South Florida lizards are helping to reshape Darwin’s theories
What do the finches of the Galapagos Islands and the lizards of a tiny island at Fairchild Tropical Botanic Garden have in common? They’ve both shaped ideas on how Charles Darwin’s theory of evolution works.
But the lizards of Fairchild are shaking things up.
A recent study by Georgia Tech evolutionary ecologist James Stroud shows that evolution can happen much faster than Darwin thought.
Lizards, lassos and a paradox
To figure this out, Stroud and his team took advantage of South Florida’s proliferation of fast-breeding lizards, and the fact that four species of them — native green anoles, and non-native brown anoles, bark anoles and Cuban knight anoles — live relatively isolated lives on an island about the size of a football field at Fairchild, just south of Miami.
Few lizards leave or come to the island, and they all face the same predators and other pressures. They also inhabited different niches in the habitat — brown anoles are ground hunters, bark anoles prowl lower tree trunks, greens are up in the tree branches and canopy, and the much larger Cuban knight anoles are at the treetops.
Stroud and his team of intrepid lizard catchers used tiny lassos to capture every lizard — 1,000 or so — on the island, tagged them with miniscule 3 mm markers so they knew who they were, measured traits such as how long or short their legs were, then set them free. They recaptured them every six months for 3.5 years to see what traits the survivors possessed — long or short legs, big or small sticky toe pads.
It took about a month to catch them all. “What’s crazy is we’d catch 90% of the lizards in the first seven to 10 days, and the last 10% took 2.5 weeks. Some lizards are a lot easier to catch than others,” Stroud said.
What they discovered would have surprised Darwin. Stroud found that one of Darwin’s prevailing ideas — that evolution happens very slowly over long periods of time — wasn’t quite accurate.
That notion has been disrupted over the past 50 years or so, he said, as contemporary evolutionary biologists have figured out that evolution can happen really really quickly.
Part of Darwin’s idea is called “stabilizing selection,” the idea that moderate traits are the fittest.
“We see nothing of these slow changes in progress,” Darwin wrote in 1859, “until the hand of time has marked the long lapse of ages.”
One can forgive Darwin. Stroud said that some fossil records don’t change at all for 50 million years. “Slow gradual change through the fossil records was the evidence Darwin used to think that evolution moved at a really, really slow pace,” Stroud said.
But Stroud’s lizards showed rapid changes almost constantly.
For example, when Stroud’s first lizard survivors showed a common trait of long legs, Darwin would suggest the next generation would have long legs. But they didn’t. Short-legged lizards had an advantage and won out.
“Stabilizing selection suggests that lizards with average legs would be the most successful. What we saw was that that changed from year to year. Some years the ones with long legs were better, some years the ones with shorter legs were better. Evolution was happening all the time. It wasn’t just standing stable. But it was kind of canceling itself out, which led to this (long-term) pattern of stasis, this very stable pattern, but just not in the way that most evolutionary biologists thought about it.”
Stroud said they don’t know what environmental forces prompted the better survival of long- or shortlegged lizards in each generation.
One of the reasons the study was so informative is that brown anoles (the most common species) can typically reproduce at around six months.
Males in the wild rarely survive more than 12 months and females 12 to 18 months. In other words, the ecologists were able to observe three generations in their 3.5-year study.
Species have the capacity to change very quickly, but they often don’t. This has been a paradox to scientists for decades.
Stroud said the paradox is: “Why do most things not change over long periods if they have the potential to change very quickly?”
Stroud’s work proves that they can, and if faced with strong enough environmental change, the species will change through natural selection.
Sticky toes
Another example of quick evolution, also known as directional evolution, can be seen in a separate study from 2014.
Anole populations living on islands off the coast of Florida took just 15 years to grow larger, stickier toe pads for climbing when confronted with non-native brown anoles, which prefer the ground. Meanwhile, the green anoles on islands without brown anoles did not develop the larger, stickier pads.
“That’s when species are interacting in a new way,” Stroud said. Green anoles had lived in Florida for 5 million to 7 million years before humans brought brown anoles from Cuba 40 or 50 years ago, he said.
“When the brown anoles were not there, green anoles were everywhere. … Green anoles freaked out and shot up to the tops of the trees to maintain space between them. … So the consequence of them having to move up and live at the top of the trees, there was directional evolution for larger, sticky toes. So stabilization was disrupted.”
“Florida is an exciting place for this research,” Stroud said. “With climate change we’re very interested in how species are going to adapt to a changing climate. This whole natural experiment with python or iguanas or Caribbean anoles is playing out right in front of us in Florida. Not only can we learn a lot from it scientifically, but it can also inform us on invasive species success.”