Dave's Lizards

I keep saying I'll write up the darn lizard paper, yet every time I start, it slips through the cracks. Tonight, no more! With a good half hour until House starts, I've sat down and given this a good discussion.

Evolution! If you don't think it's a trick by Satan, chances are you think that evolution takes place on a geologic timescale. Surely nothing we can witness, eh? Well, not so. By being clever naturalists, me and you can ferret out the signature of evolution, and catch it in the process of doing its thing.

Previously, I talked about speciation, and how we can catch it in the process with fish in Lake Victoria, but while there was some morphological change with that - the fish got a little different in their colour - it wasn't very flashy. When we think of evolution, even most scientists think of anatomical change. We want them to look really different, darn it!

That sort of evolution is much, much slower to come about. Adaptive evolution is a slow process of variation and selection. Sometimes it might take sudden leaps, but that's fairly rare (Sorry Gould). However, if selection is strong - meaning that some variants are much `better` than others - then evolution can occur rapidly.

We've previously seen examples, but few are very gee-wiz amazing. Channel Island deer mice showed rapid change of head characters and body size, Darwin's Finches showed rapid evolution of beak and body size, and Black Snakes showed rapid adaptation to an invasive toad, the Cane Toad. Harrel et. al 2008 are about to blow all their competitors socks off.

In 1971, a couple of scientists took 5 male lizards, and 5 female, and transplanted them on an island called Pod Mrcaru. They hadn't been there before, and where they were, before (Pod Kopiste), they were small bug-munchers, with males who kept territories. Thus far, a boring experiment. But then war broke out.

Said war went on for a bit, and prevented people from really heading back to the island to check on what's up with them. It took them about, oh, 36 years for them to get in on back and check, when all was said and done. And in that time, these lizards (Podarcis sicula) were in a novel environment, with new pressures to survive, and new food sources available. Natural selection did its thing.

When they did come back, they found that when it comes to shape, the lizards had signifigantly wider, taller and longer heads. Further, the lizards on the new island ate a large portion of vegetation, from 4-7% to about 34-61% (spring-summer). And the vegetation were things like leaves and stems.

Not so visible from the outside, the lizards underwent a rapid change in gut structure, with a whole new structure that wasn't really there before: They evolved caecal valves. This is huge. This is amazingly big. It would be like humans suddenly starting to grow articulated, functional tails again. In about 30 generations, they went from insectivorous to having plant-adapted digestive tracts.

Oh. And the males? Went from territoral to not. This seems to have changed the sprint speed, limb length, and god-knows-what-else-we-haven't-measured.

30 generations. To put that in context, 30 human generations would be about, oh, 900 years. So it'd be like if in the time since 1100CE, humans changed shape. It seems absurd, but under strong selective pressure, that's what happens.

Oh, I suppose that begs the question, `why don't you see that in other species? Why are all humans pretty much the same, when we've been separated for about 400 generations?`

Well, the selective pressure needs to be big - the difference in success between the lizard with the features and the one without needs to be pretty serious. Second, there needs to be not a lot of wiggle room - this actually goes to point A. Humans, for example, are amazingly plastic. We don't need many physiological changes, when we can accommodate so much by just changing our learned behaviour just slightly.

Third, the variation needs to happen - if no one gets the mutation giving them that third eye, third eyes aren't going to evolve, no matter how cool they are. This is actually a bigger deal than most people would guess. Finally, there can't be much geneflow. Remember whole thing about speciation? It lets groups adapt to their micro-enviroment without getting genes from individuals outside that environment. Geneflow carries information around, and makes groups more similar. So it has the tendency to wash out local adaptation.

These lizards had big selective pressure, not a lot of plasticity, the variation happened, and because they were on an island, the geneflow didn't happen. So what we saw was rapid divergence, and a really neat paper in PNAS. ;)

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Herrel, A., Huyghe, K., Vanhooydonck, B., Backeljau, T., Breugelmans, K., Grbac, I., Damme, R., and Irschick�, D. (2008) Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource. Proceedings of the National Academy of Science, v.105(12), pp. 4792–4795.