Q: When is a species not a species?

A: When it’s a subspecies. While there are many other potential answers to the question I posed, this is the answer I’m going to explore today.

As a taxonomist, I got quite upset yesterday by the many news outlets describing Lonesome George’s death as “the end of a species”. Because it wasn’t and it is not correct to say so. This isn’t really surprising though, given that sub-editors who write headlines on science articles often aren’t fussed with accuracy. Lonesome George was a particular subspecies of Galapagos tortoise (Chelonoidis nigra abingdonii (Gunther, 1877)). While it was almost certainly the end of a subspecies, the species proper will continue as long as there are Galapagos tortoises to propagate it.

Firstly, the reason why Lonesome George was (is?) important is more to do with history, than anything that George himself had to offer. The Galapagos tortoises, Chelonoidis nigra (Quoy and Gaimard, 1824), as a group, helped Charles Darwin figure out the theory of natural selection. They are fairly important creatures from that point of view. Put very simply, Darwin noticed that the tortoises on different islands of the Galapagos group had distinctly different physical characteristics (traits). But as they must have come from a common ancestor or geographical place, how did these changes happen?

When can we have a subspecies?

We know now that evolution is a continuous process, and we apply artificial dividers to designate species that are all, really, in a state of flux. New species are the result of different traits being selected for via breeding, over a very long time (or not, if the organism reproduces quickly). As generations of tortoises are born with particular traits, their overall appearance changes and they begin to look less like their ancestors and more like their own version. When they are deemed to be different enough, they are designated as their own species. But it’s the intermediate part that gets a bit weird, and has been subject to a range of controversy over the years. The arguing about species has resulted in two camps of taxonomists: the ‘lumpers’, who don’t like subspecies and prefer to bunch all together under species level regardless of population variation; and the ‘splitters’, who do the opposite of the lumpers.

A subspecies is a valid form of taxonomy. It is the name given to a species that has two or more discernible versions (‘races’) but that aren’t morphologically or genetically distinct enough to warrant separate species status. Often subspecies are the result of geographical boundaries, for example the water between all the Galapagos islands. There can never be one subspecies though. It’s either one species, or two or more subspecies. There is no real limit to the number of subspecies that can be recognised, as long as there is enough valid evidence to support the designation (Mallet 2001). Modern molecular techniques can reveal a new level of difference between races of a species, and is a major driver in the increase in species complexes (or often called cryptic species) being discovered.

The tortoise case study

Phylogenetic analysis of the Galapagos tortoises revealed that there is three distinct lineages within the subspecies found on Santa Cruz, which was already part of a species complex. The combination of morphological characters and genetic data indicated that one of these lineages should be described as a new taxon (presumably at subspecies level) (Russello et al. 2005). Interestingly, however, I found a paper from 2012 where each subspecies of Galapagos tortoise had been raised to species level (Poulakakis et al. 2012). While this arrangement may suit sub-editors who write bad news headlines, it is not correct, though. Consultation with the most recent annotated checklist of turtle taxonomy (van Dijk et al. 2011) revealed that the Galapagos tortoises are still being treated as a species complex – i.e., a species that comprises numerous subspecies.

Is it the end?

Yes. The world has indeed lost a subspecies – but it’s not really a big deal and we lose subspecies all the time. For this reason, incidentally, you don’t hear much about ‘species conservation’ anymore. The loss of a subspecies means even less to the public than the loss of a species, despite each subspecies contributing to biodiversity. However, with increasing sophistication and precision of molecular techniques, we may find more subspecies of Galapagos tortoises within the extant and extinct populations. Lonesome George may have had more cousins that we originally thought.

References:

Mallet (2001) Subspecies, semispecies, superspecies. In: Encyclopedia of biodiversity. SA Levin (ed). Academic Press.

Poulakakis et al. 2012. Unravelling the peculiarities of island life: Vicariance, dispersal and the diversification of the extinct and extant giant Galapagos tortoises. Molecular Ecology 21, 160-173.

Russello et al. 2005. A Cryptic taxon of Galapagos tortoise in conservation peril. Biology Letters 1, 287-290.

van Dijk et al. 2011. Turtles of the world, 2011 update: Annotated checklist of taxonomy, synonymy, distribution, and conservation status. Chelonian Research Monographs no. 5.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s