The Succulent Karoo is an arid region, situated along the west coast of southern Africa. Floristically this region is part of the Greater Cape Flora and is considered one of the Earth's 25 biodiversity hotspots. Of about 5,000 species occurring in this region, more than 40% are endemic. Aizoaceae (ice plants) dominate the Succulent Karoo both in terms of species numbers (1,750 species in 127 genera) and density of coverage. Here we show that a well-supported clade within the Aizoaceae, representing 1,563 species almost exclusively endemic to southern Africa, has diversified very recently and very rapidly. The estimated age for this radiation lies between 3.8 and 8.7 million years (Myr) ago, yielding a per-lineage diversification rate of 0.77-1.75 per million years. Both the number of species involved and the tempo of evolution far surpass those of any previously postulated continental or island plant radiation. Diversification of the group is closely associated with the origin of several morphological features and one anatomical feature. Because species-poor clades lacking these features occur over a very similar distribution area, we propose that these characteristics are key innovations that facilitated this radiation.

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We use a relatively densely sampled phylogeny to obtain preliminary estimates for the ages of the major clades identified in Euphorbia, with which we show that the succulent species of Euphorbia have diversified over the last 36 million years into many of the semi–arid, tropical parts of the world. Many major clades have subclades from widely separated regions, often on different continents. Our results imply that these distributions arose by long–distance dispersal after the break–up of Gondwana. In the case of Indian/South–east Asian succulents there appears to have been a single, relatively recent dispersal event from Africa. We have included many species from the Arabian Peninsula and Socotra and we show that these are nested within other, mainly African clades. In some cases Arabian and Socotran taxa have their closest relatives in adjacent parts of North–east Africa and most often this is true in recent clades, while in more ancient clades their closest relatives may be in Macaronesia or in the Namib Desert of Southern Africa so that these are typical 'Rand Flora' elements. In one case, closest known relatives are in North America. We find that Socotran taxa vary between 16 and 3 Ma old. The major diversifications of succulents in temperate Southern Africa (the crown clades in subg. Rhizanthium and more minor clades in subg. Chamaesyce) and in tropical East Africa (the crown clades in subg. Euphorbia) occurred in the last 20–3 Ma. In the Greater Cape Flora of the western part of Southern Africa the diversity in Euphorbia is mainly derived from one lineage in subg. Rhizanthium and one in subg.Chamaesyce. In contrast the diversity of Euphorbia in the Arabian Peninsula is derived from the invasion (mainly from Africa) of many separate lineages. We show that large, succulent trees are only found in subg. Euphorbia and only occur in the Old World. Most of them fall within a single clade and have evolved relatively recently.

The Aizoaceae is the largest family of leaf succulent plants, and most of its species are endemic to southern Africa. To evaluate subfamilial, generic, and tribal relationships, we produced two plastid DNA data sets for 91 species of Aizoaceae and four outgroups: rps16 intron and the trnL-F gene region (both the trnL intron and the trnL-F intergenic spacer). In addition, we generated two further plastid data sets for 56 taxa restricted to members of the Ruschioideae using the atpB-rbcL and the psbA-trnH intergenic spacers. In the combined tree of the rps16 intron and trnL-F gene region, three of the currently recognized subfamilies (Sesuvioideae, Mesembryanthemoideae, and Ruschioideae) are each strongly supported monophyletic groups. The subfamily Tetragonioideae is polyphyletic, with Tribulocarpus as sister to the Sesuvioideae and Tetragonia embedded in the Aizooideae. Our study showed that the group consisting of the Sesuvioideae, Aizooideae, and Tetragonioideae does not form a monophyletic entity. Therefore, it cannot be recognized as a separate family in order to accommodate the frequently used concept of the Mesembryanthemaceae or "Mesembryanthema," in which the subfamilies Mesembryanthemoideae and Ruschioideae are included. We also found that several genera within the Mesembryanthemoideae (Mesembryanthemum, Phyllobolus) are not monophyletic. Within the Ruschioideae, our study retrieved four major clades. However, even in the combined analysis of all four plastid gene regions, relationships within the largest of these four clades remain unresolved. The few nucleotide substitutions that exist among taxa of this clade point to a rapid and recent diversification within the arid winter rainfall area of southern Africa. We propose a revised classification for the Aizoaceae.

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We present a phylogeny for Mesembryanthemoideae (Aizoaceae) based on sampling of nearly all species and subspecies of the subfamily and analysis of cptrnL-F, rbcL-atpB, rps16, nrITS1 and morphology. The larger genera Phyllobolus and Mesembryanthemum are not monophyletic. Although some clades can be circumscribed with morphological (often homoplasious) synapomorphies, several clades are impossible to characterise morphologically. We recognise a single genus, Mesembryanthemum, in Mesembryanthemoideae. The genera Aptenia, Aridaria, Aspazoma, Brownanthus, Caulipsolon, Dactylopsis, Phyllobolus, Prenia, Psilocaulon, Sceletium, and Synaptophyllum are reduced to synonymy. Mesembryanthemum, which now consists of 101 species without recognised sections, can be distinguished by several uniquely derived morphological characters. Mesembryanthemum longipapillosum, which had recently been reduced to synonymy, is reinstated.