An Anolis sagrei lizard clings to a tree branch in order to evade predators (Image: Jonathan B Losos) A brave Anolis sagrei takes his chances on open ground (Image: Jonathan B Losos)
Natural selection, the keystone of evolution, can switch direction in a matter of months, a novel experiment on lizards reveals.
Jonathan Losos at Harvard University in Cambridge, Massachusetts, US, and colleagues visited a dozen tiny isles in the Bahamas. They tagged hundreds of tiny Anolis sagrei lizards, which show natural variation in the length of their legs.
In half of the islands, they introduced a larger lizard species, Leiocephalus carinatus, which preys on A. sagrei.
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The tiny islands are each about 750 square metres (around the size of a baseball diamond) and located only about 100 metres away from land where L. carinatus naturally live.
These predatory lizards regularly colonise the tiny islands, but routinely die out because they are entirely ground-based and can be wiped out when hurricanes cause flooding. For this reason, Losos says it is ethically acceptable to introduce the L. carinatus onto the islands for experimental purposes.
Out of reach
The team predicted that introducing the predatory species would initially lead to a greater number of A. sagrei lizards with slightly longer legs, which would enable them to run faster than their shorter-legged peers, which would get caught and eaten.
However, they hypothesised that after a certain amount of time, selective pressures would shift to favour lizards with shorter legs, because such animals can climb trees better, and evade the L. carinatus in that manner.
Given time, A. sagrei would somehow learn to escape death by climbing, the researchers reasoned. “These lizards are no dummies,” Losos says.
Natural shift
In fact, all of these predictions came to pass. When the researchers returned to the islands after six months and counted the A. sagrei lizards that survived, they found a greater number had long legs. After a further six months, another survey showed that natural selection had shifted to favour lizards with short legs.
And there was a huge increase in the proportion of A. sagrei lizards that chose to dwell in trees. Normally, about 60% of these lizards are found in trees – and this was the case on the islands with no predator lizards. But in the six experimental isles, which had the introduced predator species, more than 90% A. sagrei were found in trees after one year.
Not only does the study illustrate how swiftly natural selection can act, says Losos, it also shows that the process can be experimentally induced, given the right circumstances.
Journal reference: Science (DOI: 10.1126/science.1133584)
