The exemplar species of the Pheidole roosevelti group radiation among the Fijian islands.

Pheidole roosevelti descends from 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.

The ornate armature of Pheidole roosevelti is thought to be the consequence of ecological opportunities afforded in Fiji by the absence of competing ant lineages with similar foraging strategies.