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Peppered Moths: It’s Déjà Vu All Over Again

Photo: Larvae of peppered moths, by Noor MAF, Parnell RS, Grant BS, CC BY 2.5 , via Wikimedia Commons.

An interesting article was just published in Current Biology. The larvae of peppered moths are long, slender caterpillars. According to the article, “They adopt twig-like postures and can change colour to match the branches on which they are located.” This color change appears to be “mediated by a suite of visual genes” along the larva’s body. The result is better camouflage. (The evidence does not indicate that the color change is transmitted to the adult moths.)

The authors then re-tell the classic story of peppered moths, which they call “one of the most complete examples of adaptation through natural selection.” According to the story, dark-colored (“melanic”) moths originated in the 1800s. Before then, the moths were a speckled (“peppered”) light gray. But when pollution darkened tree trunks around industrial areas, the proportion of melanic moths in such areas rose to over 90 percent.

“Darwin’s Missing Evidence”

Despite the importance of natural selection to his theory, Charles Darwin had no evidence for it. All he could offer were some “imaginary illustrations.” So in the 1950s, British physician H. B. D. Kettlewell performed some experiments in which he marked some captive light and dark peppered moths with a spot of paint. Then he released them onto nearby tree trunks in polluted and unpolluted woods. Later he recaptured some of the marked moths and noted that their proportions had changed. More light-colored moths had survived in unpolluted woodlands, and more dark-colored moths had survived in polluted woodlands. The results were consistent with a theory that “industrial melanism” was due to differences in the moths’ camouflage on light and dark tree trunks. Kettlewell called this “Darwin’s missing evidence.”

Pictures of light and dark moths on light and dark tree trunks found their way into many biology textbooks. But the classic peppered moth story was thrown into doubt in the 1980s, when biologists realized (among other things) that peppered moths in the wild rarely rest on tree trunks. Apparently, most hide in the upper branches where they can’t be seen. The textbook photos had been staged. In some cases, dead moths were glued to tree trunks. In others, live moths were manually put in place and photographed before they moved. On October 15, 2002, the New York Times (p. D1) featured the photographs as an egregious example of “scientific fakery.”

A Whopping Understatement

Nevertheless, the classic story enjoyed a brief revival in 2007, when moth expert Michael Majerus reported the resting positions of peppered moths in his back garden. After seven years he had tallied 50 moths resting on tree trunks. (Those 50 were surely just a tiny percentage of the thousands of peppered moths that must have passed through his garden in seven years.) Yet Majerus had only observed moths from the ground or by climbing part-way up the trees. In a whopping understatement, he acknowledged that “the results may be somewhat biased towards lower parts of the tree, due to sampling technique…” Nevertheless, Majerus concluded that peppered moths do rest on tree trunks, and that the classic story is “The Proof of Evolution.” Not only that! The “fact of Darwinian evolution” shows that humans invented God and there will be “no second coming; no helping hand from on high.”

Yet even if the classic peppered moth story were 100 percent true, it would not be proof of Darwinian evolution, much less of the human invention of God. At most, the story shows a change in the proportions of two varieties of the same species. If Darwin had written a book titled How Existing Species Change Over Time, the peppered moth story would be relevant. But Darwin wrote a book titled The Origin of Species, because he thought he had discovered the mechanism whereby unguided natural processes could produce not only new species, but also new organs and body plans. The peppered moth story doesn’t come close to proving any of that.

The authors of the recent Current Biology article don’t claim that color changes in larva support the peppered moth story, much less Majerus’s absurd theological claims. But they do re-tell the story and place it in the context of evolutionary theory. And they do so without mentioning any of the problems that led to its demise.