There is no denying that the advent of molecular analysis revolutionised the world of plant phylogeny. Previously an uncertain landscape of shifting sands, beset by the eroding forces of convergent evolution and morphological plasticity, the higher relationships of flowering plants have begun to resolve into a much clearer view than before. But some of the revealed vistas have been unexpected, and have led to quagmires of their own.
The Malpighiales are one clade that has become generally recognised as a result of molecular analyses, but remain almost impossible to characterise morphologically. Part of that difficulty is a consequence of sheer diversity: the clade includes about 16,000 species worldwide. The bulk of these species are tropical; it has been estimated that 40% of the world's tropical rain-forest understory is composed of Malpighiales (Xi et al. 2012). Only a relative minority of Malpighiales are found in more temperate parts of the world, though that minority still includes such familiar plants as violets, willows and spurges. The ranks of Malpighiales include some of the most bizarrely modified of all flowering plants: the endoparasitic Rafflesiaceae and the aquatic Podostemaceae.
Though molecular analyses have been fairly consistent in supporting the Malpighiales as a whole, relationships within the Malpighiales long proved more recalcitrant. As a result, its species have been placed in up to 42 different families, these families varying wildly in diversity. At one end of the scale, the Euphorbiaceae has been home to over 5700 species, even in its modern restricted sense (earlier botanists recognised a Euphorbiaceae that was considerably larger). At the other end, the Malesian vine Lophopyxis maingayi and the jellyfish tree Medusagyne oppositifolia of the Seychelles have each been considered distinctive enough and of uncertain enough affinities to be placed in their own monotypic families. Many of these families could only be placed within the Malpighiales as part of a great polytomy, an unresolved mess of relationships at the base of the clade.
A major advance in our understanding of malpighialean phylogeny was made just recently by Xi et al. (2012), who were able to obtain a more resolved phylogenetic tree than previous studies through the use of data from a large number of genes (they also ignored the Rafflesiaceae; those guys just cause trouble). Their results suggested a division of the Malpighiales between three basal clades. The smallest of these includes relatives of the families Malpighiaceae and Chrysobalanaceae. Few members of this clade are familiar outside the tropics. Some are known for their edible fruit, such as the coco plum Chrysobalanus icaco, the nance Byrsonima crassifolia, the Barbados cherry Malpighia emarginata and the butter-nut Caryocar nuciferum. In contrast, the southern African gifblaar Dichapetalum cymosum contains toxic sodium monofluoroacetate and is regarded as a serious threat to livestock.
The next clade includes families relatied to the Clusiaceae, Ochnaceae and Erythroxylaceae. The latter family is closely related to (and sometimes synonymised with) the Rhizophoraceae, a small but significant family that dominates among the tropical mangroves. The Erythroxylaceae is itself most notorious for including the coca plant Erythroxylum coca, the source of the drug cocaine*. The clusioid families include the Clusiaceae, Hypericaceae and Calophyllaceae, treated in older sources as a single family Guttiferae but currently treated as separate families owing to the paraphyly of such a grouping to the families Bonnetiaceae and Podostemaceae. The name 'Guttiferae' refers to the production of resin by many clusioids. In some species, these resins are produced in the flowers in lieu of nectar and are collected for nest-building by visiting bees. Economically significant clusioids include the mangosteen Garcinia mangostana and the St John's wort Hypericum perforatum, which has been grown commercially in some parts of the world for its supposed medicinal properties but is regarded in other parts of the world as a highly undesirable weed.
The third clade, and the largest by a considerable margin, includes such families as the Euphorbiaceae, Violaceae and Salicaceae. Noteworthy examples of this clade also include the passion fruit Passiflora edulis, and the flax plant Linum usitatissimum. The Euphorbiaceae, as alluded to above, were previously considered to include taxa more recently treated as the separate families Putranjivaceae, Phyllanthaceae, Picodendraceae and Peraceae. The Putranjivaceae were placed by Xi et al. (2012) in the Malpighiaceae-Chrysobalanaceae clade, and so are not close relatives of the Euphorbiaceae sensu stricto. The remaining families are closer, but modern authors would prefer to keep them separate as the demands of monophyly would then require the Euphorbiaceae be further enlarged to include the Linaceae and Rafflesiaceae. Nobody wants a Rafflesia in their family.
Xi, Z., B. R. Ruhfel, H. Schaefer, A. M. Amorim, M. Sugumarane, K. J. Wurdack, P. K. Endress, M. L. Matthews, P. F. Stevens, S. Mathews & C. C. Davis. 2012. Phylogenomics and a posteriori data partitioning resolve the Cretaceous angiosperm radiation Malpighiales. Proceedings of the National Academy of Science of the USA 109 (43): 17519-17524.