Diversification dynamics and evolution of the Australian temperate flora

The Australian temperate flora mostly comprises sclerophyllous elements that are a dominant and characteristic feature of the southern half of the Australian continent. Despite being a characteristic component of the Australian biota, the diversification dynamics and evolutionary history of this flora remains poorly understood. In particular, studies on evolutionary drivers that gave rise to the plant species disparity observed between the southwest Australian global biodiversity hotspot region (SWA) and southeastern temperate Australia (SEA) are lacking. In this thesis, I address this gap in knowledge through assessments of (1) differences in diversification dynamics between the two Australian temperate floras at different spatial and temporal scales, (2) biogeographic patterns of different groups found in the two regions, and (3) potential evolutionary drivers that might have given rise to these patterns. I utilise a molecular phylogenetic approach and test for region-specific diversification dynamics as well as more detailed investigations on selected case study groups. In Chapter 1, a broad-scale genus-level meta-analysis across 21 plant families is used to assess differences in diversification dynamics between the two regions under several diversification models. I demonstrated that a major extinction pulse event at the Eocene–Oligocene boundary (c. 34 Ma) negatively affected the diversification of major plant groups in SEA, but not in SWA, and this in part, explains current disparities in species richness between the two regions. In Section 2, Chapters 3–5, I assess biogeographic patterns and evolutionary histories of three case study groups of Australian temperate flora: Calytrix (Myrtaceae), Pomaderris (Rhamnaceae), and Adenanthos (Proteaceae). Results indicate all of these plant groups previously had a more widespread distribution, and retreated to the mesic periphery of the continent as Australia progressively cooled in the Oligocene and became increasingly arid from the Miocene onwards. I uncovered significant incongruences between nuclear and chloroplast topologies of Adenanthos, complicating biogeographic inferences for this genus, but revealed a potential link between the radiation of Adenanthos with multiple introgression events during its early evolutionary history in SWA. The lineage-specific evolutionary responses of these case study groups indicate the need for a broader meta-analysis of multiple groups in order to better understand the evolutionary history of the wider regional flora. In Chapter 6 I collate dated phylogenies of 22 Australian temperate plant groups and use these to assess for radiation shifts, evidence of density-dependent slowdowns indicative of niche-space filling, and differences in diversification rates between SWA and SEA. In addition, I compare average region-wide diversification rates of each of the two regions with other global centres of biodiversity. These comparisons indicate that the diversification dynamics of the Australian flora are fundamentally different to most other biodiversity hotspots and species-rich biomes and that SWA that has no comparable global analogue. My thesis highlights the antiquity of the Australian temperate flora, its gradual accumulation of lineages, lack of recent radiation shifts, the persistence of ancient lineages, and low diversification rates. The lack of extreme large-scale environmental pulse events and tectonic activity across the subdued Australian landscape may explain these patterns. I conclude that evolutionary drivers for the Australian temperate flora may be fundamentally different to those seen in other biodiverse regions of the world.Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 202