Published online 27 March 2008 | Nature | doi:10.1038/news.2008.695

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The rapid evolution of tuatara

Study of the 'living fossils' may challenge theory of rate of evolution in cold-blooded organisms.

Is the sluggish tuatara evolving quickly?S. KEALL V. WARD

The tuatara does not seem to be the first place one would look for an example of rapid evolution. The New Zealand reptile has hardly changed its appearance over the last 200 million years. Its metabolism is lackadaisical. And young tuatara dawdle for more than 10 years before reaching sexual maturity.

But New Zealand scientists who analysed DNA harvested from fossils up to 8,750 years old now report that tuatara seem to do one thing remarkably fast: evolution. In a paper published this month in Trends in Genetics1, the researchers show that the rate of molecular evolution in the reptile is among the fastest yet observed for any vertebrate.

The results contradict the theory that cold-blooded animals with slow metabolisms evolve more slowly than their warm-blooded counterparts. Proponents of this theory have argued that higher rates of DNA replication and reactive-chemicals production by metabolism can damage DNA, amplifying the probability that a mutation will arise. Studies of other reptiles and mammals suggest this is true.

It will take more than this new result to overturn that theory, some experts say, particularly since the data come from ancient DNA that might be damaged by the passage of time. But lead author David Lambert, a molecular biologist at Massey University in Auckland, New Zealand, argues that there is much to learn about the rates of evolution from such studies.

Living fossils

The tuatara once inhabited the main islands of New Zealand, but now it is only found on the country’s smaller islands. The endangered reptile is often called a ‘living fossil’, in part because of its dedicated adherence to a body plan laid down hundreds of million years ago. To look at a tuatara is to look back in time to the Late Triassic period, when the reptile's ancient relatives skittered among dinosaurs and giant ferns.

The tuatara family contains only two nearly-identical species, Sphenodon punctatus and the even rarer S. guntheri, which are confined to Brothers Island off New Zealand. As these are the only surviving species of the order Sphenodontia, it is difficult to track the animal’s evolutionary history. Researchers often combine knowledge from the fossil record with genetic information from living relatives to determine how rapidly DNA is mutating.

For instance, if two species are known to have split apart from a common ancestor at least 50 million years ago, researchers can sequence the same fragment of DNA from living animals in several branches of the family tree, compare the sequences, and calculate the rate at which the DNA has changed since the common ancestor. “People often use living relatives to calibrate rates of molecular evolution,” says Lambert. “If you don’t have those living relatives, that’s going to be hard.”

Ancient DNA

Instead, Lambert and his team resorted to sequencing fragments of DNA from 33 fossilized bones of various ages along with 41 modern samples of tuatara. They arrived at an estimate of 1.56 sequence changes per base every million years — placing the reptile among the fastest of any animal yet tested. Ancient DNA from brown bears, by comparison, have an estimated rate of molecular evolution roughly one third as fast.

In 2002, Lambert and his colleagues published a similar analysis of Adélie penguin fossils, and reported a rate that was as much as seven times faster than previous estimates2. This rate is not statistically different from what he observed in the tuatara, which Lambert thinks isn't a coincidence. He suggests that rates of molecular evolution in all vertebrates might be found to be very similar.

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But others have questions about the accuracy of calculations based on fossils. Analyses of ancient DNA frequently come up with fast rates of molecular evolution: sometimes orders of magnitude larger than rates calculated using modern samples. “Both estimates cannot be correct,” says David Hillis of the University of Texas in Austin. “It is a lot easier for me to think of reasons why the ancient DNA comparisons might be suspect than the reverse.” Ancient DNA can be damaged, for instance, complicating sequence analysis.

Whatever their rate of evolution, there is one way in which the tuatara is indisputably quick: it is a nimble sprinter. “If you want to catch them to get a blood samples, they run bloody fast,” says Lambert. 

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