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Peppered Moths, an Evolutionary Icon, Are Back

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Readers of Evolution News are probably familiar with — perhaps even bored with — the classic story of peppered moths. The story lay dormant for several years, but now it’s in the news again.

In brief, the classic story is that peppered moths went from being mostly light-colored to being mostly dark-colored (“melanic”) during the industrialization of 19th-century England. The shift was called “industrial melanism,” and it was attributed to natural selection: Dark-colored moths were supposedly better camouflaged on pollution-darkened tree trunks and thus more likely to survive predatory birds. After air quality legislation was passed in the mid-20th century, light-colored moths became more common again, consistent with the story.

Darwin himself had no evidence for natural selection, but peppered moths were said to provide it. The story was called “Darwin’s missing evidence,” and it was used for years in biology textbooks to teach Darwinian evolution — usually accompanied by photographs of light- and dark-colored moths resting on light- and dark-colored tree trunks.

The story had various problems, but the most serious emerged in the 1980s when biologists discovered that peppered moths don’t normally rest on tree trunks. Instead, the vast majority of them probably rest high up in trees where they can’t be seen. Almost all of the textbook photographs had been staged, many with dead moths pinned to tree trunks.

After the publication of Icons of Evolution in 2000, many textbooks stopped using the classic story. It enjoyed a brief resurgence in 2007, but then it mostly dropped from sight again.

Of course, even if the classic story were true, it would not demonstrate Darwinian evolution. Darwin didn’t write a book titled How Existing Species Change Over Time. He wrote a book titled The Origin of Species. His argument was that natural selection produces entirely new species, organs, and body plans. A temporary shift in the proportions of light- and dark-colored peppered moths is irrelevant to that argument.

Now peppered moths are back. Some scientists in England have identified a mutation that may explain why some peppered moths are dark-colored. The scientists found a transposable element, or “jumping gene,” that is present in 105 of the 110 dark-colored moths they studied, but absent from 283 light-colored moths. Although it is not clear how the stretch of DNA where the transposable element was found affects wing color, the scientists concluded that this was “the mutation event giving rise to industrial melanism in Britain,” and that their findings “fill a substantial knowledge gap in the iconic example of microevolutionary change.”

(In the 1930s, evolutionary biologist Theodosius Dobzhansky distinguished between minor changes within existing species, which he called “microevolution,” and the origin of new species, organs and body plans, which he called “macroevolution.”)

This discovery was based on some impressive experimental work. Now it is accompanied by some impressive hype.

According to the journal Nature (in which the study was published), “there is perhaps no better example of natural selection in action than the case of the peppered moth.” Unfortunately, some “technical criticisms” of the classic story “have leaked, out of context, into the propaganda of creationists. In response, some textbooks have — heaven forbid — evolved to not include the peppered moth story at all.” But “there is enough in the pages that follow to update those textbooks.”

The Science Daily news service quotes one of the scientists involved in the study to the effect that “this discovery fills a fundamental gap in the peppered moth story,” which Science Daily calls the “iconic textbook example of evolution by natural selection.” Another scientist involved in the study says, “These findings provide an opportunity to further develop peppered moth industrial melanism as a tool for teaching evolutionary biology.”

Repeating the discredited classic story, BBC News reports that “peppered moths are nocturnal and spend their days dozing on tree trunks or walls,” so the newly identified mutation “gave them a much better chance of hiding from hungry birds in the smoke-stained world of industrial England.”

According to the Washington Post, just as peppered moths “once helped illuminate the macro process of natural selection, the researchers hope their studies will illuminate how evolution happens at a microscopic level.” But peppered moths never illuminated macroevolution. The Post continues by quoting one of the scientists involved in the study:

My hope is that by adding more scientific evidence, more modern scientific evidence, to this case study that eventually it will kind of make it easier for people to appreciate at different levels this isn’t a made up story. There’s more and more layers of evidence.

But how relevant is that evidence? These reports (and the scientists quoted in them) assume that the main problem with the classic story was that the genetic basis of melanism was unknown. But that was never the main problem. The fact that peppered moths don’t normally rest on tree trunks makes it doubtful that industrial melanism was due to camouflage and bird predation, and the fact that a temporary shift occurred in the proportions of light- and dark-colored moths provides no evidence for Darwin’s theory of the origin of species.

Maybe it’s time for the peppered moth story to drop from sight once again.

Photo credit: Martinowksy from nl [GFDL or CC-BY-SA-3.0], via Wikimedia Commons.