Tuatara: one species or two?

Photo / Mark Mitchell

By Dr Hilary Miller
11:29 AM Monday Aug 23, 2010

This post originally appeared on sciblogs.co.nz.

New Zealand's most iconic reptile, the tuatara, is currently regarded as two separate species - Sphenodon guntheri, which is found naturally only on North Brother Island in Cook Strait, and Sphenodon punctatus, which are found on other islands in Cook Strait and off the north-east coast of the North Island.

But recent research online in Conservation Genetics shows that Sphenodon guntheri is not as genetically distinctive as first thought, and suggests tuatara should be regarded as one species.

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 of 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. This occurs 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.

Read more: http://www.nzherald.co.nz/opinion/news/article.cfm?c_id=466&objectid=10668497