The Lizards Of Pod Mrcaru

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There are two small islets off the Croatian coast called Pod Kopiste and Pod Mrcaru. In 1971 a population of common Mediterranean lizards, Podarcis sicula, which mainly eat insects, was present on Pod Kopiste but there were none on Pod Mrcaru. In that year experimenters transported five pairs of Podarcis sicula from Pod Kopiste and released them on Pod Mrcaru. Then, in 2008, another group of mainly Belgian scientists, associated with Anthony Herrel, visited the islands to see what had happened. They found a flourishing population of lizards on Pod Mrcaru, which DNA analysis confirmed were indeed Podarcis sicula. These are presumed to have descended from the original five pairs that were transported. Herrel and his colleagues made observations on the descendants of the transported lizards, and compared them with lizards living on the original ancestral island. There were marked differences. The scientists made the probably justified assumption that the lizards on the ancestral island, Pod Kopiste, were unchanged representatives of the ancestral lizards of thirty-six years before. In other words, they presumed they were comparing the evolved lizards of Pod Mrcaru with their unevolved 'ancestors' (meaning their contemporaries but of ancestral type) on Pod Kopiste. Even if this presumption is wrong - even if, for example, the lizards of Pod Kopiste have been evolving just as fast as the lizards of Pod Mrcaru - we are still observing evolutionary divergence in nature, over a timescale of decades: the sort of timescale that humans can observe within one lifetime.

And what were the differences between the two island populations, differences that had taken a mere thirty-seven years or so to evolve?* Well, the Pod Mrcaru lizards - the 'evolved' population - had significantly larger heads than the 'original' Pod Kopiste population: longer, wider and taller heads. This translates into a markedly greater bite force. A change of this kind typically goes with a shift to a more vegetarian diet and, sure enough, the lizards of Pod Mrcaru eat significantly more plant material than the 'ancestral' type on Pod Kopiste. From the almost exclusive diet of insects (arthropods, in the terms of the graph opposite) still enjoyed by the modern Pod Kopiste population, the lizards on Pod Mrcaru had shifted to a largely vegetarian diet, especially in summer.

Why would an animal need a stronger bite when shifting to a vegetarian diet? Because plant, but not animal, cell walls are stiffened by cellulose. Herbivorous mammals like horses, cattle and elephants have great millstone-like teeth for grinding cellulose, quite different from the shearing teeth of carnivores and the needly teeth of insectivores. And they have massive jaw muscles, and correspondingly robust skulls for the muscle attachments (think of the stout midline crest along the top of a gorilla's skull).* Vegetarians also have characteristic peculiarities of the gut. Animals generally can't digest cellulose without the aid of bacteria or other micro-organisms, and many vertebrates set aside a blind alley in the gut called the caecum, which houses such bacteria and acts as a fermentation chamber (our appendix is a vestige of the larger caecum in our more vegetarian ancestors). The caecum, and other parts of the gut, can become quite elaborate in specialist herbivores. Carnivores usually have simpler guts than herbivores, and smaller too. Among the complications that become inserted in herbivore guts are things called caecal valves. Valves are incomplete partitions, sometimes muscular, which can serve to regulate or slow down the flow of material through the gut, or simply increase the surface area of the interior of the caecum. The picture on the left shows the caecum cut open in a related species of lizard which eats a lot of plant material. The valve is indicated by the arrow. Now, the fascinating thing is that, although caecal valves don't normally occur in Podarcis sicula and are rare in the family to which it belongs, those valves have actually started to evolve in the population of P. sicula on Pod Mrcaru, the population that has, for only the past thirty-seven years, been evolving towards herbivory. The investigators discovered other evolutionary changes in the lizards of Pod Mrcaru. The population density increased, and the lizards ceased to defend territories in the way that the 'ancestral' population on Pod Kopiste did. I should repeat that the only thing that is really exceptional about this whole story, and the reason I am telling it here, is that it all happened so extremely rapidly, in a matter of a few decades: evolution before our very eyes.

PodUdpisle l*adh1rcarj

Summer diet of lizards on two Adriatic islands

PodUdpisle l*adh1rcarj

Summer diet of lizards on two Adriatic islands

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