The species of Moringa occupy the well-known arid area distribution stretching from southern Angola and southwestern Africa across to Rajasthan with the addition of Madagascar.
The greatest diversity of Moringa species is found in the tropical dry lowlands of northeastern Africa, especially the area where Kenya, Ethiopia, and Somalia meet. Moringa arborea, borziana, longituba, peregrina, pygmaea, rivae, ruspoliana, and stenopetala all grow in this area. Moringa borziana is the southernmost of the northeast Africa species, growing west of Mombasa in southeastern Kenya. It has never been collected in Tanzania as far as I know. The westernmost of the northeast African species is M. stenopetala, which grows in the Kenyan Rift Valley in northwestern Kenya. To the west of the Kenyan drylands lie the wet forest of the Congos and Gabon. No moringas there. To the south we only find moringas again in souther Angola and northern to central Namibia, as well as in Madagascar.
What gives? What causes this pattern, in which related dryland plants are found in southwestern and northeastern Africa but nowhere else?
Graduate student John Zaborsky, in Ken Systma's lab at the University of Wisconsin-Madison, is using Moringa samples from the International Moringa germplasm Collection, plus a bunch of other plant lineages, to examine this issue. John says
It is hypothesized that an African Arid Corridor existed across the humid, wet interior of Africa numerous times in the past that allowed these plants to move north and south. I want to test whether the divergence times of these taxa align with the known/hypothesized timing of the corridor or whether long distance dispersal can explain these distributions.
Because of its classic northeast-southwest distribution, Moringa is a key group for John's study. We sent him samples of M. borziana, M. concanensis, M. drouhardii, M. longituba, M. oleifera, M. ovalifolia, M. peregrina, M. rivae, M ruspoliana, and M. stenopetala. The crucial split in the family occurs between the southern species (M. drouhardii, M. hildebrandtii, both Madagascar, and M. ovalifolia, of Namibia) and M. stenopetala + the rest of the genus (northeast Africa, Arabia, and the Indian subcontinent.
Long-distance dispersal seems unlikely for Moringa given that the seeds are very large and have adaptations for wind dispersal (the wings on the seeds of most species) or possibly even water (the spongy tissue in M. stenopetala seeds), which would only get them relatively short distances. These features also make them unlikely candidates for ingestion and later excretion by birds. They germinate very fast, making long distance floating in salt water seem unlikely. So my bet is that the M. ovalifolia-rest of the genus split falls at a time coincident with the Africa Arid Corridor. We'll keep an eye on John's progress and see what he finds!
Verdcourt, B. 1985. A synopsis of Moringaceae. Kew Bulletin 40: 1-23.