Molecular Evolution and “Living Fossils”

A short, interesting, and busy paper in Trends in Genetics looks at molecular evolution among tuataras and comes up with some interesting results.

Tuataras live only in New Zealand and are closely related to snakes and lizards. Currently, there is only one surviving species, placed in the Family Spenodontidae. They are the remnants of a group that obtained its maximum diversity in the Triassic and Jurassic. As with all “living fossils” there are some differences between the current species and its distant relatives. In the case of the tuatara this is best seen in the the orientation of the palatine teeth, which are set parallel, rather than obliquely, to the maxillary dentition.
In the meantime, there has been a considerable amount of research looking at the effects of metabolism, body size, and molecular evolution (you can go here and here for examples). In that tuataras have low body temperatures, slow rates of growth with long generation times, slow reproductive rate, and relatively small body size they would be an interesting test case. To that end, the current paper examined the molecular evolution of tuataras, specifically the mtDNA (they used HVR I and HVR II from the mitochondrial control region). There is, however, on glitch. As Hay et al put it:

Traditionally rates of molecular evolution have been estimated using phylogenetic methods, in which levels of genetic divergence among closely related species have been calibrated against time points in the fossil record. However, this is not possible in tuatara because there are no closely related species, one of the characteristics of ‘living fossils’.

In order to get around this hurdle Hay et al used ancient DNA from 33 tuatara fossils (which range in date from 650-8750 years BP). The results indicate that the tuatara has a molecular evolution rate some 50% faster than other vertebrates. In effect this means that previous studies of metabolic rate and molecular evolution can’t be generalized to all species. The result is also in line with previous studies on the nucleotide diversity in other “living fossils” such as the coelacanth and the horse-shoe crab. Hay et al’s conclusion is that:

Previous studies on living fossils such as the coelacanth and the horseshoe crab have suggested a substantial nucleotide diversity in these phylogenetically distinct species…, perhaps indirectly suggesting a high evolutionary rate. However, this is the first study to directly estimate the rate of evolution in a living fossil and also the first attempt to quantify the neutral evolutionary changes in a species having such extreme physiologic and life history traits. The results of our study support the hypothesis that rates of neutral molecular and phenotypic evolution are decoupled… Hence, life history and physiologic parameters do not explain all differences in rates of neutral molecular evolution.

Update 1: As the comment points out, there are two species still surviving.
Update 2: Palaeoblog has a nice write up as well.