An iguana appetizer

January 16, 2010 • 1:07 am

by Greg Mayer

No, it’s not a reptilian hors d’oeuvre. It’s pictures of a Galapagos land iguana, Conolophus subcristatus, to whet your appetites for those Jerry will have when he gets back. I toured the Galapagos 20 years ago, and took loads of pictures, but they’re Kodachromes (which I haven’t scanned), so the pictures of our saurian friend below are from my colleague and fellow evolutionary biologist Joe Balsano, who visisted in 2007, and then kindly regaled my Darwin class with tales and pictures of his adventure. (More Galapagos reptile photos, at the Galapagos Conservancy, here.)

The Galapagos land iguana, Conolophus subcristatus (Joe Balsano).

The first link above for the Galapagos land iguana, from the Galapagos Conservation Trust (the UK companion to the US-based Galapagos Conservancy) is slightly out of date when it says there are two species of Galapagos land iguana: there are three. The common, or just Galapagos, land iguana, Conolophus subcristatus, is shown above. The Barrington land iguana, C. pallidus, occurs only on the island of Santa Fe (also known as Barrington). The two species differ fairly subtly in color and scalation (pallidus being less colorful, with a more distinctive crest of spines; see the original description by Edmund Heller here [go to Proceedings of the Washington Academy of Sciences in the left sidebar], and the classic paper by van Denburgh and Slevin on Galapagos iguanid lizards from the California Academy Expedition here [go to Proceedings-California Academy of Sciences 4th series in the left sidebar]). These subtle differences are the sort of differences between allopatric populations (i.e., populations inhabiting distinct, nonoverlapping, geographic areas) that can lead to long and inconclusive arguments as to whether the populations should be recognized as species, or subspecies, or not named at all. These arguments are a common, and not at all unexpected, issue when dealing with organisms living on islands. (The evolutionary process issues involved, although not the taxonomic issues, are dealt with comprehensively in Jerry’s and Allen Orr’s Speciation.) But the newly discovered species the pink land iguana of Volcan Wolf on Isla Isabela (Albemarle), Conolophus marthae, is not one of these wishy-washy, is-it or is-it-not-a-species, cases: it’s a new species, alright.

The pink land iguana, Conolophus marthae. From Gentile, G., et al. 2009. An overlooked pink species of land iguana in the Gal√°pagos. Proceedings of the National Academy of Sciences 106:507-511.

It is amply distinct, both morphologically and genetically, from the other two species, including in coloration and form of the nuchal crest, as you can see from the pictures above. But, more importantly, it is also sympatric (i.e., living together in the same place) with the common land iguana. This is important because the truest test of species status is the test of sympatry: whether two forms interbreed when they co-occur in nature. In this case, the two species live together side by side, and reproductive isolating barriers, such as differences in male behavior (see the original species description by Gabriele Gentile and Howard Snell), keep them genetically isolated from one another. (Gentile and colleagues did find a single individual which showed evidence of some genetic mixing, but it is evidently insufficient to breakdown the genetic isolation of the forms.) This is a really remarkable and exciting discovery, given how many scientists, park rangers, and even just tourists, have traversed these islands. (I have been to Isabela, not far, at least as the crow flies, from where the new species was discovered.)

Although I think it’s fair to say that interested scientists have been delighted by the discovery of the pink land iguana, a number have been disturbed by what Gentile and Snell did, or rather didn’t do, in naming the species: they did not collect a specimen to document the species, but relied upon blood samples and photos. Usually, when a new species of animal is described, a particular specimen is designated the holotype, and preserved and deposited in the collection of a museum that will make the specimen available for study by other scientists. The specific identity of the holotype fixes the application of the name, and study of the holotype helps resolve any questions or confusions concerning the status or identity of the species, as well as contributing to further knowledge of the species’ biology. But if there is no holotypic specimen, then other scientists are unable to check the describer’s claims, or test their conclusions, or advance the study of the species in any way. Gentile and Snell were aware that what they were doing was problematic, and addressed the question in their paper. They even designated a particular iguana as the holotype, but left it in the wild, hoping that at some later time it might be retrieved using a radio tag they put in it. They did not collect it out of concern that loss of even a single individual might drive the species extinct.

Alain Dubois of the Museum nationale d’Histoire naturelle and Andre Nemesio of the Universidade Federal de Minas Gerais, Brazil, have led the criticism of Gentile and Snell, while acknowledging that there may be times when it is not wise to collect a specimen. See papers by them here, here, and here; Thomas Donegan of Fundacion Proaves supports what Gentile and Snell did. In the bad old days of systematic zoology, species were often named without holotypes, and this led to much confusion. Lately, there have been several species named for which holotypes have not been collected, for the same reasons advanced by Gentile and Snell, and this has led to much controversy; many of the key papers are cited in the woks of Dubois, Nemesio, and Donegan, or in works cited therein.

Some people might ask, what’s wrong with a photo? Well, I think it should be evident that there are many things you can’t determine from a photo, but perhaps a mention of the most famous species named on the basis of a photograph will make some of the problems clear: that species is the Loch Ness monster, Nessiteras rhombopteryx (abstract only without subscription). To put it only a bit too simply, specimens are what separate zoology from cryptozoology, science from pseudoscience. More on this in a later post.