- Scientists have observed that a Florida lizard species has moved much higher in the trees when faced with competition from a new competitor, an invasive lizard species from Cuba.
- Living higher in the trees involves different physical demands. Most importantly, the branches are younger, making them thinner and smoother.
- A recent experiment has shown that in just 15 years’ time, green lizards evolved anatomical changes to match their new niche in the upper regions of trees.
The life of the green anole
Small lizards called anoles are found throughout the Caribbean islands and nearby mainland areas. The coastal and swamp regions of Florida are brimming with a species of green anoles. Many tourists and locals mistakenly refer to them as chameleons because anoles, too, can change their skin coloring.
Anoles have long been a favorite subject of evolutionary biologists because of their relatively recent geographical isolation and subsequent evolutionary divergence. Like Darwin’s finches in the Galapagos Islands of South America, green anoles have experienced adaptive radiation when populations of these lizards ended up on remote islands. Each island or mainland area has its own unique set of anole species that are well suited for the particular habitat and lifestyle in which they live.
Most anoles live exclusively or mainly in trees, eating insects, spiders, and whatever little creepy-crawlies they can find.
The arboreal life has led anoles to develop very habitat-specific anatomical adaptations. Across the many species of anole in the American tropics, each is well suited for the type of trees and shrubs on which they live. For example, woody branches require rugged feet pads. Slick surfaces require more surface contact with the sticky feet. Shorter, more muscular digits are best for thick branches, and so on.
Anoles are considered “hyper-adaptable,” because they appear to adapt to precise habitats and niches relatively quickly. There are many ideas about why some species hyper-adaptable, but most involve the notion that some species experience or tolerate higher mutation rates than others, at least in certain “hot spots” in their genes. Dogs are another example of hyper-adaptability, which helps to explain how humans established the wide variety of dog breeds using only selective breeding.
New competition forces changes
In the past few decades, the green lizards common in Florida and nearby islands, Anolis carolnensis, have faced intense competition from a new rival, an invasive species of brown lizards, Anolis sagrei, from Cuba. The invasive species threatens the Florida green anoles, not by attacking them, but by competing with them for the same food in the same habitat.
In the field of evolutionary biology, the precise location and lifestyle of a species is called its niche. The competitive exclusion principle states that two species cannot occupy the exact same niche for very long. When this happens, even a slight advantage in one species will mean the extinction of the other. In practice, the vanquished species does not often go instinct. Instead, it seeks out a new niche to avoid direct competition with the more robust species.
When a species is forced into a new niche, they usually find themselves imperfectly adapted. This creates selective pressure and, over the course of many generations, the species will evolve and become better adapted to the new niche.
This is what has happened to the green anoles in Florida. Since the 1970s, scientists have been reporting that they now live considerably higher in the trees than they previously did. While everyone suspected that competition from the brown anoles was what drove the green anoles further up the tree, scientists had no way to be sure. Until now.
Movin’ on up
A new study published in Science magazine reports the results of a clever experiment. On several islands off the coast of Florida, green anoles had been living peacefully as before, with no competition from invasive brown anoles. Then, in 1995, scientists intentionally introduced brown anoles and observed the effect on the green anoles.
As suspected, rather than starve to death, the green anoles simply moved up, literally speaking. Facing competition in the shrubs and low tree branches, the green anoles moved to branches higher up in the trees.
Generally speaking, the higher up you go in the tree, the less bugs are to be found, especially the favorite meals of anoles, young crickets, grasshoppers, spiders, and palmetto bugs. This is why the green anoles have always shirked the high branches until recently. Nevertheless, because the brown anoles are even less adapted to high branches than the green anoles are, there was an opening for the green anoles to avoid extinction.
Intense pressure can induce rapid evolution
As the green anoles first transitioned to the new niche, their mal-adaptation would have been felt immediately. High branches are markedly thinner and smoother than low branches. Young bark has negligible scarring, cracks, and fissures to provide grip for tree lizards. Thus, the green anoles were likely slipping and sliding as their stubby digits provided too little surface area for sticking to the slippery young branches.
But not for long. Scientists have reported that the green anoles of Florida have evolved longer, wider, flatter digits on their feet. The increased length of the toes helps the lizards wrap their toes more completely around the young thin branches, while the wider and flatter feet increase the surface area for contact with the smooth branches. More contact surface area means they can generate more adhesive strength to better stick to the slippery branches.
These adaptations are not at all surprising. This is exactly what we would predict as the green anoles were pushed into their new niche. What is surprising is the rate at which this adaptation has taken place. Measurable differences in dactyl anatomy have evolved in just fifteen years, which is around twenty generations for these lizards. In evolutionary terms, that is practically overnight.
Remember that this change is genetic. Their feet have not changed as a result of exercise or stretching. These lizards are adapting in the true biological sense of the word: through mutation and differential survival. And they are doing it in record time.
In keeping with the spirit of the 1970s, when the green anoles first encountered the competition from brown anoles, they are stayin’ alive by movin’ on up.
[Why I am writing about green lizards in a blog about human evolution? I have chosen to highlight this research for two reasons. First, it is yet another example that can be mentioned whenever someone who doesn’t know biology claims that “evolution has never been observed in real-time.” Yes it most certainly has. Second, the idea of rapid evolutionary change is essential to understanding human evolution. Over the last few million years, our ancestors underwent several periods of intense selection and rapid evolutionary change. The massive expansion of the cerebral cortex is one such example. Recent changes in skin pigmentation among racial groups is another. Observing how rapid adaptation works in other species helps us understand how it happened with us. After all, the forces of natural selection operate the same way in humans as they do in lizards. We are not so different.]