The tuatara, a New Zealand reptile, is the last survivor of an ancient order called Sphenodontia. Its last known relatives are believed to have died out sixty million years ago.
The slow-moving creature hit the news recently because of a discovery made by David Lambert and his colleagues from the Allan Wilson Center for Molecular Ecology and Evolution at Massey University in New Zealand: The tuatara's DNA undergoes 1.56 changes per nucleotide per million years. - the fastest rate of molecular (genetic) evolution observed so far.
That in itself would not be so remarkable except that the tuatara has changed little in 200 million years.
Just how slow is the tuatara?
The apparent paradox was certainly not lost on the researchers. "Of course we would have expected that the tuatara, which does everything slowly -- they grow slowly, reproduce slowly and have a very slow metabolism -- would have evolved slowly. In fact, at the DNA level, they evolve extremely quickly," says Prof. Lambert.
The tuatara, of which there are two known species, is thought by some to resemble a dinosaur. It is believed to have separated from other reptiles in the Upper Triassic Period, 200-225 million years ago.
Its name, which originated in the Maori language, references the row of spines on the back of both sexes, the male's row being more prominent. The males grow to about 1/3 metre in length and weigh about 1 kg. The females are somewhat smaller. Unlike many reptiles, tuataras do not have visible ear openings, and the male tuatara does not have a penis.
According to the Kiwi Conservation Club tuataras have a slow metabolism, even for an exothermic (poikilothermic) animal. They have one of the slowest growth rates of any reptile. They do not reach sexual maturity until about 15 years of age, though they continue to grow until they are about 35. They breed every two to five years. After a gestation period of eight to nine months, the female lays and buries six to ten eggs in a sunny area. The young hatch after 11-16 months.
The tuatara life span averages 60 years but individuals may live over a century. Their diet is insects, lizards, eggs and, chicks. The sheer slowness of the tuatara's growth and reproduction is a source of concern for New Zealand conservationists, because any significant decline in numbers would take along time to reverse itself, even with protection.
How did scientists determine the rate of the tuatara's evolution?
Jennifer Viegas of Discovery News Channel reports that comparing the tuatara's rate of evolution to closely related animals was not possible because its close relatives are all extinct:
For the study, Lambert and his team amplified and sequenced DNA from the bones of 33 ancient tuataras, dating from more than 8,750 years ago to 650 years ago, as well as blood samples from 41 modern individuals.
The tuatara turned out to be changing its DNA faster than such animals as the Adelie penguin, aurochs, Mappin's moa, bison, brown bear, cave bear, cave lion, ox and horse.
How can there be evolution without change?
Ironically, New Zealand biologist Allan Wilson, after whom the center at which Prf. Lambert works is named, had proposed - controversially - forty years ago that "the rate of molecular evolution was uncoupled from the rate of morphological evolution," a position with which Lambert now agrees.
British physicist David Tyler, comments in his column at Access Research Network,
The paper points out that their findings challenge many hypotheses and notions about evolutionary change. The most substantial conclusion that can be drawn is that it supports the general hypothesis that the rate of molecular evolution is not coupled to the rate of morphological [body form] evolution. But this, of course, challenges the neoDawinian synthesis, which insists on small incremental changes acted on by natural selection.
[ ... ]
The evidence is accumulating that much contemporary evolutionary theory is not supported by data. If you want a prediction arising out of the tuatara research, it is that the gulf between the genome and the development of form will widen, and the association of molecular evolution with morphological evolution will be weakened.
In short, the tuatara's sluggish exterior conceals a swiftly changing genome that never got around to doing anything for two hundred million years. That in turn raises the question, as Tyler notes, of just what influence the genome does have on animal form (morphology) or evolution.
Also, here is another article on problems in molecular evolution: Molecular clock keeps good time - twice a day?
Resources:
Journal article: Rapid molecular evolution in a living fossil." Researchers include Jennifer M. Hay, Sankar Subramanian, Craig D. Millar, Elmira Mohandesan and David M. Lambert, Trends in Genetics. March 2008. (http://dx.doi.org/10.1016/j.tig.2007.12.002)
Abstract: The tuatara of New Zealand is a unique reptile that coexisted with dinosaurs and has changed little morphologically from its Cretaceous relatives. Tuatara have very slow metabolic and growth rates, long generation times and slow rates of reproduction. This suggests that the species is likely to exhibit a very slow rate of molecular evolution. Our analysis of ancient and modern tuatara DNA shows that, surprisingly, tuatara have the highest rate of molecular change recorded in vertebrates. Our work also suggests that rates of neutral molecular and phenotypic evolution are decoupled.
"Tuatara, the fastest evolving animal", EurekAlert, 20 March 2008
Tuatara images from Google