Diversification in Biodiversity Hotspots
Part of my research has focused on trying to understand why biodiversity hotspots (the regions of the globe with the highest diversity of species) harbour so many species. In order to answer this question, I have worked with molecular phylogenies of plant and animal lineages from the tropics, Mediterranean-climate regions and islands, investigating them in a temporal and biogeographical context.
The 5 Mediterranean-climate regions of the Earth harbour 20% of the known species of flowering plants. However, there are strong differences in diversity between different Mediterranean climate regions, with the Cape Floristic Region being by far the richest. I co-authored a review paper where we examined the causes for the differences in diversity between the Mediterranean Basin and the Cape. We found that the Cape harbours older plant lineages and that diversity in the Cape does not appear to be bounded by an upper limit. In contrast, the Mediterranean Basin is home to young clades, but many of its lineages have diversified at rapid rates. This work was done in collaboration with Dr. Pablo Vargas.
In order to compare diversification patterns in the Cape and the Mediterranean Basin in more detail, I built the phylogeny of Gladiolus (>260 species, Iridaceae), a clade that has >100 species in the Cape, but only seven species in the Mediterranean Basin. I showed that diversification rates had been in higher in South Africa than in Europe.
Given that Gladiolus has one of the most diverse pollination systems known in any genus, I then investigated whether pollination shifts could have driven diversification of Gladiolus in southern Africa. I compiled one of the largest animal-pollinator interactions datasets for a single genus and analysed it in a molecular phylogenetic context. The results of this study defied the longstanding view that speciation in the Cape was mostly associated with pollination shifts, and revealed the most dynamic pattern of plant-pollinator evolution documented to date. It was also the first study to show a significant effect of type of pollination system (rather than pollination shifts per se) on rates of diversification.
"Explosive" plant radiations
I have studied in detail several hyperdiverse plant clades that have diversified at rapid rates. I used diversification models to show that European carnations (Dianthus, > 100 species) have formed new species at the fastest rate ever documented in a group of terrestrial organisms (they arose at the rate of 2.2–7.6 species per million years). This surprising finding challenged the idea that diversity evolves faster in oceanic islands and the tropics, and revealed that plant diversification can proceed very rapidly in temperate clades.
I also investigated the radiation of Carex (>2000 species, Cyperaceae), one of the four most diverse genera of angiosperms in the world (in collaboration with Dr. Marcial Escudero) and of the ice plants (Aizoaceae), a clade of over 1500 species in the arid regions of the Greater Cape Floristic Region which has diversified very rapidly (Valente et al. under review).
Diversification in the tropics
I have been involved in phylogenetic studies of tropical clades with the aim of investigating the causes of high species diversity in these regions. I conducted analyses of diversification in Orchidaceae plants from the Neotropics and the charismatic guenons, one of the largest radiations of primates.
Guenons (photos by K. Guschanski).