It turns out that a recently discovered population of land iguanas on the Galapagos is probably a new species that represents the basal (original) form of Galapagos land iguana. Moreover, this iguana is found in an unexpected place, according to a paper just coming out in the Proceedings of the National Academy of Sciences (PNAS).
And it’s pink.
In 1984, Hickmann and Lipps said the following about the various pieces of evidence giving age estimates for the Galapagos islands:
… all are less than about 2 million years old. This age, together with independently determined geologic ages, indicate that the islands emerged from the sea relatively recently and that all evolution of the islands’ unique terrestrial biota occurred within the past 3 to 4 million years.
Subsequently, it was realized that the present day islands are not the whole story. It is not the case that the evolutionary divergences among species we see on these islands had to happen in the few million years that the present day Islands seem to have been in existence. The Galapagos island chain includes a number of sea mounts which were islands in the past that have disappeared beneath the sea. The geology is somewhat more complex here than in other similar island chains, but the principles are the same.
There is an oceanic plate moving south east across a hot spot. The hot spot produces volcanoes. As the plate moves past the hot spot, volcanism ceases locally and the island formed by the volcano stops growing. (If the hot spot is still hot, a new volcano, ‘upstream,’ will start to form.) Then, the top of the island, now volcanically dormant, begins to erode down and the base and core of the volcano shrinks. These forces convert the volcano into a smaller and smaller island, until finally it submerges beneath the sea and becomes a sea mount.
There are probably sea mounts in the Galapagos dating back to about eight million years, maybe over ten million, but the extant islands seem to be only a few million years old. Any understanding of the biogeography of the animals on the Galapagos has to be understood in this dynamic geological context.
Now that we have that context worked out, enter the Pink Iguana. The Pink Iguana is both a species of Galapagos land iguana and a kind of martini. The Galapagos lizard version was discovered only recently, around the time that scientists were saying that all the evolution that happened in the Galapagos had to occur over a few million years time. Even more recently, a group of scientists have done a study of the genetic relationships between the Pink Iguana and the other land iguana’s on the Galapagos, and have come to a few remarkable conclusions. But first, let us address the remarkable confusions we face in looking at Rosada.
B and C are the rosada variety, the others are standard yellow models.
There is not clear taxonomy of the Galapagos land iguanas. They are all of the genus Conolophus. Several years ago, Snell, Snell and Tracy described the status of the land iguana’s and indicate that at the time there were thought to be perhaps five or six populations known historically, at least two of which were extinct, and perhaps one relict as a very small group of remnants. The remaining iguanas were thought to be divided into three populations, two of which have at some time or another been considered to be distinct species. These species were called Conolophus pallidus and Conolophus subcrislalus. Typically, one sees the Galapagos land iguana referred to simply as Conolophus subcrislalus. The pink iguana, believed to be a new species, seems to be unnamed.
From the new study by Gentile and others, just out in PNAS:
The most surprising result was the deep divergence of the rosada lineage at the basis of the Conolophus clade. This species alters the current thinking about the timing of diversification of land iguanas, which was previously supposed to have occurred in the Pleistocene Epoch …. our estimate sets the origin of this relict lineage back to a period when at least some of the present-day islands had not yet formed. In fact, the oldest extant islands in the archipelago, San Cristo´bal and Espan˜ola, are at least 2.35 and 3.3 million years old, respectively, … given its present distribution, the rosada form clearly represents a conundrum because it occurs only on Volcan Wolf, which is considered younger than Volcan Sierra Negra (0.53 million years…) …
So, the pink iguana, which is probably a new species first noted by park employees about twenty years ago, may represent the primordial land iguana of this island chain, and thus may represent the form that split with the marine iguana quite a bit earlier in time. Even so, the timing of diversification of the Galapagos land iguanas is recent in relation to the island chain itself, and because of the marine-terrestrial split, we know (or at least have strong confidence) that the land iguanas have been here for a very very long time.
What does all this mean? No one knows, but here is what I think:
The Galapagos are way over eight to ten million years old, and some time back around then were in habited by iguanas from somewhere. Now keep in mind that the landscape looked pretty different back then. North and South America were not joined by the Panama land bridge, and there was probably a lot of movement of fauna and flora between what is now the eastern equatorial Pacific and the Caribbean.
Land iguanas have always moved from island to island on a very low level in the Galapagos. Islands have slowly formed, grown large, then shrank, and slipped beneath the sea over time. But the temporal scale of island formation and disappearance is probably much slower than the temporal scale of iguana movement between islands. Furthermore, it is possible that inter-island movement becomes more likely when the islands become smaller, for ecological reasons.
In the end, I’m not surprised by what the authors of this recent paper are surprised by. I would expect only a vague correspondence between island history and animal movement. The question that I would like to see addressed is this: Why are the basal iguanas rare and the derived iguanas more common?
G. Gentile, A. Fabiani, C. Marquez, H. L. Snell, H. M. Snell, W. Tapia, V. Sbordoni (2009). An overlooked pink species of land iguana in the Galapagos Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0806339106
Snell, H.L., Snell, H.M., Tracy, C.R. (1984). Variation among populations of Galapagos land iguanas (Conolophus): contrasts of phylogeny and ecology Biological Journal of the Linnean Sociey, 185-207
C. S. HICKMAN, J. H. LIPPS (1985). Geologic Youth of Galapagos Islands Confirmed by Marine Stratigraphy and Paleontology Science, 227 (4694), 1578-1580 DOI: 10.1126/science.227.4694.1578
This is also discussed here, at Not Exactly Rocket Science.