How the Tree Frog Has Redefined Our View of Biology- page 3 | Science | Smithsonian
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A beloved symbol of biodiversity, the red-eyed tree frog, shown here in Panama, has evolved a flexible strategy for survival. (Christian Ziegler)

How the Tree Frog Has Redefined Our View of Biology

The world’s most charismatic amphibian is upending the conventional wisdom about evolution

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(Continued from page 2)

Plasticity not only lets frogs cope with challenges in the moment; it might even buy time for evolution to happen. Warkentin has found that tadpoles also hatch early if they’re at risk of drying out. If the rainforest gradually became drier, such early hatching might become standard after countless generations, and the frog might lose its plasticity and evolve into a new, fast-hatching species.

One of the mainstays of evolutionary thinking is that random genetic mutations in an organism’s DNA are the key to adapting to a challenge: By chance, the sequence of a gene changes, a new trait emerges, the organism passes on its altered DNA to the next generation and gives rise eventually to a different species. Accordingly, tens of millions of years ago, some land mammal acquired mutations that let it adapt to life in the ocean—and its descendants are the whales we know and love. But plasticity offers another possibility: The gene itself doesn’t have to mutate in order for a new trait to surface. Instead, something in the environment could nudge the organism to make a change by drawing on the variation that is already in its genes.

To be sure, the theory that plasticity could actually give rise to new traits is controversial. Its main proponent is Mary Jane West-Eberhard, a pioneering theoretical biologist in Costa Rica affiliated with STRI and author of the influential 2003 book Developmental Plasticity and Evolution. “The 20th century has been called the century of the gene,” West-Eberhard says. “The 21st century promises to be the century of the environment.” She says mutation-centric thinking is “an evolutionary theory in denial.” Darwin, who didn’t even know genes existed, had it right, she says: He left open the possibility that new traits could arise because of environmental influence.

West-Eberhard says Warkentin’s group has “demonstrated a surprising ability of tiny embryos to make adaptive decisions based on exquisite sensitivity to their environments.” That kind of variation, West-Eberhard says, “can lead to evolutionary diversification between populations.”

Although not everyone agrees with West-Eberhard’s theory of how plasticity could bring about novelty, many scientists do now think that phenotypic plasticity will emerge when organisms live in environments that vary. Plasticity may give plants and animals time to adjust when they’re dumped in a completely new environment, such as when seeds are blown to an island. A seed that isn’t as picky about its temperature and light requirements might do better in a new place—and might not have to wait for an adaptive mutation to come along.

Also, many scientists think that plasticity may help organisms try out new phenotypes without being entirely committed to them. Early hatching, for example. Different species of frogs vary greatly in how developed they are when they hatch. Some have a stumpy tail and can barely swim; others are fully formed, four-limbed animals. “How do you get that kind of evolved variation?” Warkentin asks. “Does plasticity in hatching time play a part in that? We don’t know, but it’s quite possible.”

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The town of Gamboa was built between 1934 and 1943 by the Panama Canal Company, a U.S. government corporation that controlled the canal until 1979, when it was handed over to Panama. Gamboa, on the edge of a rainforest, is part ghost town, part bedroom community for Panama City and part scientific summer camp. Quite a few residents are scientists and staff at STRI.

When I visited, Warkentin’s team had up to a dozen people, including several undergraduates she refers to as “the kids.” One morning a posse of vigorous-looking young people in knee-high rubber boots, backpacks and hats departs Warkentin’s lab and strides across the field behind the school, past the tennis courts.

James Vonesh, a professor at Virginia Commonwealth University, who did a postdoctoral fellowship with Warkentin and still collaborates with her, points out his favorite sign in town, a holdover from the Canal Zone era: “No Necking.” It’s painted on the front of the stands at the old swimming pool, now part of the local firefighters’ sports club. Then he explains to one of the kids what “necking” means.

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