As soot from coal-fired factories blackened trees and buildings in 19th century England, naturalists noticed that peppered moths were also trading in their light-colored wings sprinkled with black specks for a sleek, all-black stealth-bomber look known as the carbonaria form. Within a few decades of their first appearance near Manchester, the black moths dominated, making up 90 percent or more of the peppered moth population in local urban areas.
Biology textbooks often cite peppered moths as a classic example of adaptation to changing environmental conditions. The trouble is, no one really knew which molecular changes led the moths to switch wing color. It was an open debate whether the change, which presumably allowed moths to blend better into the increasingly grimy background and avoid bird predators, was due to one mutation or many, and if the adaptation occurred once or several times.
Now, researchers led by Ilik Saccheri, an ecological geneticist at the University of Liverpool in England, report online April 14 in Science that they have traced the mutation responsible for the funereal look to a single page in the moth’s genetic instruction book. That page is a region of a chromosome that contains genetic instructions for creating color patterns on wings in butterflies and other related species. This region of the butterfly and moth genome is an adaptation hot spot — one in which mutations produce hundreds of different wing color patterns in many species, including variations that allow edible butterfly species to mimic foul-tasting species, and mutations that control the size of eyespots on butterfly wings.
“The fact that the carbonaria mutation maps to the same region as butterfly wing pattern genes is amazing,” says Robert D. Reed, an evolutionary developmental biologist at the University of California, Irvine, who was not involved in the study. “Presumably it takes hundreds of genes to make a wing pattern, so why does this region appear over and over again?”
As yet, no one has identified the precise DNA changes that lead to the hundreds of different color patterns, but scientists are actively scouring the region for the pattern-changing mutations.
Likewise, Saccheri and his colleagues don’t yet know which genes or regulatory elements are altered by the carbonaria mutation. What they do know is that the black moths they collected from 80 sites in the United Kingdom share some key genetic signposts, suggesting that the carbonaria mutation involves only one spot in the genome and happened just once, probably shortly before the first reported sightings in 1848 near Manchester.
“I think we have fairly strong evidence that industrial melanism in the U.K. was seeded by a single recent mutation,” Saccheri says. That might not settle the matter, though. “Until we find the causal mutation it’s still open to some debate.”
Peppered moths in continental Europe and the eastern United States also went dark during the industrial revolution. Saccheri does not know if those moths have mutations in the same region as the British moths or if mutations elsewhere produced the same color pattern.
Notably, once the air was cleaned up in Britain, the black moths declined in numbers while the peppered form increased. The carbonaria form now account for only a few percent of peppered moths in England and Wales, Saccheri says.
Image: An all-black form of peppered moths (left) nearly replaced the typical light-colored form (right) during the industrial revolution in England. Researchers bred this black male and typical-form female to help determine that a single mutation that happened in the late 1840s led to the famous adaptation. (Ilik Saccheri/Science/AAAS)