Sorry for the length of this post.. the question involves laying some framework.

In 2008 there was a study done on Podarcis sicula, a species of lizard, that had been relocated to the island of Pod Mrcaru in 1971.  Among other variations in existing morphology,  the lizards had apparently developed Cecal Valves in their gut.  Before being moved to the island, they primarly fed on insects which were easier to digest, but with their new plant diet, the lizards developed the ability to store the plant food in the cecal valves where it could be broken down by microbes.


I have a couple questions about this in relation to the evolutionary process.

From what I understand the central mechanism of Evolution is Random Mutation + Natural Selection.  While the environment has a direct influence on which traits are passed on in a population through various pressures,  the genetic mutations themselves are not influenced by the environment, correct?

However, it seems that the morphology of these lizards are directly responding to their environment by producing the intestinal structures, and also elements like increased bite force due to having to chew up thick plant material, which also explains why such major physical changes took place in only 35 years.

I looked into this more and found a phenomena called "Phenotypic Plasticity" which is the ability of an organism to change its phenotype in response to changes in the environment.  The researchers who conducted the lizard research alluded to this as a possible explanation themselves, even pointing out that the cecal valves are present in juvenile stages of the lizards' development.

So it sounds to me that if a brand new population of the original wild type
Podarcis sicula were moved to this island, then in approx. 35 more years we'd have the same morphological changes occurring as a direct response to the environment, which would be hard to view as anything random.

My question is whether or not this is actually the evolutionary mechanism in action, or simply a fixed response built into the species (phenotypic plasticity).  How do biologists weigh in on this issue?

Thank you for reading this lengthy post.

- Bill

It would be interesting to do the experiment you suggest -- move a new population of lizards to the island (ensuring that they can't interbreed with the existing ones) and see whether they evolve a similar system. If they do, then that is evidence that the base population from which both groups are drawn has already evolved some of the prerequisuites.

Phenotypic plasticity refers to the different ways that organisms with the same genetic code can develop under different environments. The important point to understand here is that however much change there is to the phenotype, that does not change the genotype: in other words, environment can shape the creature, but not its code. So it does not contribute directly to evolution.

Interesting as Mike says.

I have no idea of the generational reproduction time of a lizard but if we assume a maximum of 5/year then at most 150 generations could have elapsed to cause denovo muttations that cause cecal valves that are selected for as the diet charged - that seems too low to me. In contrast, Bill's idea of prolongation into adult of the embryonic valves fits very well with that timeline and would probably need less mutations.