Science Literature

According to National Geographic News, a remarkable evolutionary change has occurred in a population of lizards:

"Italian wall lizards introduced to a tiny island off the coast of Croatia are evolving in ways that would normally take millions of years to play out, new research shows. In just a few decades the 5-inch-long (13-centimeter-long) lizards have developed a completely new gut structure, larger heads, and a harder bite, researchers say."

The experiment started in 1971, when 5 adult pairs of Podarcis sicula were introduced to the tiny island of Pod Mrcaru. Then came the Croatian War of Independence which lasted until the mid-1990s. Tourism did not start again, however, until about 2004, and this is when researchers returned to the island to see what had happened in the intervening years.

"Lizards Evolve "Overnight" on Island" says National Geographic News (to enlarge, click here)

What had happened to the introduced species? What impact had this had on the native lizards? "What they found, however, was shocking." No trace was found of the lizard species Podarcis milisellensis, which had previously inhabited the island. The introduced species were everywhere, at higher densities than their homeland. Furthermore, they had developed an appetite for eating plant food (34% in the Spring and 61% in the summer) whereas plant consumption in the source population was low (4%-7%). Their social structure was different and the lizards no longer defended territories.

Detailed analysis revealed three apparently related morphological changes: "head morphology, bite strength, and digestive tract structure". They appear to be adaptive, as all are related to the increased diet of plant materials.

"Physically, however, the lizards were not built to digest a vegetarian diet. Researchers found that the lizards developed cecal valves - muscles between the large and small intestine - that slowed down food digestion in fermenting chambers, which allowed their bodies to process the vegetation's cellulose into volatile fatty acids. "They evolved an expanded gut to allow them to process these leaves," Irschick said, adding it was something that had not been documented before. "This was a brand-new structure.""

In the paper, the authors note that <1% of all currently known species of squamates have cecal valves, and that: "These valves are similar in overall appearance and structure to those found in herbivorous lacertid, agamid, and iguanid lizards and are not found in other populations of P. sicula or in P. melisellensis." Herbivory explains why cecal valves are useful to these lizards. Also, "along with the ability to digest plants came the ability to bite harder, powered by a head that had grown longer and wider." All these changes came in a 36 year time window.

So, the question must be asked: how are we to understand the observations? The harder bite and the larger head are two aspects of the same phenomenon. Observations like this can be related to trends observed in artificial selection and they are not surprising. We have no reason to think that natural selection acting on natural variations over 30 generations cannot deliver this outcome.

The cecal valve finding is, however, dramatic. This is the finding that drew the comment that the animals "are evolving in ways that would normally take millions of years to play out". There can be no rationale for a Darwinian mechanism here - involving incremental assembly of the cecal valve. There is no time for this, even if a gradualist route could be found. No, the relevant genetic information must be present in the ancestors and epigenetic factors can be inferred to have activated the relevant mechanisms to make the structure. This research is revealing that organisms have a capacity for variability that goes significantly beyond their current phenotype. This implication has not escaped the attention of creationist biologists, who find this research a vindication of their view that animal radiations are rapid and the expression of innate variability embedded in the genome. It would be an interesting and educational activity for students to evaluate this theoretical model alongside others - although we can already be confident that Darwinism would not fare well in the exercise.

Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource
Anthony Herrel, Katleen Huyghe, Bieke Vanhooydonck, Thierry Backeljau, Karin Breugelmans, Irena Grbac, Raoul Van Damme, and Duncan J. Irschick
Proceedings of the National Academy of Sciences, 25 March 2008 105: 4792-4795 | doi 10.1073/pnas.0711998105

Abstract: Although rapid adaptive changes in morphology on ecological time scales are now well documented in natural populations, the effects of such changes on whole-organism performance capacity and the consequences on ecological dynamics at the population level are often unclear. Here we show how lizards have rapidly evolved differences in head morphology, bite strength, and digestive tract structure after experimental introduction into a novel environment. Despite the short time scale (~36 years) since this introduction, these changes in morphology and performance parallel those typically documented among species and even families of lizards in both the type and extent of their specialization. Moreover, these changes have occurred side-by-side with dramatic changes in population density and social structure, providing a compelling example of how the invasion of a novel habitat can evolutionarily drive multiple aspects of the phenotype.

See also:

Johnson, K., Lizards Rapidly Evolve After Introduction to Island National Geographic News (April 21, 2008)

Lizards Undergo Rapid Evolution After Introduction To A New Home, ScienceDaily (Apr. 18, 2008)