Male green (left) and brown (right) anoles displaying their neck sails, or dewlaps. Credit: Todd Campbell and Adam Algar.
Green anoles are small lizards native to Florida in the United States. “The green anole is about 3-6 cm long,” said Doctor Yoel Stuart, lead author of the study, “it’s active during the day, it tends to eat arthropods: insects and spiders and things”. These lizards tend to live on the ground and around the bases of tree trunks.
Another anole species, the brown anole, has begun invading islands in Florida. The brown anole has the same diet as the green anole, is active during the day, and also lives on the same parts of trees.
An evolutionary theory suggests that closely related species, like the brown and green anole, in the same area will compete most intensely for similar resources, thus forcing each other to change (or to go extinct). This theory is called ‘character displacement’.
Todd Campbell, a co-author of the current study, tested the character displacement theory back in the mid-90s, using the green and brown anoles. Dr. Campbell measured the perch heights of green anoles on six islands and then artificially introduced brown anoles to three of them. When faced with the brown anoles, the green anoles showed a rapid increase in their perch heights.
Yoel explained the next step, “so we saw this behavioural response [after brown anole introduction] and then 15 years later, we’re like…‘let’s go back to these islands and see if there’s evolution going on’”.
Before going to the islands, the team made some predictions. Science uses theories (hypotheses) to make bold predictions; if those predictions fail, the theory must be re-evaluated; if those predictions hold true, the theory must continue to be challenged.
“So we predicted that perhaps the green anole would evolve larger toe pads given that habitat-use shift,” said Yoel. “Because we know that…species that are living higher up in trees also tend to have larger toe pads and we also know that having larger toe pads makes you better at clinging”.
First, the team returned and confirmed that the green anoles on the invaded islands were indeed still perching higher. To test the next prediction, the team used mini-fishing rods (with mini-lassos) to collect lizards from the islands. They took them to a makeshift lab, anaesthetised them, scanned their feet, and then returned them to their trees.
A scan of the green anole’s toe pads. Credit: Yoel Stuart/University of Texas at Austin
“From those images we were able to measure their toe pads, where we'd get both the number of scales and the area of those toe pads. And they got bigger on those [invaded] islands,” said Yoel.
In just 15 years and 20 generations, the green anole’s toe pads were significantly larger on the invaded islands. The predictions of character displacement theory were fulfilled; competition with the brown anole drove changes in the green anole’s behaviour and then body.
But how do the researchers know that toe pad enlargement on invaded islands was evolution and not a coincidence or another influence? The scientific method, of course.
“[Another] possibility is that invaded islands are different somehow in their environment so that green anoles aren't necessarily responding to the invader, but responding to some difference in the environment,” suggested Yoel. “Say, if there were some kind of tree on the invaded islands that allows the green anole to live up higher - and that tree is not present on the uninvaded islands.”
To rule out that possibility, the researchers surveyed every study island, and counted and measured every tree and shrub species. “We couldn't find any appreciable differences [between] invaded and uninvaded islands,” said Yoel.
If the lizards didn’t evolve, the other most likely alternative was ‘plasticity’. To address this, the researchers did a ‘common garden experiment’. Yoel explains, “the idea was to test whether the divergence that we'd seen was really a genetic change - or if it was the result of plasticity or a plastic response”.
“One example of a plastic response during growth is that professional tennis players tend to have longer serving arms than non-professional tennis players,” said Yoel. This is not due to genetics, but due to muscle, ligament, and bone reinforcement over tens of thousands of serves, the body’s own plasticity.
“We took female [green anoles] from invaded islands, females from uninvaded islands, brought them back to the lab, collected the eggs as they laid them, hatched the eggs, raised them up past the 6 months. And what we wanted to ask was, are those differences that we saw in the wild, maintained in the lab,” said Yoel.
“In the common garden, [the raised] lizards of mothers from invaded islands had larger toe-pads than lizards from mothers from uninvaded islands,” said Yoel, “[this] suggests there's some genetic, evolved component to those differences”.
If several populations of one species are separated and unable to interbreed, once they are introduced to each other, they may compete and drive each other further down evolutionary forks. In this way, species might drive their own evolution through a positive feedback cycle.
Through this rigorous and comprehensive experimental design, Yoel and his co-authors were able to rule out alternative influences to conclude that anole evolution had occurred as predicted by the character displacement theory, a phenomenon that may very well have contributed to the great diversity of life we see today.