 mainly Caribbean and South American species is sister to the ca. 1,200 species in the genus. This thesis explores the phylogenetic relationships, morphology, and biogeography of this smaller clade using DNA sequence analyses. The second chapter deals in detail with Croton alabamensis, a member of this smaller clade and a rare plant found in Texas and Alabama. AFLP analyses along with parsimony, likelihood, and Bayesian analyses of DNA sequence data were performed to discriminate phylogenetic structure and quantify genetic diversity. Most of the molecular variance is accounted for by variance within populations. Approximately equal variance is found among populations within Texas and Alabama and between populations from the two states. Both sequence and AFLP data support the same phylogenetic relationships between C. alabamensis and its relatives, while the AFLP data provide resolution among the various populations. The third chapter reconstructs a phylogenetic hypothesis for the smaller clade of Croton species using DNA sequence data from the nuclear ITS region and the chloroplast rbcL, ndhF, and trnL-trnF regions. Overlaying the morphological characters that have been used historically in Croton taxonomy onto the molecular phylogeny identifies which are homoplasious and which are diagnostic for the main divisions within the genus. A South American origin and a mesic habitat are indicated as ancestral for the genus. The four chloroplast markers showed concordance with one another but were significantly incongruent with the ITS data. An explanation for the incongruence between the genomes is a possible introgression event. Croton poecilanthus , the only tree endemic to Puerto Rico, is identified as a potential hybrid given that chloroplast genome analyses places it closely related to members of section Moacroton, which it greatly resembles, while nuclear ITS analyses place it in an unresolved position at the base of Croton. Chapter five takes all Croton species from mainland Central and North America, which are not necessarily expected to make a monophyletic group, and applies molecular tools to aid in identifying the sections that are represented and assign all species to section. |
