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Peppered Moth Now Reverts Back to Gray: Evidence of Oscillating Selection?

Casey Luskin

In the world of peppered moths, gray is the new black. The “peppered moth” became famous after textbooks started using it as an iconic example of evolution. It’s still employed in some current textbooks: Douglas Futuyma’s 2005 edition of Evolution states, “By the 1930s, however, examples of very strong selection came to light. One of the first examples was Industrial Melanism in the peppered moth (Biston betularia). … There is considerable evidence, obtained by several independent researchers, that birds attack a greater proportion of gray than black moths where tree trunks, due to air pollution, lack the pale lichens that would otherwise cover them.” (p. 393) While Futuyma is right to further note that “other factors also appear to affect the allele frequencies,” debates have raged over whether moths really do rest on tree trunks where they are predated on by birds, whether birds are the main cause of changes in the relative proportion of dark and light moths in populations, and how much the colors really changed.

All that aside, a recent article in the London Daily Telegraph now reports a new chapter in this story, as moth populations are now reverting from black back to gray / white:

The Peppered moth, which changed its colour from white to black in areas of Britain with heavy pollution, is now reverting to its original appearance. … Now in post-industrial Britain, 200 years after Darwin’s birth, the moth is changing back to its original white colour.

“We have seen these moths making a big swing back to their original colour,” said Richard Fox, project manager of Moths Count. “It has been happening for decades as air pollution is cleaned up and with the demise of heavy industry in the big cities.[“]

(Moth turns from black to white as Britain’s polluted skies change colour, London Daily Telegraph, June 19, 2009.)

The idea is that in the absence of industrially-generated soot on trees, black moths are easier to spot and the gray moths now have an advantage in camouflage. Of course, this hypothesis depends entirely on the highly disputed classical moth story being true–namely that moths rest on tree trunks where they are commonly eaten by birds. Moth researchers are also hoping to understand, sadly, “why the moth has been declining so dramatically since the 1960’s.” (Facetiously, a cynic might suggest it’s due to all the collecting of moths for evolutionary research!)

So what does this mean for the debate over evolution?

If you’re an evolutionist, this is now becoming at best a case of oscillating selection, much like what has been observed in the oscillating sizes of beaks of the Galapagos finches, which grow slightly larger during a drought but revert back to their original size when the drought is over. Regarding those finches, Jonathan Wells observes in Icons of Evolution that evolutionists overstate their case when “evidence for oscillating natural selection in finch beaks is claimed as evidence for the origin of finches in the first place.” (p. 174)

Are such extrapolations warranted? Phillip Johnson comments:

To make the story look better, the National Academy of Sciences removed some facts in its 1998 booklet on Teaching About Evolution and the Nature of Science. This version omits the flood year return-to-normal and encourages teachers to speculate that a “new species of finch” might arise in 200 years if the initial trend towards increased beak size continued indefinitely. When our leading scientists have to resort to the sort of distortion that would land a stock promoter in jail, you know they are in trouble.

(Phillip Johnson, “The Church of Darwin,” Wall Street Journal August 16, 1999.)

Wells likewise critiques portrayals of the finch story: “Rather than confuse the reader by mentioning that selection was reversed after the drought, producing no long-term evolutionary change, the booklet simply omits this awkward fact.” (Icons, p. 174-175)

What will happen now that the peppered moth is turning out to be a case of oscillating selection, “producing no long-term evolutionary change”? Will textbooks tell the whole story or will they continue to perpetuate the peppered myth?


Casey Luskin

Associate Director, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.



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