Classic sooty-moth tale bolstered by new results

Backyard experiment strengthens scenario for evolutionary changes due to pollution

A recently criticized textbook example of evolutionary forces in action, the dark forms of peppered moths that spread with industrialization in Britain, may be on its way back.

Results from a large experiment support the much-debated original hypothesis that industrialization favored a dark form (left) of the peppered moth over the lighter one (right) because hungry birds found light wings easy to spot against sooty backgrounds. J. Mallet

Results of an ambitious experiment on the moths (Biston betularia) support the original hypothesis that their dark-colored forms spread in soot-coated landscapes because they are more difficult for hungry birds to spot, says evolutionary biologist James Mallet of Harvard University. He and three colleagues have published the final peppered moth experiment of Michael Majerus, who spent six years monitoring the fates of a total of 4,864 moths, presented his conclusions at a conference but died before publishing them. The study appears online February 8 in Biology Letters.

The moth story not only makes “a compelling example of evolution in action,” but it’s “a terrific case history of how science works,” says evolutionary biologist Scott Freeman of the University of Washington in Seattle. “Majerus raised questions; he and his colleagues did the hard work required to answer them.”

The moths, which usually have salt-white wings sprinkled with pepper-black, have long played a role in evolutionary biology. In the early years of genetics, breeding experiments established that a single gene can create a black form. It showed up in Manchester, England, in 1848, and by 1895, 98 percent of the region’s moths were dark. Moths went dark in similarly industrializing areas, and when clean-air regulations began to clear the pollution, dark forms went into decline.

Experiments in the mid-20th century supported the idea that industrial grime provided better camouflage for dark wings, but that work drew escalating challenges starting in the 1990s. Majerus and other scientists raised questions about those studies’ methods, such as whether the high densities of moths released had altered the results and whether the tree trunks where moths were placed were a normal resting place. Debates over those studies ignited reputation bashing, charges of fraud and a firestorm of creationist glee.

Majerus, who lived in a relatively unpolluted hamlet near Cambridge, England, set out to answer some of the questions himself. On spring and summer mornings he rose before dawn to climb ladders and set out eight to 10 moths on realistic spots in the trees behind his home. He then checked to see if they survived the first four hours of daylight.

After six years of moth patrol, he established that dark forms had only about 91 percent the survival rate of the light moths. That kind of predation pressure was strong enough to account for the shift in moth forms, he concluded.

Majerus also established that a noticeable portion (36 percent) of moths do choose to rest on tree trunks, and he tested, and rejected, the alternative hypothesis that bats instead of birds were differentially catching light or dark moths. Birds indeed were eating the moths; Majerus witnessed many of the fatalities from his window.

The storied moth may one day find its way back into textbooks, though perhaps in new forms. Freeman says he considered including it in his introductory textbook with new material on dark moth forms spreading in gritty North American districts. And author Kenneth Miller of Brown University says he’s thought about discussing peppered moths along with color forms of rock pocket mice to explain natural selection’s influence on dark-pigmented forms.

“In short,” Miller says, “the peppered moth story is and always was a fine example of natural selection in action.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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