There are several species of Adenium, but the exact number is still unresolved. A recent DNA analysis of 5 mitochondrial, nuclear, and chloroplast segments (Dimmitt and Edwards 2021) identified seven well-defined species in three distinct clades (groups), and at least one more and perhaps several more species that could not be resolved. The three clades are shown in the graph below left, and their geographic distributions in the map below right.
The horizontal lines in the cladogram are approximate measures of the distance between taxa (species or varieties) - the shorter the line, the more closely related the taxa. For example, in the Southern African group, A. boehmianum and swazicum are more closely related to each other than they are to oleifolium, and all 3 are quite distinct from the other 2 clades.
The geographic distributions are drawn from limited herbarium records and the personal observations of John Lavranos. Adeniums do not occur continuously within these areas; there are many isolated populations.
The taxa in the Southern African and Arabian clades comprise six clearly defined species. The 3 Southern African species can be defined by their genetic differences alone, indicated by the fairly long horizontal lines between them. They are further distinct in having nonoverlapping geographic ranges. Adenium swazicum and A. boehmianum have very similar flowers, but these two species have very different growth forms, and occur on opposite sides of the continent. They're also geographically separated by A. oleifolium, which is phenotypically very different from the other two.
Adenium swazicum (left) and A. boehmianum (center) have very similar flowers. They are unique in the genus in having uniformly colored petals (same shade from tip to throat), dark throats, and very short anther appendages. Both usually but not always have round flowers. But swazicum's leaves are narrow while those of boehmianum are very broad. The flowers of A. oleifolium (right) are quite different: small with nonoverlapping, pointed petals and long anther appendages.
The 3 species in the Arabian clade are more closely related to one another than the taxa in the Southern African clade are related. Genetically (according to the 5 sequences analyzed) they could be considered a single species. But there are sufficient vegetative phenotypic and geographic differences to justify separating them. The flowers, however, have widely overlapping traits and are not reliably diagnostic. According to John Lavranos' field experience, Adenium "arabicum" (above left) and A. dhofarense (above middle) on the Arabian Peninsula are separated by a 100 km gap where no adeniums occur. They are also quite different in vegetative form. A. socotranum (above right) is isolated on a small, distant island, and can grow several times larger than any other adenum. The DNA clearly shows that this island giant is derived from the Arabian plants and not from A. somalense.
Of the four taxa in the East African clade, Adenium multiflorum (above) is a distinct species. (Even though it is geogrphically in Southern Africa, it's genetically related to the other three far to the north.) Adenium "obesum", somalense, and crispum are a taxonomic problem. They were barely distinguishable by the 5 DNA segments that were analyzed, indicated by the very short horizontal lines between them. They appear to be a single genetic species. Plants of these 3 taxa in cultivation are phenotypically quite different and so are usually easily recognized. However, photos of wild plants show much more variation than we see in cultivation. Some examples of questionable identity are shown in the Other Taxa page. Furthermore, no adeniums from the vast Sahel region are known to be in cultivation, so their DNA could not be sequenced. The few photos available show that they have different vegetative and flower forms from the plants in East Africa. See the Other Taxa page. At this time it is not possible to determine whether the adeniums in this extensive region from East Africa through the Sahel are one highly variable species, or perhaps a multitude of isolated, closely related species.
Flowers and leaves of Adenium "obesum" (left), somalense (center), and crispum (right) in cultivation look quite different. The plants are also distinctive (see species pages). A. "obesum" is an erect or more often a spreading shrub with a modest caudex, and obovate leaves with faint veins. A. somalense has a massive conical trunk (but which is underground in sandy soil), and has elliptical leaves with prominent whitish veins (center inset). A. crispum is a dwarf shrub with linear, crisped leaves that also have prominent veins. But wild populations show much more variation than is seen in cultivation. See the Other Taxa and Unknowns page for details.