Diversification on an ancient Darwinian island : Evolutionary history of caddisflies (Trichoptera) and other organisms on New Caledonia

Abstract: Islands are either of continental or oceanic origin, and the biota of such islands are a result of vicariance and dispersal, respectively. New Caledonia, in the South Pacific, is a part of former Gondwana, but the origin of its biota is heavily debated. Geological studies show that the island was submerged in the Eocene, and that oceanic crust was obducted onto New Caledonia leaving a thick layer of ultramafic rocks on top of the island. We tested the null hypothesis that New Caledonian biota are a result of old Gondwanan vicariance by fitting diversification models on phylogenies from the literature and calculating the gamma statistic. According to the null hypothesis diversification is gradual over time. If on the other hand the biota are a result of recent dispersal or have survived in refugia we would expect slowing of diversification over time. Based on our results we can reject the null hypothesis of old vicariance for the New Caledonian biota. The caddisfly (Trichoptera) fauna on New Caledonia is exceptionally rich with around 600 species, and more than 99.9% are endemic. Molecular phylogenetic hypotheses for three monophyletic caddisfly radiations, Xanthochorema, Caledopsyche et al. and Agmina, are presented. Age estimates agrees with the geological evidence. Three new species of Xanthochorema are described and more than 63% of the Agmina species are new to science. The ultramafic rock substrate, with high heavy metal concentration, has led to high levels of plant adaptation on New Caledonia. We show that caddisflies adapted early to this substrate and subsequently dispersed to non-ultramafic substrate several times, with significantly fewer dispersal events back to the toxic substrate, indicating that adaptation to ultramafic substrate is required and recolonization is difficult when this adaptation has been lost. Ultramafic substrate and other environmental factors have been important in driving the diversification of New Caledonian caddisflies.