A mutation in the gene cortex is responsible for darkening the wings of some peppered moths.

Ilik Saccheri

‘Landmark study’ solves mystery behind classic evolution story

The story of the peppered moth is a classic example of evolution in action, right up there with Darwin’s finches. As coal soot and smoke blackened the trees of industrial England in the late 1800s, a rare, dark variant of the peppered moth flourished, quickly supplanting its common, white peers by blending in with the newly darkened tree bark. By the 1950s, 90% of all peppered moths in the region near Manchester were dark, not white. When air quality started to improve in the 1970s, the white moths made a comeback; today, they constitute more than 90% of the population.

But the story had an unsatisfying ending: Despite decades of research, scientists didn’t know the exact mutation responsible for the once-unusual dark wings. Now, a new study pinpoints the location and identity of the mutation. A second study—also new—reveals that the same gene where the moth mutation occurs controls the colorful patterns in some butterfly wings, too. Surprisingly, the mutation is not in a gene known to be related to coloration. Given the exalted status of the peppered moth, the first paper is a “landmark study,” says evolutionary biologist Peter Holland of the University of Oxford in the United Kingdom, who wasn’t involved in the research. “We don’t get a real understanding of evolution until we actually see the mutation.”

By 2011, a team of researchers led by Ilik Saccheri, an ecological geneticist at the University of Liverpool in the United Kingdom, had narrowed down the likely location of the mutation to a single chunk of chromosome. In the new study, Saccheri and his colleagues sequenced and compared that region in both forms of the moth. The 87 differences were candidates for the dark color mutation. Next, they looked at each of those locations in the DNA of 283 pale peppered moths and 110 dark peppered moths.

The scientists found a single genetic variation in 95% of the dark moths that was missing in every pale moth they tested, they report online today in Nature. The mutation—the insertion of a portion of DNA that can “jump” to a new location within a genome—occurs within cortex, a gene known to affect cell division and egg development in fruit flies. “It’s surprising that it’s this gene, rather than something that’s more recognizably wing pattern–related or color pattern–related,” Saccheri says.

The gene’s influence isn’t limited to moths. A different team of researchers compared sections of DNA from distinctly patterned forms of three species of the tropical Heliconius butterfly. The scientists singled out cortex as the genetic driver behind the vivid wing variation, they report today, also in Nature.

Researchers aren’t sure how cortex shapes the color of moths and butterflies, but it may have something to do with the growth of the tiny scales that cover their wings like a mosaic. Scales of different hues develop at different speeds, says evolutionary geneticist Nicola Nadeau of the University of Sheffield in the United Kingdom, co-author of the butterfly study. The cortex gene “could be controlling this developmental rate difference, and that is presumably how it’s controlling the butterfly color patterns,” she says.

Considering the differences between moths and butterflies—they’ve been following separate evolutionary pathways for about 100 million years—the fact that the same gene plays an important role in the wing coloration of both groups is an intriguing result, Holland says. He adds that the next step will be to find the mechanism by which cortex works. This latest addition to the peppered moth tale, he says, “is going to be in the textbooks for decades to come.”