Progress and prospects for understanding evolution and diversity in the southwest Australian flora

The flora of southwest Australia has often been described as remarkable and special, both in an Australian and a global context, particularly because of its high species richness and endemism. Many explanations for the special characteristics of the region’s flora have been proposed, most invoking a special evolutionary history. Relatively few studies, however, have explicitly compared either the floras or histories of southwest Australia and southeast Australia, a useful comparison as both may be assumed to have started with a similar flora and to have related histories. Such comparisons may be useful in discriminating the many factors, both historical and ahistorical, that may explain differences in richness and endemism. We analyse here flowering plant checklists from geographically comparable areas in southeastern and southwestern Australia to describe and quantify floristic differences, confirming that southwestern Australia has higher species richness but lower generic and family richness than southeastern Australia, and review previous explanations for these differences. We conclude that, while much has been achieved since Joseph Hooker first formally described these differences in 1859, much remains to be understood and knowledge gaps and paradoxes remain. Current explanations, while plausible, remain unproven, and differences in histories may or may not be the best explanations. Framing investigations of the special characteristics of southwestern Australia around null hypotheses may help provide a necessary rigour to such analyses.