Iguanas have often been spotted rafting around the Caribbean on vegetation and, ages ago, evidently caught a 600-mile ride from Central America to colonize the Galapagos Islands. But for long distance travel, the Fiji iguanas can't be touched.
A new analysis conducted by biologists at the University of California, Berkeley, and the University of San Francisco (USF) suggests that sometime after about 34 million years ago, Fiji iguanas landed on the isolated group of South Pacific islands after voyaging 5,000 miles from the western coast of North America — the longest known transoceanic dispersal of any terrestrial vertebrate.
Overwater dispersal is the main way newly formed islands get populated by plants and animals, including humans, often leading to the evolution of new species and entirely new ecosystems. Understanding how these colonizations happen has fascinated scientists since the time of Charles Darwin, the originator of the theory of evolution by natural selection.
The new analysis, to be published next week in the journal Proceedings of the National Academy of Sciences, suggests that the arrival of the ancestors of the Fiji iguanas coincided with the formation of these volcanic islands. The estimated time of the arrival, 34 million years ago or more recently, is based on the timing of the genetic divergence of the Fiji iguanas, Brachylophus, from their closest relatives, the North American desert iguanas, Dipsosaurus.
Previously, biologists had proposed that Fiji iguanas may have descended from an older lineage that was more widespread around the Pacific but has since died out, leaving Brachylophus as the sole iguanids in the western Pacific Ocean. Another option was that the iguanas hitchhiked from tropical parts of South America and then through Antarctica or even Australia, though there is no genetic or fossil evidence to support this.
The new analysis puts those theories to rest.
"We found that the Fiji iguanas are most closely related to the North American desert iguanas, something that hadn't been figured out before, and that the lineage of Fiji iguanas split from their sister lineage relatively recently, much closer to 30 million years ago, either post-dating or at about the same time that there was volcanic activity that could have produced land," said lead author Simon Scarpetta , a herpetologist and paleontologist who is a former postdoctoral fellow at UC Berkeley and is now an assistant professor at USF in the Department of Environmental Science.
"That they reached Fiji directly from North America seems crazy," said co-author Jimmy McGuire, UC Berkeley professor of integrative biology and herpetology curator at the Museum of Vertebrate Zoology. "But alternative models involving colonization from adjacent land areas don't really work for the time frame, since we know that they arrived in Fiji within the last 34 million years or so. This suggests that as soon as land appeared where Fiji now resides, these iguanas may have colonized it. Regardless of the actual timing of dispersal, the event itself was spectacular."
While sailors today can take advantage of favorable winds to reach Fiji from California in about a month, an iguana — or more likely a group of iguanas — would probably have taken much longer to ride flotsam through the doldrums and across the equator to Fiji and Tonga, where this group of iguanas is found. Luckily, iguanas are large and herbivorous and used to long periods without food and water. And if the flotsam consisted of uprooted trees, the raft itself would have provided food.
"You could imagine some kind of cyclone knocking over trees where there were a bunch of iguanas and maybe their eggs, and then they caught the ocean currents and rafted over," Scarpetta said.
The Fiji iguanas are an outlier
All told, there are over 2,100 species in the suborder Iguania, a large group that also includes animals such as chameleons, anoles, bearded dragons and horned lizards. What most people think of as iguanas are the Western Hemisphere family of lizards, Iguanidae, that include and mostly look like the widespread green iguana of Central and South America that Carl Linnaeus described as Iguana iguana in 1758. There are 45 species of Iguanidae living in the Caribbean and the tropical, subtropical and desert areas of North, Central and South America. These include the well-known marine iguanas of the Galapagos Islands, but also the chuckwallas of the American Southwest.
The Fiji iguanas are an outlier, sitting all alone in the middle of the Pacific. The four species on Fiji and Tonga are listed as endangered, primarily because of habitat loss, predation by invasive rats and exploitation by smugglers feeding the exotic pet trade.
Biologists had speculated, based on a few fossils found in east Asia, that an ancestral population of iguanids, now extinct, lived around the Pacific Rim and somehow made their way to the middle of the Pacific, island-hopping along the way. They may have journeyed by land and sea from America via the Bering Land Bridge and on through Indonesia and Australia or down along the Pacific coast of the Americas and through Antarctica. Or they could have rafted from South America with the Humboldt Current, gyring into the South Pacific.
Previous genetic analyses of a few genes for iguanid lizards were inconclusive about the relationship of the Fiji iguanas to all the rest. Scarpetta, while a postdoctoral fellow with McGuire a few years ago, embarked on a comprehensive survey of all genera in the Iguania to clarify the family tree of the group.
"Different relationships have been inferred in these various analyses, none with particularly strong support," McGuire said. "So there was still this uncertainty about where Brachylophus really fits within the iguanid phylogeny. Simon's data really nailed this thing."
Scarpetta collected genome-wide sequence DNA from more than 4,000 genes and from tissues of more than 200 iguanian specimens housed in museum collections around the world. As he began comparing these data, one result stood out clearly: The Fiji iguanas are most closely related to the iguanas in the genus Dipsosaurus. The most widespread of these is the North American desert iguana, Dipsosaurus dorsalis, which is adapted to the searing heat of the deserts of the American Southwest and northern Mexico. The other species in the genus is native to Santa Catalina Island in the Sea of Cortez.
"Iguanas and desert iguanas, in particular, are resistant to starvation and dehydration, so my thought process is, if there had to be any group of vertebrate or any group of lizard that really could make an 8,000 kilometer journey across the Pacific on a mass of vegetation, a desert iguana-like ancestor would be the one," Scarpetta said.
The analysis determined that the two lineages, Brachylophus and Dipsosaurus, diverged about 34 million years ago, which doesn't fit with earlier theories of the origin of the Fiji iguanas.
"When you don't really know where Brachylophus fits at the base of the tree, then where they came from can also be almost anywhere," McGuire said. "So it was much easier to imagine that Brachylophus originated from South America, since we already have marine and land iguanas in the Galapagos that almost certainly dispersed to the islands from the mainland."
With the new analysis, a South American origin can be ruled out. And because the Fiji Islands emerged from the sea also about 34 million years ago, the iguanas may have serendipitously intersected the islands not long after. Other islands aside from Fiji and Tonga could also have harbored iguanas, Scarpetta noted, but it is the nature of volcanic islands to disappear as readily as they appear. Evidence of other Pacific Island iguanas, if they existed, has probably been lost.
Scarpetta, who has been enamored with salamanders, snakes and lizards since before high school, continues to analyze genome-wide data for Iguanian lizards to learn more about their evolutionary relationships and to infer their movements and interactions through time and space.
Other co-authors of the paper are Robert Fisher of the U.S. Geological Survey in San Diego, Benjamin Karin and Ammon Corl of UC Berkeley, Jone Niukula of NatureFiji-MareqetiViti in Suva and Todd Jackman of Villanova University in Pennsylvania. Scarpetta was supported by a National Science Foundation postdoctoral research fellowship.
