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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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Karen Warkentin, wearing tall olive-green rubber boots, stands on the bank of a concrete-lined pond at the edge of the Panamanian rainforest. She pulls on a broad green leaf still attached to a branch and points out a shiny clutch of jellylike eggs. “These guys are hatchable,” she says.

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Red-eyed tree frogs, Agalychnis callidryas, lay their eggs on foliage at the edge of ponds; when the tadpoles hatch, they fall into the water. Normally, an egg hatches six to seven days after it is laid. The ones that Warkentin is pointing to, judging from their size and shape, are about five days old, she says. Tiny bodies show through the clear gel-filled membrane. Under a microscope, the red hearts would just be visible.

She reaches down to wet her hand in the pond water. “They don’t really want to hatch,” she says, “but they can.” She pulls the leaf out over the water and gently runs a finger over the eggs.

Sproing! A tiny tadpole breaks out. It lands partway down the leaf, twitches and falls into the water. Another and another of its siblings follow. “It’s not something I get tired of watching,” Warkentin says.

With just a flick of her finger, Warkentin has demonstrated a phenomenon that is transforming biology. After decades of thinking of genes as a “blueprint”—the coded DNA strands dictate to our cells exactly what to do and when to do it—biologists are coming to terms with a confounding reality. Life, even an entity as seemingly simple as a frog egg, is flexible. It has options. At five days or so, red-eyed tree frog eggs, developing right on schedule, can suddenly take a different path if they detect vibrations from an attacking snake: They hatch early and try their luck in the pond below.

The egg’s surprising responsiveness epitomizes a revolutionary concept in biology called phenotypic plasticity, which is the flexibility an organism shows in translating its genes into physical features and actions. The phenotype is pretty much everything about an organism other than its genes (which scientists call the genotype). The concept of phenotypic plasticity serves as an antidote to simplistic cause-and-effect thinking about genes; it tries to explain how a gene or set of genes can give rise to multiple outcomes, depending partly on what the organism encounters in its environment. The study of evolution has so long centered on genes themselves that, Warkentin says, scientists have assumed that “individuals are different because they’re genetically different. But a lot of the variation out there comes from environmental effects.”

When a houseplant makes paler leaves in the sun and a water flea grows spines to protect against hungry fish, they’re showing phenotypic plasticity. Depending on the environment—whether there are snakes, hurricanes or food shortages to deal with—organisms can bring out different phenotypes. Nature or nurture? Well, both.

The realization has big implications for how scientists think about evolution. Phenotypic plasticity offers a solution to the crucial puzzle of how organisms adapt to environmental challenges, intentionally or not. And there is no more astonishing example of inborn flexibility than these frog eggs—blind masses of goo genetically programmed to develop and hatch like clockwork. Or so it seemed.

Red-eyed tree frog hatchlings were dodging hungry snakes a long time before Warkentin started studying the phenomenon 20 years ago. “People had not thought of eggs as having the possibility to show this kind of plasticity,” says Mike Ryan, her PhD adviser at the University of Texas in Austin. “It was very clear, as she was doing her PhD thesis, that this was a very, very rich field that she had sort of invented on her own.”

Karen Martin, a biologist at Pepperdine University, also studies hatching plasticity. “Hatching in response to some kind of threat has been a very important insight,” Martin says. “I think she was the first one to have a really good example of that.” She praises Warkentin’s sustained effort to learn big biology lessons from frog eggs: “I think a lot of people might have looked at this system and said, ‘Here’s a kind of a quirky thing that I could get some papers out of, and now I’ll move on and look at some other animal.’ She dedicated herself to understanding this system.”

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