The Evolution of Regional Species Richness: The History of the Southern African Flora

Abstract

The flora of southern Africa is, for its latitude and area, very species-rich. Although the hyperdiverse Cape flora contributes almost half of this richness, three other radiations (desert, grassland, and woodland) contribute significantly to the botanical wealth of southern Africa. Each radiation occurs in a different ecological setting and has a different diversification history. Such parallel radiations can develop in suitably complex environments, given gradual change through time and no region-wide catastrophes. These four radiations cross-seeded each other, with clades spawning subclades in other radiations, thus linking all four into one complex radiation. This led to an increase in the number of diversifying clades in each radiation. Such complex radiations accumulate diversifying lineages over a long time, spawn daughter radiations on other continents, and become powerhouses of global-biodiversity generation. We suggest that several of the most species-rich regions may harbor such complex radiations.