There are now 25 known species, of which 20 come from Madagascar, where isolated landscapes and micro-environmental conditions have produced highly specialized species. The species count continues to grow as "Pachypodium menabeum" has been resurrected from invalid taxonomy and "Pachypodium makayense" added newly to the list. One can speculate that in regions such as Madagascar, there might still be unidentified species that are confined to a single rocky outcrop or an inselberg. The family Apocynaceae before it included Asclepiadaceae had 3 genera that can be considered succulent plants: "Adenium", "Pachypodium", and "Plumeria". The first two genera ("Pachypodium" and "Adenium") are generally assumed to have a close association with each other. Studies; however, of these two genera reveal that they are not as intimately close as once thought. However, a study of key characteristics of the taxon and a cladistic study of the subfamily Apocynoideae and the family Asclepiadaceae (before its merging with the Apocynaceae), demonstrates that this closed association is not warranted. True, both are succulent plants and pachycaul. According to Leeuwenberg however, "Adenium" is maintained in the subtribe Neriinae, placed underneath the tribe Wrightieae whereas "Pachypodium" is placed beside them in the subtribe Pachypodiinae, within the tribe Echiteae. Though related, these taxa means that the two are not intimately related. "Pachypodium" are native to Madagascar and continental Southern Africa, i.e. Angola, Botswana, Mozambique, Namibia, South Africa, Swaziland and Zimbabwe. In elevation, "Pachypodium" in both mainland Africa and Madagascar grow between an altitude of sea level, where some species grow in sand dunes, such as "Pachypodium geayi", to for "Pachypodium lealii" in southern Africa and for "Pachypodium brevicaule" in Madagascar. In continental southern Africa, the extreme temperatures range from in some locations to as much as . Whereas in Madagascar, with not such a great temperature amplitude, the temperature ranges from . A generalization about precipitation regimes for both southern Africa and Madagascar does not have much meaning because the habitats of "Pachypodium" vary so greatly with a moisture regime.