THERE IS A new article published online at PNAS reporting the results of a 36-year experiment on the Italian wall lizard Podarcis sicula. A population of lizards was already living on a small islet called Pod Kopiste, and the authors moved five females and five males to the nearby islet Pod Mrcaru. This was originally an experiment in “competitive exclusion”, as the islet was already inhabited by the Dalmatian wall lizard, Podarcis melisellensis, which was apparently outcompeted and is now extinct on that islet.
Upon examination of the descendants of the transplanted lizards 36 years (about 30 generations) later, the authors report changes in head size and shape that increased the lizard’s bite force. This was apparently an adaptation to a diet based heavily upon plants instead of upon arthropods, since cutting leaves and woody stems requires more force than needed to crush arthropods. But the changes in the skull fade to insignificance when compared to the strangest adaptation discovered, the evolution of cecal valves in the intestine to slow food passage and allow for fermentation to break down otherwise indigestible cellulose. Similar valves are seen in other herbivorous lizards, but are not present in the parent population on Pod Kopiste. These lizards seem to have evolved a key anatomical innovation in under 30 generations.
The switch to a vegetarian diet provided more plentiful food to the lizards, resulting in a higher population density, and seems to have resulted in behavioral changes, as the researchers say the lizards no longer defend territories. The lizards are also now larger, shorter-legged, and slower than their parent population.
In order to confirm these lizards are related to the parent population they carried out mitochondrial DNA testing that shows the lizards on Pod Mrcaru are indistinguishable from those on Pod Kopiste. However, the authors do not mention if this DNA testing was capable of ruling out extraction of the lizards from other local Italian wall lizard populations. I am not sure what other steps the authors took to control the outcome of this experiment, but I wonder if they poked around at any other nearby islets looking for another population of Italian wall lizards with unusual herbivorous traits that might have produced emigrants that colonized Pod Mrcaru.
Other experiments have shown similar rapid changes in phenotype, so most likely the authors are correct that these lizards have been reproductively isolated for only 36 years. This is now the third article published studying this population of lizards since their transplantation, so no doubt we will see further validation in future.
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 DOI: 10.1073/pnas.0711998105