This Moth's Fast Color Change Is an Evolutionary Tale
The peppered moth has long been one of the most popular stories in all of evolution—for Darwinians and creationists alike. The Darwinians have always treated the sudden appearance in the mid-19th century of dark-winged moths of this species (Biston betularia) as the first evidence of evolution taking place within a single human lifetime. Creationists have countered that this supposed slam dunk for natural selection was just a product of biased scientific research bordering on fraud.
A study published this week in the journal Nature finally resolves this bitter debate with irrefutable genetic evidence. So which side wins? Is it the textbook case for Darwinism? Or was it all a terrible mistake, as the creationists have alleged?
I’m going to make you hold your breath for a bit while I fill in the background.
Until the mid-19th century, the common form of peppered moth had a pale coloration suited to hiding on the bark of light-colored or lichen-coated tree trunks. The theory was that this camouflage enabled it to avoid being eaten by birds. But in 1848, in the industrial city of Manchester, England, a specimen with black wings turned up. By the end of the 19th century, this version of the peppered moth was everywhere, and the paler, mottled version had disappeared, almost becoming extinct.
The shift was no accident, according to scientists. Reliance on coal for heating and industrial production had blackened skies and forests. An editorial in the same issue of Nature quotes an 1851 railroad guide to the English industrial midlands: “The pleasant green of pastures is almost unknown, the streams, in which no fishes swim, are black and unwholesome…the few trees are stunted and blasted.” These blackened tree trunks no longer camouflaged pale-colored moths, and birds presumably devoured them.
But the random appearance of the black form of the same species conferred a distinct advantage, because those moths were much harder for hungry birds to spot. It was natural selection in action. (The same shift occurred in the same species at about the same time in the United States, particularly around pollution-blackened Pittsburgh. But most of the research in the new study took place in Britain.)
To test the Darwinian version of events, an Oxford University scientist named Bernard Kettlewell in the 1950s introduced moths of both colorations in polluted and unpolluted forests. He verified the theory that camouflage suited to local conditions—whether dark or light—saved those moths from being gobbled up by birds. Additional circumstantial evidence for natural selection developed after the 1950s, when antipollution laws began to un-blacken the landscape. That proved advantageous to the lighter form of peppered moths, which began to make a broad resurgence.
Other scientists later quibbled over some of Kettlewell’s methods: Was it a valid test to pin dead moths to tree trunks or to flood a forest with an unnatural abundance of living moths? Creationists then treated this healthy scientific scrutiny of methods as if it were a debate about the fundamental science—and blew it out of proportion, often with the help of selective quotation. In the normal course of science, other researchers tested the original proposition using better methods and verified Kettlewell’s results in meticulous detail. But creationists have generally ignored that evidence and stuck with the original quarrel.
This brings us to the new genetic study, by a team of scientists largely working in the laboratory of Ilik Saccheri at the University of Liverpool. Applying “next-generation sequencing technology to open up what were previously treated as genetic black boxes,” they tracked the change in peppered moths to a specific gene. Then they narrowed the change to a specific random mutation on that gene, exactly as Darwinian theory had predicted. The mutation involved what is popularly known as a “jumping gene” (a transposon, or transposable element, to scientists): a mobile segment of DNA that can change position within a genome and alter the expression of other genes. The researchers not only found the specific mutation, but they dated its appearance to about 1819, early in the modern era of heavy reliance on coal.
Because creationists have continued to use the peppered moth “to further an antiscience agenda,” said Saccheri in an interview, “I think it’s important to respond with additional layers of evidence. And so here we have in some sense the ultimate piece of evidence that’s now written in stone.”
Jerry Coyne, the University of Chicago biologist whom creationists love to quote for his past criticisms of peppered moth research, praised Saccheri’s work. “I’m satisfied that the bird predation story is sound and the original experiment was right,” he said. The new study “completes this story. We now know what happens all the way from the mutation itself to the ecological forces involved.”
Saccheri recommended that creationists—or the people who call themselves “creation scientists”—go to any natural history museum with a collection of peppered moths and extract the DNA from both lighter and darker varieties. “You can do the test that we describe in the paper, and you will find the transposon”—that is, the mutation—“in the black ones. And further, you will find that not a single light-colored moth has that. We’re no longer relying on historical records that people can cast aspersions on. You look inside modern biological material and there is only one conclusion you can draw.”
But I’m going to make a prediction: Creationists will not draw that conclusion. They will instead ignore the science, because what they care about has nothing to do with science, or facts, or even truth—and everything to do with blind religious faith.