Podarcis sicula, a case for rapid evolution.

This week's science comes from a few years ago, so some may have heard about this already. Podarcis sicula, or the Italian Wall-Lizard, was the focus of a study that started nearly 40 years ago. Between 1971 and 2000 no one was allowed to the island, so study could not be conducted. When they went back they were in for a surprise. The result of this study shows us why we must keep in mind both biological and environmental definitions of a "species". In a short, 36 year, timespan (approx. 30 generations of this species) the introduced population have evolved a cecal valve in their intestines. Keep reading to find why this blows my mind!

So to get a feel of this experiment, the main species in this study P. sicula, come from a small island Pod Kopiste (0.09 km^2) in the south Adriatic Sea. Five mating pairs from this ancestral population were placed on the island of Pod Mrcaru (0.03 km^2), where P. melisellensis lived. After 36 years the endemic species was extirpated and P. sicula became common. Keep in mind the size of these islands as this is very important for later analysis.

Further P. sicula was primarily insectivorous with less than 10% of it's diet being vegetation.

Using mitochondrial DNA they were able to determine that the two populations are indeed still the same species. However this is mitochondrial DNA, not cellular DNA.

After 36 years on Pod Mrcaru, the study found that the lizards now have a diet consisting of about 35% vegetation in the winter, to over 60% in the summer. Along with this was the finding of heads that were longer, deeper, and all around larger with more jaw muscle for extra chomping power. If you didn't know, these are very large changes in and among themselves. Evidence of evolution, I would think so! (Recall evolution is the change in allele frequencies over time.)

What huge new trait came about though? The existence of cecal valves, structures in the gut used to slow the progression of food for better absorption. Why so important? Cecal valves are present in <1% of all known squamates! Let me repeat that for full understanding, less than 80 of all known lizards and snakes have this structure. This wouldn't be too interesting if it weren't for the fact that it isn't present in the Pod Kopiste population nor is it present elsewhere in the genus. This is an entirely new structure! Because of the incredibly low frequency of these valves in other lizards and utter lack of them in amphibians, it is a safe conclusion to say that these structures are not ancestral.

What does this all mean though? Simply, evolution can occur even faster than previously suspected. With a simple selective pressure, changes occurred within the life span of a human. Typically this is seen in insects or bacterium, but now it has been seen in vertebrates. If only they could have seen this development in action!

Remember I said to keep in mind of the size of the island? With that small of an island, genes get passed around very quickly within the population, this means that there is fast genetic flow. So when a positive trait occurs, it can get through the population in a relatively short amount of time.

Original paper -> Rapid large-scale evolutionary divergence.