While insular radiations are documented for many terrestrial arthropods, few examples are known for eusocial insects. The purpose of our study was to ascertain whether the spinescence observed among Fijian Pheidole ants was inherited from overseas ancestors or is evidence of ecological release upon colonizing an island depauperate of interspecific competitors. We analyzed molecular data to test three hypothesis previously proposed for the Pheidole roosevelti group: (i) the group is monophyletic; (ii) spinescence is a plesiomorphic trait inherited from an overseas ancestor; and (iii) the group is most closely related to the New Guinean P. cervicornis group. The analysis included fragments of two mitochondrial genes (COI, cytb) and two nuclear genes (H3, EF1a-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 lineage of ants that colonized a small, oceanic island in the Miocene, and proceeded to radiate across the emerging islands of the archipelago into niche-space occupied elsewhere in the Pacific by distantly related spinescent congeners. Specifically, we predict that 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.
Evan Economo and I are now using these Pheidole ants as a model for test predictions of the taxon cycle hypothesis.
Economo, EP & EM Sarnat (Submitted) The old regional and new global taxon cycle in ants.
Sarnat, EM & CS Moreau (2011) Biogeography and morphological evolution of a Pacific island ant radiation. Molecular Ecology 20:114-130. [pdf]
Sarnat, EM (2008) A taxonomic revision of the Pheidole roosevelti-group (Hymenoptera: Formicidae) in Fiji. Zootaxa 1767:1-36. [pdf]