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.