Systematics and evolution of Xanthophyllum (Polygalaceae)

The present work comprises the first revision of all species of Xanthophyllum; 93 species (22 new) have been distinguished with 5 subspecies (1 new) and 2 varieties (both new). Seven subgenera are proposed (4 new) of which one has been divided into 2 sections and 2 subsections. Keys to all taxa have been included. In the General Part the (sub)generic and (sub)sectional characters are discussed separately in order to find arguments regarding the direction of the evolution of those characters in the ‘Hennigian’ way of reasoning. From this it has been concluded that the Polygalaceae are derived from the Malpighiaceae- Vochysiaceae-Trigoniaceae-complex and secondly that Xanthophyllum belongs to a derived tribe of the Polygalaceae (and not to a separate family Xanthophyllaceae). The lack of information on the genomes of the species appeared to be a serious problem in the reconstruction of the evolution within Xanthophyllum: one subgenus with ‘gigas’-characters may represent an old allopolyploid hybrid; it is suggested that hybridization may have been important in the evolution of the genus. Although only two species, endemic to N. Queensland, do not occur in Malesia-Southeast Asia, it is shown that Australia must have been the centre of origin of the genus. The fact that Wallace’s Line is still respected by all species is regarded as an indication that West Malesia is a secondary centre of speciation.