Is south-western Western Australia a centre of origin for eastern Australian taxa or is the centre an artefact of a method of analysis? A comment on Hakea and its supposed divergence over the past 12 million years

Abstract

Lamont et al. (2016) concluded that the Australian sclerophyllous genus Hakea (Proteaceae) arose 18million years ago in the South West of Western Australia (SWA) and dispersed 18 times to eastern (EA) and central Australia (CA) only 12million years ago (mid-Miocene). Their explanation of the biogeographic history of Hakea was based on the following: accepting a fully resolved molecular phylogenetic tree, although ~40% of nodes had posterior probability values below 0.95; using all nodes including geographically paralogous nodes to determine ancestral area probabilities; and applying a strict clock to estimate clade divergence times. Our re-analyses of the same dataset using a relaxed clock model pushes the age of Hakea to 32.4 (21.8–43.7) million years ago relative to its nearest outgroups, and the age of the divergence of two major clades (A and B) to 24.7 (17.2–33.7) million years ago. Calibration based on a new finding of Late Cretaceous fossil Banksia pushes these dates to 48.0 (24.3–75.2) million years ago and 36.6 (18.5–55.4) million years ago respectively. We confirm that each of the two main clades includes lineages in SWA, CA and EA. At the basal node of Clade A, two eastern Australian species form the sister group to three SWA scrub–heath–Eremaean species. These two groups together are sister to a large, mostly unresolved clade of SWA, CA and EA taxa. Similarly, at the base of Clade B is a polytomy of lineages from the SWA, CA and EA, with no resolution of area relationships. There is no evidence of a centre of origin and diversification of the genus is older than the mid-Miocene, being at least Oligocene, and probably older, although calibration points for molecular dating are too far removed from the ingroup to provide any great confidence in the methodology. Consideration should be given to the possibility of vicariance of multiple, widespread ancestral lineages as an explanation for lineages now disjunct between EA and SWA.