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.


tuatara02.jpg 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.

About these ads

5 Responses

  1. I thought that it was old news that neutral evolution cannot be generalized and that neutral evolution rates are highly variable. Not that I don’t think this study is cool, but that neutral evolution rates are highly non-constant isn’t exactly news.. I’m a biology undergrad and this was pointed out to us right at the onset.

  2. I thought that it was old news that neutral evolution cannot be generalized and that neutral evolution rates are highly variable. Not that I don’t think this study is cool, but that neutral evolution rates are highly non-constant isn’t exactly news.. I’m a biology undergrad and this was pointed out to us right at the onset.

  3. One species? Did the Brothers Island tuatara go extinct or something?

  4. Cameron. Different islands have populations that are slightly different genetically but there is argument over whether the differences are great enough to warrant classification as separate species. Of course the local Department of Conservation likes to emphasise the differences to aid funding for preservation of the separate gene pools but I think all the subspecies could easily form fertile hybrids. But in deference to a poster at Dienekes I am forced to admit hybrids may suffer some level of outbreeding depression.

  5. I’ve always thought “neutral evolution” an oxymoron. I mean, if the sequence wiggles around but produces no measurable changes in body plan, behavior, etc., how can that be “evolution”?!
    In specific reference to this article, I’m also bothered by the grand interpretations of “evolutionary rate” for a SPECIES when all they looked at was mitochondrial DNA. Wouldn’t one expect the mutagenic pressures– as well as editing and repair– to be quite different in mitochondrial and nuclear chromosomes? Researchers have documented natural selection of mitochondria within a single individual and even within single CELLS of that individual! (specifically, myocytes; large, multinucleate cells) So why couldn’t it be the case that tuatara’s mitochondria have a high mutagenic rate without that bearing one iota on the rate of what really counts: nuclear DNA?

Comments are closed.

Follow

Get every new post delivered to your Inbox.

Join 54 other followers

%d bloggers like this: