Biologists who released lizards on several uninhabited islands in the Bahamas have been able to observe an evolutionary effect in action.
Jason Kolbe, a biologist at the University of Rhode Island and his team have found that the lizards' genetic and structural traits were determined by both natural selection and a phenomenon called the founder effect. This is the loss of genetic variation that occurs when a new population is established by a very small number of individuals.
"We rarely observe the founder effect as it happens in nature, but we know that it occurs because islands are colonized by new species over time," says Kolbe.
"What we didn't know was how these evolutionary mechanisms interact with each other."
Kolbe's team randomly collected brown anole lizards from a large island near Great Abaco in the Bahamas, and released one pair on each of seven nearby islands whose lizard populations had been cleared by a recent hurricane.
The scientists predicted that natural selection would lead the lizards to develop shorter limbs. Great Abaco is forested, while the other islands have short, scrubby vegetation, and previous research had found that anole lizards living in forests had longer hind limbs than those found in scrub habitat.
Lizards with longer legs can run faster on the broad perches available in forests, while short-limbed lizards are better at moving on the narrower perches found in lower vegetation.
The scientists revisited each of the islands over the next four years to measure the lizards' limb length and collect tissue samples for genetic analysis. All the new populations survived and increased an average of 13-fold in the first two years, before leveling off.
"We noticed a founder effect one year after starting the experiment, which resulted in differences among the lizards on the seven islands. Some of the islands had lizards with longer limbs and some had lizards with shorter limbs, but that was random with respect to the vegetation on the new islands," says Kolbe.
"Over the next four years, the lizards on all the islands experienced a decrease in leg length that is attributable to natural selection. But those that started out with the longest hind limbs still had the longest hind limbs."
The fact that the populations maintained their order from longest to shortest limbs throughout the experiment means that both the founder effect and natural selection contributed to their current differences.
The next step will be to determine how long the founder effect persists before other factors erase its signature.