Natural selection does a handbrake turn – quick evolution at work

Evolution works over centuries and millennia. But occasionally, scientists can stumble across situations where evolution happens quickly enough to be experimentally tested. Islands provide such opportunities as invading species can drastically change or reverse the evolutionary pressures on the locals.

Our lifespans of decades and, with luck, centuries seem like vast stretches of time to us. But to the forces of evolution, they are mere temporary blips. The common knowledge has it that evolution occurs over geological timescales – thousands and millions of years.

As such, evolutionary biology takes a lot of criticism for being a ‘descriptive science’, being less open than other fields to that fundamental aspect of science – experimentation. Those who study evolution must be content to observe snapshots of life, either present or entombed in rock, and make inferences from there.

A brown anole lizardBut this is not always so. Occasionally, evolution happens at astonishingly fast rates, as epitomised by the case of the peppered moths.

Today, canny scientists are on the look-out for similarly speedy evolutionary events, that are more amenable to testing. Jonathan Losos and colleagues at Washington University, St Louis, have found one such example in a small Caribbean lizard.

The brown anole lives in the Bahamas and spends much of its time foraging on the ground. But occasionally, its island homes are invaded a larger predatory species, the curly-tailed lizard.

Losos’s earlier work had shown that these invasions caused the anole populations to head for the trees, abandoning their vulnerable land-based activities over a few generations. He spotted the signs of quick evolution at work and set about testing it.

Losos deliberately introduced curly-tailed lizards to islands containing brown anoles. A year later, and the percentage of brown anoles caught on the ground fell from about 40% to under 10% in a year, but not in other islands untouched by the curly-tails.

In the first six months, the anole populations on invaded islands shifted towards individuals with longer legs, who were better at outrunning the predators. But six months later, and the survivors were those with much shorter legs, which allowed them to hide from curly-tails in narrow and irregular tree-top spaces.

Within a single generation, Losos had shown that the evolutionary forces, or ‘selection pressures’, acting on the anoles went through very quick reversals.

As the lizards’ behaviour changed and they started to leave the ground, traits that had once been gifts became hindrances. Natural selection, it seems, is a fickle master.

Over more generations, the persisting threat of the curly-tailed lizards will drive the evolution of shorter and shorter legs in the anole population. The endpoint of this process can be seen on other islands, where some lizards species have evolved very short legs indeed and become ‘twig specialists’.

These rare sightings of ‘microevolution’ help to show us the essence of a process that takes several of our lifetimes. In doing so, they greatly enrich our appreciation of how life on earth became as rich as it is today.

Losos, Schoener, Langerhans & Spiller. 2006. Science 314: 1111.

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4 Responses

  1. This article is distressing for me. You mentioned the discredited peppered moths as an example of fast working evolution when, even if it weren’t a fabrication, it still wouldn’t be evolution because there was no change in the moths themselves, but rather certain ones were supposedly not so easily spotted by birds and therefore not eaten. So there existed more of a certain moth not because of any change in the moth, but because their predators couldn’t see them.
    But it’s all beside the point because it was faked.
    This anole situation is kind of like the peppered moth except that, as far as we know, it wasn’t faked. Still, this is not evolution, it’s math. If I have 30 anoles, 15 with long legs, 15 with short and I subtract 5 with short legs and two with long, what do I have left? I have more with long legs, that’s what. Not because anything changed in the anole, but because I subtracted more with short legs than long.
    Now suppose after taking away those with the short legs they are placed up on a high shelf where I can’t reach them, but I can still reach the ones with the long legs. Obviously they are going to disappear. Not because of any genetic change in the lizards themselves, but because I can’t remove what I can’t get to.
    This IS as you say, microevolution, but even that term is confusing because it has nothing whatsoever to do with macroevolution. Microevolution rightly named would be adaption.
    What I’m saying is, these lizards already have the genetic coding for long and short legs. By removing the long legged lizards we are merely removing the coding for long legs. This will NEVER result in these lizards developing feathers, wings and aviation lungs because that requires new genetic information that does not yet exist in the creature.
    I’m trying hard not to be antagonistic. As a creationist I am constantly told that creation is unscientific and dishonest, and I have to say that I view articles such as this one (which I’ve also read at LiveScience.com, and National Geographic) to be dishonest in their portrayal of what evolution actually is.
    I would really appreciate a response because I’d like to understand your views on what I’ve said here. I may be wrong or missing a vital piece of information and if so I’d like that info. But based on what I do know, I can’t help but be personally offended by articles like this.

  2. I think the point to this paper, which I did try to convey in the article, is not that evolution itself is happening over a short space of time, but that the *selection pressures* acting on the anoles are rapidly changing, which *in time* will lead to true evolutionary change.

    This is why the article’s title uses the phrase “natural selection” rather than evolution. If the distinction has not been made clearly elsewhere in the article, that’s my bad.

  3. Daryle Henry also mentions that the peppered moth story was ‘discredited’ but it has been anything but. For more on this, have a look at this New Scientist article or the homepage of Michael Majerus.

  4. [...] on evolution in action: Natural selection does a handbrake turn – quick evolution at work Of flowers and pollinators – a case study in punctuated [...]

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