The majority of Aloe species occur naturally on mainland Africa, in tropical and subtropical latitudes. The genus is found almost throughout the African continent south of the Sahara Desert, except for the moist lowland forest zones and the western end of West Africa.
As will be seen in Figure 1.1 the majority of species occur in southern Africa and on the eastern side of the continent. Many other species are found on the Arabian Peninsula and on Madagascar and a few, mostly formerly in the genus Lomatophyllum, are known from some of the smaller Indian Ocean islands. The Arabian species have clear relationships with the species of northeast Africa. Madagascan species appear not to be closely related to those of mainland Africa and so active speciation seems to have occurred since the separation of these two land masses. Likewise, the former Lomatophyllum species form a group not represented on the African mainland.
Some species are very widespread in distribution. Reynolds (1966) cites A. buettneri as the most widespread species, with a range of at least 5,600km, from Mali to Zambia and the species has since been recorded in Namibia. However, Carter (1994) regards this as a West African species only, with two related species in the rest of the range reported by Reynolds. Another very widespread species is
A. myriacantha, with a range of about 4,800 km from Kenya and Uganda to the Republic of South Africa. Most other widespread species have more modest distribution ranges, amounting only to some hundreds of kilometres. At the other end of the scale there are species with very restricted distributions. They may be known only from one limited area (e.g. A. reynoldsii Letty) or only from the type locality (e.g. A. murina L.E.Newton). Some are known only from single isolated mountains (e.g. A. kulalensis).
Many countries have some endemic species, as shown in Table 1.1. The highest rate of endemism is in Madagascar and isolated Indian Ocean islands. Naturally, it is to be expected that very large countries, such as South Africa, and isolated islands, such as Madagascar, would have many endemic species. Conversely, it is not surprising that very small countries, such as Burundi and Rwanda have no endemic species. Of the small countries on the African mainland, only Lesotho and Swaziland have endemic species, with one each.
Table 1.1 Geographical distribution of the genus Aloe.
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