Biotic interactions and species diversification in Southern African biodiversity hotspots

Abstract

The south-western tip of Africa holds unique levels of species richness and endemism, and contains two hotspots of biodiversity: the Cape Floristic Region and the Succulent Karoo. Several hypotheses have been proposed to explain the radiation of the region’s diverse flora. However, due to the lack of comprehensive, comparative studies, the major forces that drive plant diversification have remained unclear. My thesis combines near-complete specieslevel molecular phylogenies with detailed biological, ecological and biogeographical information to investigate the evolutionary processes generating southern Africa's exceptional plant diversity. I demonstrate that in the genus Babiana (Iridaceae) climatic niches retain a high degree of phylogenetic conservatism, and show that species of Babiana were only able to successfully extend their range into new biomes with the establishment of more favourable climates. Additionally, results indicate that floral characters in Babiana evolve according to a new 'reversible shift' model, which better explains the evolution of pollination systems through multi-directional transitions in a diverse pollinator environment. These findings challenge the commonly held idea that floral specialisation is an evolutionary dead-end and offer new perspectives towards our understanding of plant-pollinator interactions. Analyses of a comprehensive data set of four large Cape clades show that the temporal dynamics of plant radiations in southern Africa confirm that the flora represents a combination of ancient and young radiations, and that diversification rates have remained constant through time. Finally, I reveal that although several biotic and abiotic factors contribute to the diversity, soil-type shifts is the most important driver of plant diversification in southern Africa. Together with complex geomorphological conditions, this factor, rather than pollinator specialisation or phenological divergence, has given rise to the exceptional diversity found in this region today. Comparisons with other biodiversity hotspots, especially those with Mediterranean climates, will reveal whether this is a global scenario for the evolution of hyper-diverse floras.