THE EVOLUTION OF GROWTH-FORMS IN THE MACARONESIAN GENUS AEONIUM (CRASSULACEAE) INFERRED FROM CHLOROPLAST DNA RFLPS AND MORPHOLOGY

Phylogenetic relationships among species of Aeonium were studied using morphological characters and chloroplast DNA RFLPs. Cladistic analysi s of weighted morphological data indicates that the small, herbaceous and least woody species are basal in the genus. Chloroplast DNA data g ave similar results, supporting the separation of the herbaceous or sm all, woody species from the large, hapaxanth rosettes, rosette trees, and branched subshrubs with yellow, white or red flowers as well as th e only (herbaceous) species with axillary inflorescences. The relation ships among the species descending from a polytomy that comprises ail species of the genus as well as a polytomy which comprises 18 of the 2 6 species studied, are only very incompletely resolved, except for two monophyletic clades that contain the branched subshrubs with yellow f lowers (A. sect. Aeonium) and the branched subshrubs and rosette trees with white or red flowers (A. sect. Leuconium), respectively. Cladist ic analysis of the combined morphological and chloroplast DNA data imp roved resolution considerably. Four monophyletic clades are distinguis hed, each of which, except for three species, comprises only one of th e five main growth-form types. Although Aeonium is generally regarded as an outstanding example of adaptive radiation, this mode of speciati on seems to have been of minor significance in the evolution of the ge nus, because each growth-form apparently evolved only once. Instead, i sland speciation in the absence of major ecological shifts, is probabl y more important in the evolution of the genus.