Biogeography and morphological evolution in a Pacific island ant radiation.

While insular radiations are documented for many terrestrial arthropods, few examples are known for eusocial insects. This study seeks to ascertain whether the spinescence observed among Fijian Pheidole ants was inherited from an oversea ancestor or is evidence of ecological release from interspecific competitors. We broaden our understanding of morphological convergence, insular radiation and Pacific biogeography by testing three hypotheses proposed previously for the Fijian Pheidole roosevelti group: (i) the group is monophyletic; (ii) spinescence is a plesiomorphic trait inherited from an overseas ancestor; and (iii) the group is closely related to spinescent New Guinean relatives. The analysis included the fragments of two mitochondrial genes (COI, cytb) and two nuclear genes (H3, EF1α-F2) from 66 taxa, including all members of the roosevelti group, representatives from the spinescent subgenus Pheidolacanthinus, Fijian congeners and widespread Pacific congeners. Our results yield new insights into the biogeographic history of Fiji, reveal a fascinating example of convergent evolution and serve as a novel example of ecological release occurring within an insular eusocial insect lineage. These findings recover the history of a presumably unremarkable ant species that colonized a remote oceanic archipelago in the Miocene (17-10 Ma) and radiated across the emerging islands into niche-space occupied elsewhere in the Pacific by distantly related spinescent congeners. We propose the radiation of Fijian Pheidole into spinescent morphotypes was the consequence of ecological opportunities afforded by the absence of competing ant lineages with conspicuous epigaeic foraging strategies.