April 2025

Tropical islands like Hawaii, Tahiti, and Fiji offer locals and tourists alike a range of enticements – lush foliage, balmy weather, and beaches galore. But for biologists, who view such places not just through tinted sunglasses, but also through the lens of evolution, these vacation destinations offer intriguing mysteries as well. Such islands were produced by volcanic activity, arising out of the ocean devoid of life. So each represents a compelling test case of evolution in action starting from scratch. How did the ancestors of the living things that inhabit these islands today get there in the first place? How did each newly arrived species change the environment for the next? How did they all evolve once they arrived? How did waves of human migration shape the evolutionary trajectories of these isolated ecosystems? Last month, biologists announced the results of a study that solves one of these mysteries: how iguanas arrived on Fiji.

Where's the evolution?

The islands of Fiji are located in the South Pacific, around 1000 miles north of New Zealand and nearly 2000 miles from the coast of Australia. They are volcanic, like Hawaii, but much older. The Hawaiian island of Kauai, for example, is around five million years old, while Fiji has existed for around 34 million years. Nevertheless, in terms of Earth history, that is relatively young. When Fiji arose, Pangaea had long since broken up, the non-avian dinosaurs had been extinct for 30 million years, and mammals and birds had already diversified.

Today, four species of iguana are found on Fiji. But they are outliers in their part of the world. Australia and Asia have no native iguanas, nor have iguana fossils been found on these continents. Nearby Tonga was once home to two species of iguana that have now been extinct for several thousand years, but iguanas don’t live on other islands in the South Pacific. In fact, all other iguana species on Earth are native to North and South America, a whole ocean away.

The Fijian iguanas form a clade – that is, a branch of an evolutionary tree, or a group in which all the members of the group are each other’s closest relatives. The simplest, or most parsimonious, explanation for this distribution is that that sometime in the past 34 million years, a few iguanas of one species made it to Fiji and was cut off from its source population. In isolation, the Fijian population and the source population began accumulating genetic differences from one another, eventually producing two distinct lineages in a process known as allopatric speciation. And at some point, the Fijian population further diversified into the multiple species that are found there today. This scenario (one colonization and subsequent diversification) is much more likely than the alternative – that several distinct iguana species, which all happen to be close relatives, each independently left their homes and wound up on Fiji. There’s little question about this basic scenario, but researchers wondered, what was that source population, how did the ancestral iguanas get to Fiji, and when did this all happen?

To answer these questions, researchers collected and sequenced DNA from 14 species of iguanas and used those data to reconstruct the evolutionary relationships among them. That evolutionary tree, or phylogeny, is shown below. You can see that Fijian iguanas, represented here by the species Brachylophus bulabula, are most closely related to Dipsosaurus, a group of desert iguanas from North America. These two groups are sister taxa – that is, they are each other’s closest relatives and share a common ancestor that no other species has. The researchers also reconstructed the most likely geographic ranges of the ancestral iguana lineages (indicated by the coloration of the branches on the tree below) and found that the shared ancestor of Brachylophus and Dipsosaurus almost certainly lived in North America. That means that, sometime in Fiji’s history, a group of iguanas must have traveled there from North America, a journey of at least 5000 miles.

The scientists also used the extent of the genetic divergence among different lineages, along with iguana fossils with known date ranges, to estimate the timing of the different speciation events shown on the phylogeny. This analysis found that Brachylophus and Dipsosaurus probably diverged around 34 million years ago – shortly after or right around the time that Fiji formed. Furthermore, at that time in Earth history, land near the poles would have been very cold and covered in ice. This limits the potential migration routes that the tropical iguanas could have taken. They would not have survived the long journey to Fiji by land going either north (via the Bering Strait) or south (hopscotching off Antarctica). Instead, the scientists hypothesize that, 34 million years ago, a few North American iguanas wound up rafting to Fiji across the Pacific Ocean on a mat of vegetation.

The notion of this fateful trip is not as far-fetched as it might seem. The source population of desert iguanas was probably well suited to overcome the hardships of the voyage. Dipsosaurus iguanas can survive long periods without food or water and are resistant to heat. And Earth systems modeling suggests that the direction of ocean currents could have sped their journey along. Furthermore, we’ve seen this sort of thing happen today – albeit for shorter trips. In 1995, after an ocean journey of 200 miles, 15 iguanas arrived on the island of Anguilla in the Caribbean on a massive mat of trees uprooted by a hurricane.

The new study clarifies where the Fijian iguanas came from and how they got to the island, but of course, doesn’t tell us what ecosystem they encountered when they arrived, how they adapted to the local environment, or how they diversified. For biologists, this test case of evolution in action holds many more mysteries to be unraveled.