Deciding where to draw the line between species is a common dilemma in biology. There are many different ways of defining what a species is (John Wilkins on Evolving Thoughts lists 26 different ’species concepts’). One of the most common definitions is the evolutionary species concept, which classifies a species as ’a single lineage of ancestor-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate” (Wiley 1981). Under this definition it doesn’t matter whether species are reproductively isolated (as in the Biological Species Concept), what is important is whether they remain genetically or morphologically distinct from one another over time. A major problem with this definition however, is deciding just how genetically or morphologically distinct two populations need to be before they are regarded as separate species. (See here for more on these and other species concepts).
In small, geographically isolated populations founder effects can be an additional complicating factor. A founder effect happens when populations are founded by only a few individuals, or when a large population suddenly becomes very small (a bottleneck). In these situations genetic variation can be rapidly and randomly lost, and result in the new population looking quite different from the old even if they have only been separated for a short period of time. This is particularly relevant to the North Brother tuatara population, which was almost exterminated in the 1800s and is now restricted to a 1.7ha patch of scrub on the island.
Natural populations of tuatara are now found only on off-shore islands (32 individual island populations in total), ranging from the Poor Knights Islands in the north-east, to Stephens Island in western Cook Strait (see the map below). Tuatara were first described by John Edward Gray of the British Museum, who received a tuatara skull in 1831. In 1877 Walter Buller described the population on North Brother Island as a new species, S. guntheri, because of differences ’in appearance’, especially ’colour’, and in ’habits and disposition’. However, this distinction appears to have been largely ignored until the early 1990s — taxonomic reviews in the 1950s and 1970s describe tuatara as a single species, with the North Brother Island population listed as a sub-species. In 1990 Charles Daugherty published the first genetic results on tuatara, and showed a striking divergence between the North Brother Island population and other tuatara populations. This work was published in Nature with the great headline ’Bad Taxonomy Can Kill’ and resulted in their reinstatement as a full species.
So why has this result now been overturned? The key lies in the type of genetic marker and analysis methods used. Daugherty and colleagues used protein markers called allozymes, which were the most commonly used genetic markers at the time. The levels of divergence they measured between North Brother Island and the other populations were equivalent to levels seen between species in many other vertebrates, and correlated with the morphological differences between the groups. Therefore their recommendation to reinstate the North Brother Island population to full species status was entirely justified based on the evidence available at the time. However, these days allozymes are regarded as a rather insensitive genetic marker and have fallen out of favour. In some cases they can be influenced by selection, in which case patterns of divergence will reflect environmental differences acting on a particular protein rather than reflecting overall differences between populations. The DNA-based genetic markers used nowadays evolve more quickly and are regarded as ’neutral’ (i.e. not likely to be skewed by selection), so are more likely to accurately reflect population history.
Hay and colleagues have now analysed genetic differentiation between tuatara populations with two additional types of DNA markers – mitochondrial DNA(mtDNA) and microsatellite markers, and have also re-analysed the original allozyme data using more powerful methods. The mtDNA data shows a strikingly different pattern to the allozymes (see figure below). The phylogenetic tree of mtDNA suggests that the North Brother population is closely related to the other populations in Cook Strait, but that the northern populations are distinct from all the Cook Strait populations. This reason for this discrepancy is not clear, but probably reflects the different type of marker. Unlike allozymes, mtDNA is maternally inherited (passed only from mother to offspring). The two markers also provide information on different timescales and differ in their susceptibility to selection and founder effects.
The allozyme results in the original paper by Daugherty et al were presented as a phylogenetic tree based on genetic distances between populations. The same type of analysis done on the microsatellite data shows North Brother tuatara as distinct, but the difference is not as great as for the allozyme data. Hay and colleagues have reanalysed the original allozyme data using principal components analysis, which is a more robust and sensitive method than genetic distance measures for revealing patterns in this type of data. The PCA tells a somewhat different story from the phylogenetic tree — this shows that the primary division is between all the Cook Strait populations and all the northern populations, and that the split within Cook Strait which separates North Brother is not as large.
This study adds another chapter to the tuatara’s already complex taxonomic history. However it is unlikely to impact on tuatara management in the short term, as the Department of Conservation already manages the northern, western Cook Strait, and North Brother Islands separately. Of more concern is that the North Brother Island population has extremely low levels of genetic diversity, high levels of inbreeding, and a male-biased sex ratio. Animals from North Brother have been translocated to new islands to boost their numbers, but their lack of genetic diversity has the potential to handicap future growth of these new populations. So, if tuatara are all one species, and the distinctiveness of North Brother from the rest of Cook Strait is largely the result of human-mediated founder effects — an artefact of small population size, rather than reflecting a deeper difference in evolutionary history — why not mix in a few animals from other Cook Strait islands to boost their diversity? (I’m not necessarily advocating this, but it is an interesting question).
In many cases, species definitions are kind of semantic issues, based on our human need to put things in categories. The tuatara demonstrates that when populations are geographically isolated and founded by only a few individuals, deciding where to draw the species line can be difficult. However, understanding the differences between populations is important for managing them successfully. Whether tuatara are ultimately regarded as one, two or three species, the work of Hay and colleagues gives conservation managers a much clearer picture.
Hay, J., Sarre, S., Lambert, D., Allendorf, F., & Daugherty, C. (2009). Genetic diversity and taxonomy: a reassessment of species designation in tuatara (Sphenodon: Reptilia) Conservation Genetics DOI: 10.1007/s10592-009-9952-7