The Evolution of Wahlenbergia (Campanulaceae) in Australasia
Wahlenbergia is a large genus of flowering plants within the family Campanulaceae. In this thesis the first molecular phylogeny of Wahlenbergia was reconstructed from approximately 20% of the genus, based on the nuclear ribosomal ITS (nrITS) DNA marker and the chloroplast trnL-F DNA marker, with samples from South Africa, Europe, Australia and New Zealand. Additionally a large phylogeny with increased within-species sampling focusing on addressing taxonomic questions among the 45 Australasian species of Wahlenbergia was also reconstructed based on nrITS and trnL-F, plus an additional chloroplast DNA marker, trnK. Relationships and species limits of the New Zealand species of Wahlenbergia were further analysed using amplified fragment length polymorphisms (AFLPs). Wahlenbergia was found to be polyphyletic, though most of the species form a clade. Tree topologies and molecular dating analysis showed that the genus originated in South Africa about 16.2 million years ago (mya), then dispersed to Australasia before radiating there about 3.7 mya, thus refuting the hypothesis of Gondwanan vicariance for the Australasian species. Two dispersals from Australia to New Zealand are hypothesised, one leading to a radiation of species with the rhizomatous growth from about 1.0 mya and the other leading to a radiation of species with the radicate growth form 0.49 mya, although the radicate species might not form a clade. Low levels of genetic variation among individuals from Australia and New Zealand was revealed with all markers, and the phylogenies were poorly resolved as a result. The low genetic diversity is probably due to rapid and recent evolution during a period of geological and climatic change, coupled with incomplete lineage sorting and hybridisation. Phylogenies reconstructed using AFLPs were also poorly resolved, although AFLPs were found to be useful for species delimitation, as has been shown in studies of other plant groups. Despite the poor resolution, several morphological species and subspecies were recovered as monophyletic with DNA sequence data, notably the morphologically distinctive New Zealand W. cartilaginea, W. matthewsii and W. congesta subsp. congesta. Further research into species boundaries within the W. albomarginata/W. pygmaea complex is needed. Members of the New Zealand lowland radicate W. gracilis complex may all belong to the same morphologically variable species, although further research is needed to justify such a taxonomic change. The other New Zealand radicate species, W. vernicosa, is probably a separately evolving lineage, and is not conspecific with the Australian W. littoricola.