Evolution in action has been observed in tree-living lizards that developed thicker, stickier toe pads in as little as 15 years.

The green anole lizards living on islands off Florida were forced to adapt to perching higher in trees by an invasion of scaly cousins from Cuba.

As a result, their feet evolved to become better at gripping the thinner, smoother branches found in the tree tops.

Within 20 generations, their toe pads had enlarged and developed more sticky scales.

US lead scientist Yoel Stuart, from the University of Texas at Austin, said: “We did predict that we’d see a change, but the degree and quickness with which they evolved was surprising.

“To put this shift in perspective, if human height were evolving as fast as these lizards’ toes, the height of an average American man would increase from about five foot nine inches today to about six foot four inches within 20 generations.

The degree and quickness with which they evolved was surprising

“Although humans live longer than lizards, this rate of change would still be rapid in evolutionary terms.”

Invasive brown anoles from Cuba first appeared in South Florida in the 1950s, possibly after stowing away on ships. Since then they have spread across the south-eastern US and even jumped to Hawaii.

The new study, published in the journal Science, is one of only a few well-documented examples of what evolutionary biologists call ‘character displacement’.

This is when similar species competing with each other evolve differences to take advantage of particular ecological niches.

Charles Darwin discovered a classic example in the form of two species of Galapagos finch that developed different beak shapes to adapt to different food sources.

Competition between the two lizard species for the same food and space may be driving the rapid evolution of the green anoles, scientists believe.