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.
But 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.