Your questions – How many species?

By David Winter 19/07/2013 3


Here’s the first in our series answering your questions about the project. We start with a question from Maggy Wassilieff, who wants to know about the tuatara population living on North Brother Island in the Cook Strait:

Could you comment on the current understanding of the Brothers Is Tuatara? Is it a separate species? How long has the Brothers Is tuatara been isolated from other nearby populations?

Just what is a species anyway?

If we want to know how many tuatara species there are, and whether the North Brother Island population is distinct from other tuatara, we first need to know just what we mean by “species”. It may come as some surprise to learn that there isn’t a simple answer to this question. Biologists have spent at least 150 years grappling with what we now call the “species problem”, and in that time we have managed to define at least 26 sets of rules for determining where one species ends and another starts.

Thankfully, we don’t have to review all 26 of these “species concepts”, or the endless arguments they elicit, to know whether the tuatara living on Brothers Island are a distinct species. Instead, we can use our knowledge of where species come from to help us define what a species is. We have been studying the origin of species (speciation) for a long time now, and we have a pretty good idea how it works. New species start to form when populations stop sharing genes with each other. Once that process starts, changes in one population can’t affect what’s going on in others. That is, each population has its own evolutionary trajectory and is free to change and, in time, become distinct from all other populations.

If we take “species” to mean a population that is capable of maintaining its own evolutionary trajectory, then the job for biologists is to find evidence for this sort of independent evolution. There are lots of different sources of data we can use for this process, and the history  of the North Brother Island tuatara is a lovely example of the way new data has changed the way scientists understand the world.

 

In the beginning there was one

It took scientists a while to realise just how important tuatara are. They were first introduced to the scientific world in 1831, after John E. Gray found a tuatara skull sitting on display at the Royal College of Surgeons in London (it’s not clear how the skull made it to the UK). Gray recognised the skull was interesting, but didn’t fully grasp what he was looking at. He dedicated all of 9 lines in the first volume of his Zoological Miscellany to its “peculiar structure” while confusing the tuatara for a lizard. Crucially, Gray decided the skull represented a new genus (the taxonomic rank above species) and gave it the name in Sphenodon* (meaning wedge-toothed).

A tuatara skull, like the one Gray examined. I guess the teeth are pretty wedge-like. Image is courtesy of Te Papa Collections Online, and provided under a CC ND-NC license.

Ten years after he described the tuatara skull, Gray was sent a complete tuatara specimen by Johann Dieffenbach. Again, Gray didn’t quite get what he was looking at. As he didn’t dissect the specimen,  he didn’t see the unique skull and, so, didn’t realise he was looking at another Sphenodon. Instead, the poor old tuatara was again confused for a lizard and given a new name: Hatteria punctatus. Even if science hadn’t yet worked out where tuatara fitted in the diversity of life, scientists had at least recognised one species.

It wasn’t until 1867 that Albert Gunther connected the skull called Sphenodon to the animal called Hatteria punctatus and realised the tuatara represented a distinct order of reptiles separate from lizards. By the rules of taxonomy, the first name given to a creature is always the preferred one, so in this case the one tuatara species was moved into the older genus and called  Sphenodon punctatus.

Gray, Gunther and other European taxonomists had described tuatara based on the few specimens that made their way to the Old World. Naturalists in New Zealand, who could see tuatara on many different off-shore islands, started noticing that some of the animals they found seemed a bit different than the one described under that name S. punctatus. In particular, in 1877 Walter Buller decided the tuatara living on North Brother Island in the Cook Strait where different enough that they represented a distinct species and named them S. guntheri, in honour of Albert Gunther.  There were also names given to the endemic population on Hauturu /Little Barrier Island and to an apparently extinct species known only from sub-fossil bones in Northland.

It seems, however, that no one paid much notice to these other species. When tuatara were legally protected in 1895 only S. punctatus was named in the law. For the next hundred years or so, most New Zealand biologists and conservationists treated tuatara as a single species.

 

The first wave of molecular data

Charles Daugherty headed the team that first used genetic data to determine how many tuatara species there are

As genes are passed down from one generation to the next, they record the evolutionary history of the populations through which they move. Over time, the forces of mutation, natural selection and plain old chance combine within populations to make genetic change inevitable. If two populations subject to these changes are not connected by gene flow, those changes will take populations in different directions and each will end up with a distinct genetic make-up. For this reason, genetic data is invaluable for scientists trying to determine whether apparently distinct populations represent separate species with separate histories and diverging futures.

It wasn’t until 1990 that biologists could get enough genetic data to test the idea that tuatara should be considered a single species. Charles Daugherty from Victoria University headed up a team that collected blood samples from tuatara populations on 24 off-shore islands (Daugherty et al, 1990, doi: 10.1038/347177a0).

