Darwin's Toolkit

Sean Carroll says exploring the creative power of genes is the next frontier in evolutionary biology.

A BUTTERFLY’S WING IS LIKE A POINTILLIST MASTERPIECE. Fragile, brilliant and diaphanous, it resembles a solid canvas, painted with swirls and spots of elaborate design. But each wing is in fact a mosaic, comprising hundreds of individually colored scales. From a distance, the effect is the same as viewing a Seurat mural. We take no notice of the dots and see only the whole.

In the middle of the 19th century, naturalist Henry Walter Bates looked at the wings of butterflies and saw a different kind of picture, a vision of how species evolve on the fly. On an expedition in the Amazon rainforest, where he catalogued hundreds of butterfly species never seen by Europeans, Bates began to conclude that the insects’ wing patterns were anything but fanciful decoration. In the competition to mate, eat and avoid being eaten, the odd splash of color or eye spot that deterred a predator became a life-or-death utility. Returning to England after seven years in the jungle, Bates wrote excitedly to his friend Charles Darwin: “I think I have got a glimpse into the laboratory where Nature manufactures her new species.”

Bates had the picture right, but he couldn’t see the dots. The genes and proteins that build butterfly wings and other body parts were a frontier even more remote than the Amazonian wilderness. That territory would remain unknown for another 100 years, until the discovery of DNA and the birth of molecular genetics. Now scientists such as CALS molecular biologist Sean Carroll are using those tools to answer the question that bewildered Bates: How did butterfly wings get that way?
“When you look at something as beautiful as a butterfly wing, it’s hard to imagine that there’s an explanation for why it looks the way it does,” explains Carroll. “But it’s decipherable. We know now how to peel back those layers and figure out how it’s put together piece-by-piece.”

The key lies in understanding development, the furious transition when cells divide and specialize to form everything from fish scales to tiger fur. Carroll is among a group of scientists who have figured out how to spy on the genes that do the heavy lifting during this phase, building limbs, wings and other major body parts, and then make cross-species comparisons to describe how they evolved. In studies of fruit flies, Carroll’s lab has pinpointed small changes in DNA that explain why some flies have spots on their wings while others don’t. His work has shown that in some cases flies can gain or lose the ability to create those wing patterns with the flip of a single genetic switch.

This branch of evolutionary biology, known as “evo devo” for its marriage of evolution and development, has gained scientific footing on par with big fossil discoveries. At the American Museum of Natural History, the genetic evidence for evolution shares equal billing with bones in a permanent exhibit on human origins. Carroll has begun writing a regular column for the New York Times, and he appears prominently in an episode of Nova on the genetic evidence of evolution, which airs in December.

“What Sean is doing is unlocking Darwin’s toolkit,” says Neil Shubin, a paleontologist at the University of Chicago and a frequent Carroll collaborator. “He is giving us a mechanistic understanding of how body forms are created and change over time.”

But like Darwin and Bates before him, Carroll is still compelled by what he doesn’t know. He cites the explosion of information unleashed by the sequencing of plant and animal genomes as one example of a vastly unexplored frontier. “We used to get words about a few species, and now we get encyclopedias,” he says. “And that gives us the means to investigate all kinds of new questions.”

But can molecular fossil hunting match the flesh-and-bone dramatics of the early explorers, who battled disease, famine and the occasional cannibal on the way to their discoveries? Carroll spent two years researching the exploits of evolution’s pioneers for his most recent book, Remarkable Creatures, which he says he wrote partly to capture the derring-do of the scientists from that golden age.