When Nathalie Alomar stepped into the Appalachian forest after dark, the familiar landscape transformed. "During the day, an Appalachian forest is familiar: trees, leaf litter, logs, and rocks," she recalled. "But after dark, especially after rain, the forest completely changes. Salamanders are suddenly everywhere, walking across the forest floor like they own the place." For an animal that breathes through its skin and depends on cool, moist conditions, watching them thrive felt, she said, "unforgettable."
Alomar, then a Ph.D. student at Yale, had set out to study what seemed like a straightforward case of evolutionary停滞—in English, evolutionary stasis. The common woodland salamander, genus Plethodon, has colonized forests across the eastern United States and diversified into dozens of species while keeping the same basic body shape for tens of millions of years. It's a classic textbook example: many species, little visible change.
But when Alomar and her team collected nearly 300 individual salamanders representing 30 distinct species—from Connecticut to Georgia and as far west as the Ozark Mountains—something unexpected emerged in the lab. While these salamanders looked nearly identical to the naked eye, their inner workings told a wildly different story.
Using specialized equipment, the researchers measured skin resistance to water loss, metabolic rate, heat and cold tolerance, body size, limb proportions, and skull shape. They then cross-referenced these physiological traits with climate data and evolutionary analyses. What they found turned a long-held assumption on its head: the salamanders' internal physiology had evolved rapidly and differently, adapting to local climate conditions, even as their outward appearance remained nearly unchanged.
"Many Plethodon species can look almost indistinguishable from one another, even when they come from very different regions," Alomar said. "But in the lab, I started noticing subtle differences, and it was exciting to see those qualitative impressions reflected in the quantitative patterns we found in the data."
Heat tolerance changed relatively little among species, the team discovered, but how quickly salamanders dried out when deprived of moisture varied enormously—reflecting the diverse humidity of their habitats. The findings, published in the Proceedings of the National Academy of Sciences, challenge the widespread idea that evolutionary diversity can be judged by looks alone.
"This study was all about detecting the hidden inner workings of organisms," said Martha Muñoz, an associate professor of ecology and evolutionary biology at Yale who led the research program. "I've always been fascinated by what goes on under the hood. These salamanders have a body plan that's changed little for tens of millions of years. From an evolutionary biologist's perspective, it's a puzzle. It turns out that the diversity was there all along. It just wasn't obvious from what meets the eye."
The team visited nine states over four years, enlisting a small army of students for the nocturnal fieldwork. Next, they plan to trace the salamanders' deeper evolutionary history: how they spread across North America, what happened during past glacial cycles, and what secrets still hide in their genomes. For these unassuming forest dwellers, it seems, the most important changes were never skin-deep.