Rapid evolution of lizards in the Adriatic

Enough about Ben Stein and his lies about evolution. Let's talk some truth about evolution, namely scientific truth. The fine folks at Zooillogix brought my attention to a paper that flew under my radar at a time I may (perhaps) have been more focused on basketball. In it, researchers from Harvard and Amherst studied a population of lizards on a very small islet in the Mediterranean. In less than 40 years, this population of lizards has evolved to have significantly different morphology from its parent population, a new set of endosymbionts, and novel anatomical features rarely in related lizard species. It's an interesting case of evolution in action.

Back in 1971 10 lizards of the species Podarcis sicula were transplanted from one small islet in the South Adriatic to a nearby, somewhat smaller hunk of rock. Over a three year period from 2004-2006, Herrel et al. returned to these small islands to see what became of the lizards (1). What they found is that the transplanted lizards had taken over the second islet. The lizards from the second island were still genetically very similar to those on the originating island (see their supplementary figure 5). However, there were pronounced differences in the diet. Whereas the population on the island of origin ate very little plant matter (<10% of the total diet), the transplanted lizards appeared to subsist mostly on plant matter, with some seasonal variations up and down from 50%.

The change in diet appears to have prompted some substantial changes in morphology as well. The size and mass of the lizards on the second island is significantly greater, and the head dimensions are altered. For the smaller female lizards, this translates directly into an increase in bite force. For the male lizards, however, the differences in head size alone are not sufficient to account for the observed increase in bite force, implying that there are additional adaptations of some kind. Herrel et al. speculate that the increased bite force of the lizards helps them to eat and digest leaves. This is reinforced by the observation that structural features related to the opening of the jaw are largely unchanged.

There are additional adaptations internally. For instance, the lizards on the second island have a structure called a cecal valve (and additional anatomical changes to the cecum) that are believed to aid in the digestion of plant matter such as leaves. This is particularly interesting because this valve structure does not appear in the originating population, and is rare among related species of lizards (the suborder scleroglossa). Moreover, the hindgut of these lizards contained nematodes that are absent from the parent population, suggesting the development of a novel symbiosis. The authors note that the new morphological characteristics are present in juveniles as well as adults, suggesting that the changes involved are genetic, though further experiment is required. The authors also note some interesting changes in population dynamics and behavior that appear to have resulted from the altered eating habits of these lizards.

That these new features appeared within less than 40 years is especially striking. In less than a human lifetime this population of lizards evolved adaptations such as altered jaw morphology, as well as an apparently novel internal feature. While small populations and constrained locations can accelerate the process of evolution, it is still instructive to consider this result when discussing the "likelihood" of evolution. Significant morphological adaptations can evolve very rapidly. Imagine what the power of evolution could do given hundreds of millions of years in which to work.

Oh, wait... you don't have to.

1. Herrel, A., Huyghe, K., Vanhooydonck, B., Backeljau, T., Breugelmans, K., Grbac, I., Van Damme, R., Irschick, D.J. (2008). Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource. Proceedings of the National Academy of Sciences, 105(12), 4792-4795. DOI: 10.1073/pnas.0711998105