The evolution of growth-forms in the Macaronesian genus Aeonium (Crassulaceae) inferred from chloroplast DNA RFLPs and morphology
Mes, T.H.M.; T'hart, H.
Molecular Ecology 5(3): 351-363
ISSN/ISBN: 0962-1083 DOI: 10.1046/j.1365-294x.1996.00090.x
Phylogenetic relationships among species of Aeonium were studied using morphological characters and chloroplast DNA RFLPs. Cladistic analysis of weighted morphological data indicates that the small, herbaceous and least woody species are basal in the genus. Chloroplast DNA data gave similar results, supporting the separation of the herbaceous or small, woody species from the large, hapaxanth rosettes, rosette trees, and branched subshrubs with yellow, white or red flowers as well as the only (herbaceous) species with axillary inflorescences. The relationships among the species descending from a polytomy that comprises all species of the genus as well as a polytomy which comprises 18 of the 26 species studied, are only very incompletely resolved, except for two monophyletic clades that contain the branched subshrubs with yellow flowers (A. sect. Aeonium) and the branched subshrubs and rosette trees with white or red flowers (A. sect. Leuconium), respectively. Cladistic analysis of the combined morphological and chloroplast DNA data improved resolution considerably. Four monophyletic clades are distinguished, each of which, except for three species, comprises only one of the five main growth-form types. Although Aeonium is generally regarded as an outstanding example of adaptive radiation, this mode of speciation seems to have been of minor significance in the evolution of the genus, because each growth-form apparently evolved only once. Instead, island speciation in the absence of major ecological shifts, is probably more important in the evolution of the genus.