Cold-War exclusive: Cuban lizards invade Florida
A new study of lizards along Florida’s Atlantic Coast shows that a long- lasting genetic change — evolution — can occur in just 20 generations. Scientists have seen evolution among microbes or fruitflies in the lab, but demonstrating a change in gene structure is more difficult for larger organisms living beyond the lab.
For decades, native Anolis carolinensis lizards have lived on islands built of sediment dredged during construction of the Intracoastal Waterway. In the 1940s, the closely related Cuban brown anole lizard (Anolis sagrei) entered Florida. Since the Cuban outcompetes the native for food and habitat, A. carolinensis lizards began moving higher into the vegetation.
The situation provided a natural experiment, says Yoel Stuart, a post-doctoral fellow at the University of Texas. Stuart is first author of new study in Science showing that this competition forced a change in native-lizard behavior and foot structure.
The behavior change started within months of the Cuban’s arrival. The change in the genes that control the size and build of foot pads needed to hold onto branches happened within two decades.
A natural experiment
Plants and animals, including the native anole lizard, took up residence after the artificial islands were created 60 or more years ago. Competition began with the arrival of the Cuban lizard in the 1990s.
During a 1995 survey, co-author Todd Campbell, an associate professor of biology at the University of Tampa, located six islands with only the native lizards, measured the height of their perches, then released Cubans on three of those islands. Within three months, the natives were roosting higher in the trees — but only on islands with the introduced Cubans.
Both lizard species eat insects and compete for territory, says Stuart. The competition “is a little bit of everything; they fight, eat one another and eat each other’s food.”
Was the native’s choice to cede the lower elevations to the Cubans a simple behavior change, or was it associated with a genetic change — evolution? Higher branches are smaller, and higher leaves tend to be slicker, and so before investigating the genetics, the researchers predicted that the lizards would enlarge the toepads that give them such a good grip.
The observations for the new report began in 2010, when Campbell, Stuart and their colleagues compared six islands containing both species to five islands housing only natives. Again they noticed the different perching height, and also noticed that toepads on the natives were larger than before.
So far, so good. But what about genetics? After all, environment and behavior can have physical effects with no alteration of the genetic code. “Professional tennis players have a longer serving arm, likely because of tens of thousands of serves,” says Stuart. With the lizards, “There might be some plastic changes that occur during growth that cause them to have different sized toepad just because they are living in trees, but this is not evolved.”
To test whether the enlargement was genetic, the researchers raised individuals from both the invaded and native-only islands in a controlled environment. “We found those differences maintained in the lab, which suggests that the population has evolved,” Stuart says.
Finally, using advanced sequencing, the researchers compared the displaced natives, and concluded that the larger toepads had evolved on each of the six islands they shared with the Cubans.
We remarked that 20 generations or less seemed a short time for an evolutionary change, and Stuart responded that biologists “are tending to realize that if selection is strong, it’s possible to detect evolution rather quickly.”
Finding a way to enter an environmental niche — like the upper branches — “is a really good way of partitioning resources, building up the number of species that can live in a community, and allowing species to coexist,” Stuart says.
Stuart wrote that if the same rate of change in the standard deviation (a statistical gauge of variations in data) in the toepads were applied to men, “a back of the envelope calculation” showed that “the average U.S. male would be the height of an NBA shooting guard or small forward” after 20 generations.