Focus for Ntwai's PhD-project is phylogenetic relationships and historical biogeography in the southern African Silene , see abstract below. Opponent at the seminar is Urban Olsson, Department of biological & environmental sciences.
In systematics, like other related disciplines, species is considered the most fundamental biological unit. Further reinforcing the emphasis on the importance of species is being assigned a categorical rank in the taxonomic hierarchy. Despite the importance, the concept of “species” is surrounded by much confusion due to the lack of a consensus among biologists regarding the definition of species. The criterion of monophyly has allowed systematists to develop and rigorously test species hypotheses using empirical data. Additionally, species delimitation approaches under the multispecies coalescent model offers a very precise and non-arbitrary definition of species-as-branches of phylogenetic trees. Thus, through parametrization, the model allows explicit assumptions to be made where biologists can apply data to test its fit to particular delimitations. However, for both theoretical and practical reasons, the application of such a concept is premature, and not likely to meaningfully improve biodiversity measures at this point. Diversification studies are also affected, as branching events will depend on the number of tips present in a tree. However, this effect will decrease the more ancient these events are.
We utilized the multispecies coalescent model to study phylogenetic relationships and historical biogeography in the southern African Silene L. (Caryophyllaceae). There are eight recognized species in southern Africa, taxonomically placed in three sections: Elisanthe (Fenzl ex Endl.) Ledeb., Dipterospermae (Rohrb.) Chowdhuri and Fruticulosae (Willk.) Chowdhuri. Although the taxonomy of the southern African taxa has recently been revised, their phylogenetic relationships and biogeographic history remain unclear. Thus, the aims of our study was to infer the phylogenetic position and geographic origins of the southern African taxa. We generated DNA sequences of nuclear and plastid loci from several individuals belonging to all eight species of Silene recognized from southern Africa. We used a Bayesian coalescent species tree continuous diffusion approach to co-estimate the species tree and the ancestral areas of representative members of the genus. Our results show that the perennial southern African members of section Elisanthe form a strongly supported clade with the Eurasian annual S. noctiflora and the Central Asian perennial S. turkestanica. The perennial species currently placed in section Fruticulosae form a strongly supported clade together with the annual S. aethiopica (Dipterospermae), which is nested in a larger Mediterranean clade comprising mostly annual species classified in section Silene s.l. Estimates of ancestral areas indicate a late Pleistocene dispersal to southern Africa from central and east Africa for the Fruticulosae-Dipterospermae clade. The Elisanthe clade is inferred to have colonized southern Africa through long-distance dispersal from the southwestern Mediterranean region during the late Pleistocene. Our findings support the hypothesis of a relatively recent colonization into southern Africa resulting from two independent dispersal events during the Pleistocene.