This is another wonderful paper – the result of what may be a unique translocation experiment involving Italian wall lizards (Podarcis sicula: Herrel et al., 2008). (I do read other stuff – I might tell you about some of that next time.)
Thirty-six years ago an earlier team of researchers moved 5 pairs of P. sicula from Pod Kopiste, a small island in the Adriatic Sea to the even smaller Pod Mrcaru. At the time there was another species (P. melisellensis) on the smaller island, but that has since become extinct. Analysis of mitochondrial DNA has shown that all the living lizards on the island are sicula. However, they do differ significantly – in head size & shape – from their parent population on Kopiste.
Herrel & his colleagues looked at what the Mrcaru lizards were eating. And they found significant differences in diet: compared to the Kopiste lizards, the Mrcaru lizards ate lots of veges. The amount of plant material in their diets ranged from 34% (in spring) to 62% (in summer), compared to 4-7% for Kopiste. What’s more, around 50% of this was high-cellulose material, which begs the question, how would they digest this? (Cows & other ruminants do it in the rumen, a specialised gut chamber containing huge numbers of symbiotic bacteria, which break down the cellulose. This generates volatile fatty acids – VFAs- as a by-product, & the ruminants gain a lot of energy from the VFAs.)
So the team looked at lizard guts. The P.sicula lizards on Pod Mrcaru turned out to have specialised structures – caecal valves – in their guts. Such valves are found in other vegetarian lizard species, but not in other sicula populations. The valves slow the movement of food through the gut, allowing regions of the gut to act as fermentation chambers where microbes can ferment cellulose from the lizards’ diets & release VFAs.
This means that we are looking at very rapid evolutionary change in the Mrcaru lizards. In just 36 years this population has evolved significant differences in both their behaviour (dietary preferences) and their external and internal morphology. The changes in head structure appear to be related to changes in diet – the Mrcaru lizards had a stronger bite, something that would be an advantage in biting off tough bits of plants. The move to a vege-based diet has seen the sicula population density increase markedly, compared to the ancestral Kopiste group, probably because they have access to a more reliable food source. And this in turn seems to have affected social structure – the Mcraru males are not territorial: again, unlike the Kopiste animals.
Herrel et al. concluded: …our data show how rapid phenotypic changes may affect population structure and dynamics through their effect on behavioural ecology and life history of animals. They also show that rapid evolution can result in changes in both qualitative and quantitative characters.
A. Herrel et al. (2008) Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource. Proceedings of the National Academiy of Science 105 (12): 4792 – 4795, www.pnas.org/cgi/doi10.1073/pnas.0711998105