Diversification patterns of the southwest Australian biodiversity hotspot reveal a novel macroevolutionary pathway to plant hyperdiversity.

The macroevolutionary drivers for disparities in plant species richness across Australia are understudied, hindered by lack of densely sampled comparative phylogenetic data. Here, we address this gap by analysing plant diversification dynamics and quantifying macroevolutionary trajectories of 22 plant clades (4289 species in 14 families) across two Australian regions. We show the southwest Australian (SWA) floristic region differs from southeastern Australia (SEA) in having relatively low speciation and extinction rates, fewer recent rapid radiations, declining speciation rates, relatively high rates of sympatric speciation indicative of greater niche space saturation, and an absence of mass extinction events since the Eocene. We show that low diversification rates in SWA can be attributed to the early diversification and old age of its flora. By contrast, the SEA extant flora largely diversified after the Eocene-Oligocene boundary extinction event. The paucity of diversification rate shifts across the two floras is likely linked to the lack of extreme environmental pulse events on the geologically stable Australian continent. Our results uncover the dynamics, which have shaped the SWA region as a current centre of hyperdiversity in a continental and global context and suggests an alternate pathway to diversity, not found in other biodiversity hotspots.