The Macaronesian clade (Crassulaceae) includes four genera, Aeonium, Aichryson, Greenovia, and Monanthes, that are largely endemic to the Canary Islands. The monophyly of this clade has been supported based on phylogenetic analyses of Crassulaceae; however relationships within this clade remain uncertain. To resolve relationships within this clade, parsimony analyses were conducted on DNA sequences from several chloroplast regions (trnL-trnF, psbA-trnH, and matK), the internal transcribed spacers, and morphology (Aeonium only). Phylogenetic analyses of these data sets recover three major clades. Congruence tests were conducted on each of these data sets as well as the topologies resulting from our initial parsimony analyses. Several incongruent taxa were pruned from the cpDNA data set; a combined data set was constructed; and parsimony analyses were repeated. These analyses again recovered three clades. The monophyly of Aichryson and Greenovia was supported; however, Aeonium is monophyletic only if Greenovia is included in this genus. Aeonium, the largest genus of Canary Island Crassulaceae, comprises species that are highly diverse in growth form, including rosette trees, candelabrum shrubs, highly-branched shrubs, and woody rosettes. In addition, species of Aeonium are also diverse in habit (perennial and perennial monocarpic species) and physiology, with CAM, C3, and CAM-C3 intermediate photosynthesis present in the genus. Analyses of stable isotopes of carbon were conducted from field collected leaf material to determine the degree of physiological variation present in Aeonium. The evolution of growth forms, habit, and physiology was investigated using our phylogenetic hypothesis for this genus. These analyses suggest that two subclades of Aeonium comprise taxa with similar growth forms, whereas two subclades contain taxa that display a wide range of growth form diversity. Monocarpy has arisen a minimum of four times in the genus. The ancestral physiological condition for Aeonium is C3 photosynthesis, with at least three origins of CAM photosynthesis.

Key words: CAM, Canary Islands, Crassulaceae, diversification, phylogenetics