An Evolutionary Biogeography of the Genus Aloe

The genus Aloe (Liliaceae) ranges from the southern tip of Africa to the Arabian Peninsula, and is also found on Malagasy and Socotra, in the Guinea savannas of West Africa and widely, now, in the Old and the New Worlds where it was introduced at various times over the past four thousand or so years. In so far as its native range is concerned, it is suggested that the centre of origin for the genus is in the highlands of SE Africa whence the ancestral aloes spread during the Tertiary era. Eleven secondary centres of speciation arerecognized in the 'High Africa' physiographic province of the continent which, between them, contain 82% of the recognized species. Over the generic range, species density increases directly with terrain diversity, and approximately 50% of the variance in the former is 'explained' by the latter variable. Contemporary conditions of climate, soils and vegetation cover do not increase the level of explanation of geographic variations in species density over the generic range, and it is proposed that the unexplained variance reflects the long period of time available for evolution of new species in the various centers of diversity.