“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. Red-eyed tree frog hatchlings were dodging hungry snakes a long time before Warkentin started studying the phenomenon 20 years ago. 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. Phenotypic plasticity offers a solution to the crucial puzzle of how organisms adapt to environmental challenges, intentionally or not. The realization has big implications for how scientists think about evolution.

Depending on the environment-whether there are snakes, hurricanes or food shortages to deal with-organisms can bring out different phenotypes. 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. But a lot of the variation out there comes from environmental effects.” 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. 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 phenotype is pretty much everything about an organism other than its genes (which scientists call the genotype). 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. 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. Life, even an entity as seemingly simple as a frog egg, is flexible. 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. With just a flick of her finger, Warkentin has demonstrated a phenomenon that is transforming biology. “It’s not something I get tired of watching,” Warkentin says. Another and another of its siblings follow. It lands partway down the leaf, twitches and falls into the 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. She reaches down to wet her hand in the pond water. Under a microscope, the red hearts would just be visible. Tiny bodies show through the clear gel-filled membrane. The ones that Warkentin is pointing to, judging from their size and shape, are about five days old, she says. Normally, an egg hatches six to seven days after it is laid.

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

Purple frog red tree frog