Life is adept at adapting to changes in the environment – and the environment is changing faster than ever, thanks to us. Evolution is normally thought of on the scale of millions of years, but a new study has observed how human activity has directly driven separate populations of geckos to evolve new attributes in the space of just 15 years.

The human activity in question began in 1996, with the building of the Serra da Mesa Hydroelectric Plant in Brazil. An artificial reservoir was created by flooding 656 sq mi (1,700 sq km), and in the process almost 300 new islands were now cut off from the "mainland."

Researchers from the University of Brasilia and the University of California, Davis studied the newly-separated populations of animals on these islands, focusing on the most common gecko species in the area, Gymnodactylus amarali. The team found that over 15 years, G. amarali on the islands had grown bigger heads on average than those of the same species found on the mainland.

Before the dam was built, the geckos in the area had lived mostly off termites, with larger lizard species eating the bigger bugs and leaving the smaller ones to G. amarali. But it turns out that flooding the valley had wiped out those larger lizards, and with less inter-species competition for food, G. amarali adapted to fill the niche they left behind. The geckos grew larger mouths and heads to help them chow down on the newfound bounty of bigger termites.

It's a great "Petri dish" example of natural selection at work. Essentially, those G. amarali with bigger heads had access to more food, leading to them being more successful at survival and reproduction. Over time, the big-head genes were passed down to later generations in higher numbers, until it became a common characteristic of the island-dwelling geckos. Those still on the mainland, meanwhile, still faced competition from the larger lizards and so saw no change in head size, making them a perfect control group.

While their heads grew, the lizards' bodies stayed more or less the same size. The researchers say this is most likely a matter of efficiency: bigger bodies require more energy to run, which would offset the advantage of a larger head. And as further evidence that a bigger head relative to body size was the most efficient evolutionary path, the researchers found that the trait independently became common among populations on five islands isolated from each other.

The story of G. amarali isn't necessarily a sad one, but it does highlight just how much influence human behavior has on the environment, both directly and indirectly.

The research was published in the journal PNAS.