Comparative dating of Acacia: combining fossils and multiple phylogenies to infer ages of clades with poor fossil records

The ubiquitous and highly diverse element Australian Acacia makes an ideal candidate for investigating a range of questions about the evolution of the flora of continental Australia. In the past, such efforts have been hampered by a lack of well-supported phylogenies and by the relatively poor macrofossil record, which probably reflects the depositional environment in which Acacia species are predominantly found. However, the broader subfamily Mimosoideae offers several reliably age-constrained fossils that can be used as calibrations in divergence-dating analyses of DNA sequence data. In addition, the microfossil pollen record of Acacia is relatively rich and provides a good age constraint for the entire Acacia clade. By using multiple reliable fossil constraints, we applied a combination of primary calibration points to produce a comprehensive study of divergence dates in Acacia s.s. and related mimosoid legumes. Previous dating studies included very limited samples of the diversity of Australian Acacia and experienced difficulties in identifying appropriate age calibrations for the lineage, leading to considerable variation in their results. We used novel calibration schemes and multiple nuclear and chloroplast DNA sequence markers to produce the first estimates of divergence dates for major lineages within the Australian Acacia s.s. clade and for related lineages across the Mimosoideae subfamily. We estimate average crown divergence dates for Vachellia at 13–17 Ma, Senegalia at 31.0–33.4 Ma and Acacia s.s. at 21.0–23.9 Ma. The timing of radiations within these lineages is consistent with the hypothesis that Miocene aridification in Africa, the Americas and Australia was a driver for the diversificationss of lineages in Acacia s.l.