Members of Zygophylloideae are shrubs, shrublets and herbs of arid and semiarid areas of almost all continents, and many of the species are major components of the vegetation in their areas of distribution. A phylogenetic analysis of Zygophylloideae based on noncoding trnL plastid DNA sequences and morphological data, indicates that the currently recognised genera Augea, Tetraena, and Fagonia, are embedded in Zygophyllum. A new generic classification based on six monophyletic and morphologically distinctive entities is proposed here: Augea, Fagonia, Melocarpum, Roepera, Tetraena and Zygophyllum.
The taxonomy of the genus Fagonia is revised in detail. A key to the 34 species recognised is presented, as well as descriptions and distribution maps for each species. Five new species are described and illustrated, F. densispina, F. gypsophila and F. latistipulata from Somalia, and F. hadramautica and F. mahrana from the southern part of the Arabian peninsula. Of the accepted species, 26 are restricted to the Old World and eight to the New World. The names of all four species of Fagonia currently on the IUCN Red List of Threatened Plants are put into synonymy. Instead, eight other species are proposed for this list.
Relationships within Fagonia are inferred from analysis of plastid trnL intron and nuclear ribosomal ITS sequences. The phylogenetic analysis is performed using parsimony and Bayesian model averaging. All species of Fagonia in the Old World, except F. cretica, form a weakly supported clade, and all Fagonia species of the New World, except F. scoparia, form a second, well supported clade, sister to the Old World clade. Fagonia scoparia, endemic to northeastern Mexico, is sister to all other Fagonia species. Vicariance-dispersal analysis indicated that the occurrences of Fagonia in South America and southern Africa are due to dispersals, and that the ancestor of Fagonia had a distribution in agreement with the boreotropics hypothesis.