The genus Brachylophus consists of four extant iguanid species native to the islands of Fiji and a giant extinct species from Tonga in the South West Pacific. One of the extant species, B. fasciatus, is also present on Tonga, where it has apparently been introduced by humans.
The name, Brachylophus, is derived from two Greek words: brachys (βραχύς) meaning "short" and lophos (λόφος) meaning "crest" or "plume", denoting the short spiny crests found along the backs of these species.
Brachylophus species are the most geographically isolated iguanas in the world. Their closest extant relatives (the genera Amblyrhynchus, Conolophus, Ctenosaura, Cyclura, Iguana and Sauromalus) are present in primarily tropical regions of the Americas and islands in the Galápagos and Lesser and Greater Antilles. Several of these genera are adapted to xeric biomes. Phylogenetic evidence supports the Brachylophus lineage to have diverged from the rest of Iguanidae in the latest Eocene, about 35 million years ago. They are the second most basal extant genus in the family, with only Dipsosaurus diverging earlier. The location of members of Brachylophus, so distant from all other known extant or extinct iguanids, has long presented a biogeographical enigma.
One hypothesis to account for this biogeographical puzzle, based in part on an estimated divergence date of 50 million years ago (although more recent studies have revised this to about 35 million years ago), is that these species are the descendants of a more widespread but now extinct lineage of Old World iguanids that migrated overland from the New World to Asia or Australia, and then dispersed by some combination of continental drift, rafting and/or land bridges to their present remote location. However, no other fossil or extant species of this putative lineage have been found to date in Southeast Asia, Australasia or the western Pacific outside of Fiji and Tonga.
A secondary hypothesis, and the more widely accepted one, is that these species evolved from New World iguanas that rafted west across the Pacific Ocean with the aid of the South Equatorial Current.