In those days, the genetic data evolutionary and conservation biologists could get their hands on came from a technique called isozyme electrophoresis. Many of the enzymes that keep the chemical processes in your cells ticking over come in several subtly different forms. As these forms have different shapes, and different electrical charges, these variants can be separated by loading samples of blood or tissue onto a matrix (like a gel), and applying a charge that moves each enzyme-variant according to its own charge, size and shape.

Daugherty and his colleagues looked at 25 different enzymes, and found a striking pattern. The North Brother population, the one that Buller had given a special name to, stuck out as being different from all the others. Given this genetic difference, and a number of subtle morphological differences, Daugherty and his team recommended that the North Brother population should be recognised as distinct species and the name S. guntheri was resurrected.

nature1990
Summary of the 1990 study – in which the N. Brother population stood apart from all others. (Click to enlarge)

 

The second wave of molecular data

For 20 years or so the North Brother tuatara were recognized and managed as a separate species. In that time, the ability of geneticists to create datasets grew enormously. So, in 2010 a new team, including Daugherty and headed up by Jennifer Hay who was also an author on the 1990 paper, got together to apply new data to the old question of tuatara taxonomy (Hay et al, 2010. doi: 10.1007/s10592-009-9952-7). This time, the biologists could determine the sequence of specific parts of a tuatara DNA, and use a type of short repetitive DNA marker called “microsatellites” to compare populations.

When all this new data was added to the picture, it became clear that the North brother Island tuatara weren’t quite as distinct as the isozyme study suggested. If there is a major split in the genetics of tuatara it corresponds to the geographic split between populations in Northern New Zealand and those in the Cook Strait. The North Brother population fits within the Cook Strait group.

ttDNA
Relationships among tuatara populations, as revealed by mitochondrial DNA sequences. With these data, the N. Brother population falls within the diversity of the Cook Strait population

Faced with new evidence, Hay and her colleagues did what scientists do, and updated their picture of the world. They concluded the North Brother tuatara population doesn’t represent a distinct species. That’s not to say the earlier work on these populations was wrong – it’s clear from both studies that he Cook Strait tuatara contain genetic diversity not present in populations from Northern New Zealand. This difference  probably arose in part because the many populations that used to exist between Cook Strait and Northern New Zealand are now extinct. Even so, it is important for the conservation of tuatara that the genetic diversity of these species is maintained. The 2010 paper removes species-status from S. guntheri, but makes the case that the conservation of the Cook Strait populations should be managed separately from that of the Northern tuatara.

A third wave?

Between the isozyme study, and the DNA sequences and microsattelites examined in the 2010 paper, biologists have now used about 0.001% of the tuatara genome to uncover the history of these populations. Just as different populations have their own histories, different genes can take slightly different paths through the populations that they move through. The tuatara reference genome sequence that we are creating will make it much easier for researchers to study many more genes from each remaining tuatara population, and perhaps from ancient DNA in sub-fossil bones. In addition to helping researchers to build a better picture of the recent history of tuatara in New Zealand, the reference sequence should enable research on genes relating to immune response, sex determination and other functions directly related to survival in tuatara.


*In fact, to compound his other mistakes, Gray misspelled the word as “Sphænodon”. Pretty much everyone realised Gary’s error, so the correct spelling took over and is now officially recognized.

Hillary Millar, who is a tuatara biologists herself, wrote about the taxonomic history of the tuatara on her blog. You should check it out, and stay tuned for an update from Hillary on her most recent tuatara genetic paper.

 

 

 

 


3 Responses to “Your questions – How many species?”

  • You say above: [quote]If we take “species” to mean a population that is capable of maintaining its own evolutionary trajectory, …[unquote]

    I say: Any isolated population will maintain its own evolutionary trajectory (for as long as it remains isolated), and is therefore an incipient species under the status quo. But we don’t want every population to be a species, do we? Therefore a species must be able to maintain genetic isolation (at least to a high degree, if not 100%), were it to (under natural conditions) have the opportunity to interbreed with any other given population. The tricky bit is in telling if this is the case or not, but in practice it falls back to traditional evaluation of potential reproductive barriers. These can be evaluated from morphology and/or behaviour, possibly and/or genetics IF GENETICS CLEARLY REVEALS INCOMPATIBILITY, which I’m not sure it ever does in all but the most otherwise obvious cases? I don’t know enough about genetics to say, but clearly, based on morphology and behaviour, only one species of tuatara is justifiable…

  • A minor quibble, and a bit late in the day, but isn’t NZ part of the Old World?
    Just found your blog and am looking forward to following your progress.