Sapotaceae biogeography supports New Caledonia being an old Darwinian island
Panbiogeographers suggest that the biome in New Caledonia is of vicariant origin, dating from the Cretaceous – rather than being the result of repeated dispersal since c. 37 Ma, when the area is postulated to have re-emerged after c. 15 million years of submergence. Distributions of the plant family Sapotaceae were used as a model system to test this, and to elucidate the probabilities of ancestral areas, all phrased in six hypotheses.
Australasia and the Pacific.
We used a recently published dataset with extensive sampling (168 terminals) from the subfamily Chrysophylloideae and three nuclear ribosomal DNA markers. Phylogenetic divergence times and ancestral areas were estimated in a Bayesian framework using beast, a relaxed clock method, and with fossil calibration points. Area transition probabilities were modelled using a reversible rate matrix, assigning equal prior probability to each transition between two areas.
Our analyses suggest that Sapotaceae arrived and diversified in New Caledonia nine times during the period 4.2–33.1 Ma. All crown-node radiations occurred in the Miocene or Pliocene, with stem splits reaching back into the Oligocene. Australia and New Guinea are the most likely source areas for Sapotaceae in New Caledonia, but this archipelago has never acted as a stepping stone for Sapotaceae to disperse into the Pacific.
Repeated dispersal is the only mechanism able to explain the range expansion of Sapotaceae into New Caledonia. The family has successfully colonized the main island nine times since its re-emergence in the Eocene. We reject the panbiogeographical hypotheses that representatives of Sapotaceae in New Caledonia originated in the Cretaceous, differentiated due to vicariance, and were of Pacific origin. We therefore argue that New Caledonia is an old Darwinian island. The Pacific has been colonized repeatedly and terminal lineages are never older than the islands they inhabit (except for Hawaii). Chrysophylloideae extended across Wallace's Line into Southeast Asia around 20 Ma, when the Australian continent came into juxtaposition with Eurasia.