Three species of iguanas in the genus Brachylophus are restricted to the remote Pacific islands of Fiji and Tonga, and represent the most geographically isolated iguanas in the world. For years, they were believed to be the descendants of South American iguanas that had rafted to the islands some 13 million years ago. The prospect of a pregnant iguana, or a boa constrictor for that matter, taking refuge on a mass of floating vegetation and drifting for thousands of miles across the Pacific until it ultimately makes landfall on a desert isle has long been biogeography's ace in the hole. Typically referred to as a "sweepstake" route, this type of somewhat random, long-distance dispersal can certainly be used to explain unusual distribution patterns. Got a group of organisms that are distributed on a continental mainland and on some, but not all, nearby (or not so nearby) islands? Then we can postulate that it rafted from the mainland to those islands where it's found, and not the one's where it's missing. Without question, this is undoubtedly the correct answer to some biogeographical puzzles. However, as with any ace in the hole, you have to be careful how you use it. The probability of such an event, although real, is astronomical. If we have to pull out the sweepstakes dispersal argument too often, it begins to lose its power.

The Fijian iguanas are among the most problematical of such cases - it's a long, long way from South American to Fiji. Even with drifting continents, it's been a long way for a long time. The working hypothesis had the ancestors of the islanders drifting perhaps as much as 5,000 miles. New evidence suggests that it might not have been that difficult - maybe they just walked. Brian Noonan of the University of Mississippi (that's Ole Miss for you SEC fans) and Jack Sites from Brigham Young have published work in The American Naturalist in which they use new molecular data to produce a new estimate regarding the divergence of the island iguanas from their continental relatives. Their analysis suggests a much more ancient divergence of the two lines than had previously been believed. Noonan and Site's data suggests that the Fijian iguanas diverged from their South American cousins over 50 million years ago. This changes the playing field. Literally.

In the Eocene epoch, the position of the continents was dramatically different. There was likely a land bridge (or, at least, an "island-hopping") connection between South America and Antarctica, and between Antarctica and Australia. Throw in the considerably warmer and more homogeneous climate patterns of the Eocene, and we have a much more workable hypothesis to get our iguanas to the isolated islands.

But wait... If we can explain the presence of iguanas on Fiji and Tonga as the result of a widespread distribution across the Southern Hemisphere dating back to the Eocene, then why aren't they found on many other islands in the South Pacific? Noonan and Site have a plausible explanation for that as well. Fossil evidence suggests that iguanas did, in fact, inhabit other South Sea islands, but disappeared. In most cases, that disappearance seems to coincide with the arrival of humans on the islands. And yes, the islanders apparently had a sweet tooth for iguana flesh.

So why haven't the species found on Fiji and Tonga disappeared yet? Well, simply because humans haven't been on those islands as long as an most islands in the region. But give us time...