Iguanas rafted more than 8,000 km from North America to Fiji

Abstract

Founder-event speciation can occur when one or more organisms colonize a distant, unoccupied area via long-distance dispersal, leading to the evolution of a new species lineage. Species radiations established by long-distance, and especially transoceanic, dispersal can cause substantial shifts in regional biodiversity. Here, we investigate the occurrence and timing of the greatest known long-distance oceanic dispersal event in the history of terrestrial vertebrates-the rafting of iguanas from North America to Fiji. Iguanas are large-bodied herbivores that are well-known overwater dispersers, including species that colonized the Caribbean and the Galápagos islands. However, the origin of Fijian iguanas had not been comprehensively tested. We estimated the phylogenetic relationships and evolutionary timescale of the iguanid lizard radiation using genome-wide exons and ultraconserved elements (UCEs). Those data indicate that the closest living relative of extant Fijian iguanas is the North American desert iguana and that the two taxa likely diverged during the late Paleogene near or after the onset of volcanism that produced the Fijian archipelago. Biogeographic models estimate North America as the most probable ancestral range of Fijian iguanas. Our analyses support the hypothesis that iguanas reached Fiji via an extraordinary oceanic dispersal event from western North America, and which spanned a fifth of the earth's circumference (>8,000 km). Overwater rafting of iguanas from North America to Fiji strengthens the importance of founder-event speciation in the diversification of iguanids and elucidates the scope of long-distance dispersal across terrestrial vertebrates.

Keywords: Fiji; dispersal; island biogeography; lizards; phylogenomics.

Fig. 1.

Plot of the posterior age distribution of the Brachylophus–Dipsosaurus clade across divergence time analyses. The center symbol is the median divergence time from each analysis and the black bar is the 95% HPD interval.

Fig. 2.

Phylogenomic timetree of iguanas based on StarBeast3 analysis of 150 loci (50 AHE, UCE, RELEC) and three fossil calibrations (for brevity, only two calibrations labeled and outgroups removed), and time-stratified DEC+J analysis from BioGeoBEARS using areas allowed and manual dispersal matrices, and additional areas added to accommodate all alternative hypotheses for the origin of Fijian iguanas. Pie charts indicate the relative probability of the possible ancestral geographic ranges at nodes and at splits immediately after the corresponding cladogenetic event, and tip boxes indicate extant species ranges. Stars at nodes indicate fossil calibrations. The globe inset shows a representation of the transoceanic dispersal of iguanas from North America to Fiji that occurred at the divergence between Dipsosaurus and Brachylophus or along the Brachylophus branch.

Fig. 3.

Hypothesized biogeographic scenarios for the colonization of Fiji by Brachylophus, occurrences of fossil iguanids (i.e., Pumilia, Armandisaurus, and Queironius), and distribution of modern iguanids. The world map is set at 34 Ma on a Robinson projection. The paleogeographic map data were assembled using the R package rgplates using the plate model from Müller et al. (52, 53).

Fig. 4.

(A) Distances between island and mainland for extant iguanid lizards and (B) distances for other proposed long-distance, overwater dispersal events in terrestrial vertebrates. For the iguanids, a minimum distance was taken for each species between the mainland and the closest island inhabited by each taxon to the mainland using GoogleEarth (19). Species occurrence data were taken from ref. . Species that are not found on islands are at 0, and those that are found on both island and mainland are marked with a gray circle next to the distance bar. A dark gray line across the upper ends of adjacent bars mark taxa that (almost certainly) resulted from a single dispersal event. Bars are color coded by genus and the legend also lists the relevant island(s) for exclusively island radiations. For other vertebrate dispersal events, distances were taken from refs. , , –, , and/or measured during the relevant time period using GPlates (52). Asterisks above bars indicate the longest possible dispersal distances, and white dashes on the same bars indicate maximum distances given a steppingstone model.