| Crassulaceae are an angiosperm family of cosmopolitan distribution composed of 35 genera and about 1500 species. Although identification of the family is straightforward, defining natural groups within Crassulaceae has been extremely difficult. In part, this difficulty is due to recurrent evolution of morphological characters associated with xerophytism. matK sequences were used to estimate the phylogeny for 112 species of Crassulaceae representing 33 genera. These analyses suggest that five of six subfamilies recognized in Crassulaceae are not monophyletic. Instead, a basal split separates the Crassula clade (Crassuloideae) and the rest of the family (Sedoideae). Within Sedoideae, four subclades were recovered: Kalanchoe, Leucosedum, Acre, and Aeonium; evidence also exists for the Telephium and Sempervivum clades. Examining the distribution of three floral features that have been used to define subfamilies within Crassulaceae indicates that sympetaly and polymerous flowers have arisen multiple times in Crassulaceae, whereas haplostemony is confined to Crassuloideae.; One clade within Crassulaceae that exhibits the systematic uncertainty and morphological diversity present throughout the family is the Macaronesian Clade, which is composed of four morphologically and cytologically diverse genera: Aeonium, Aichryson, Greenovia , and Monanthes. DNA sequences from the nuclear ITS region and three chloroplast regions, as well as a morphological data set, were used to resolve relationships among 54 species from this clade. These analyses resolve three subclades within the Macaronesian clade that correspond to Aichryson, Monanthes, and Aeonium (including Greenovia). Furthermore, our data suggest that the Macaronesian clade evolved on the Canary Islands from herbaceous, annual ancestors.; Aeonium is the largest of the Macaronesian genera; species of Aeonium display remarkable morphological and physiological diversity. In fact, the genus has been termed the "botanical equivalent to Darwin's finches." The phylogeny for the Aeonium subclade was used to investigate the diversification of this genus. Carbon isotope analyses indicate that C3, CAM, and C3-CAM intermediate species are present in Aeonium. When examined phylogenetically, two of the four clades of Aeonium species possess little physiological variation, whereas two clades are diverse in physiology. Concentrated-changes tests indicate no significant association between CAM and three growth-form features, as has been previously suggested. |
