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Peppered Moths–Fraud or Textbook Case for Evolution–Face The Test

Posted by Richard Conniff on June 3, 2016

The darker version of the peppered moth (Photo: Ben Sale/Flickr)

(Photo: Ben Sale/Flickr)

by Richard Conniff/Takepart.com

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-nineteenth century of a dark-winged variety of this moth 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 instead just a product of biased scientific research, bordering on fraud.

A new study being published this week in the journal Nature finally resolves this often-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-nineteenth 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 English industrial city of Manchester, a specimen with black wings turned up, and by the end of the nineteenth century, this version of the peppered moth was everywhere. The paler, mottled version disappeared, almost becoming extinct.

The sudden shift was no accident, according to scientists. Reliance on coal for heating and industrial production in the nineteenth century 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.” Pale moths could no longer hide against blackened tree trunks, 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.

(Incidentally, 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 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 anti-pollution laws began to unblacken 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 subsequently 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 down 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

Since creationists have continued to use the peppered moth “to further an anti-science 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 that 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 ultimately has nothing to do with science, or facts, or even truth–and everything to do with blind religious faith.

2 Responses to “Peppered Moths–Fraud or Textbook Case for Evolution–Face The Test”

  1. […] and the case of the peppered moth, also provided the first evidence of evolutionary change observable on a human time scale. As […]

  2. UPDATE August 22 2018

    Study confirms truth behind ‘Darwin’s moth’

    August 17, 2018
    Source:
    University of Exeter
    Summary:
    Scientists have revisited — and confirmed — one of the most famous textbook examples of evolution in action.
    Share:

    FULL STORY

    Scientists have revisited — and confirmed — one of the most famous textbook examples of evolution in action.

    They showed that differences in the survival of pale and dark forms of the peppered moth (Biston betularia) are explained by how well camouflaged the moths are to birds in clean and polluted woodland.

    “Industrial melanism” — the prevalence of darker varieties of animals in polluted areas — and the peppered moth provided a crucial early example supporting Darwin’s theory of evolution by natural selection, and has been a battleground between evolutionary biologists and creationists for decades.

    The common pale form of the moth is camouflaged against lichen growing on tree bark. During the Industrial Revolution — when pollution killed lichen and bark was darkened by soot — a darker-winged form emerged in the UK.

    Later, clean air legislation reduced soot levels and allowed lichen to recover — causing a resurgence of pale peppered moths.

    The example has been well supported by many studies, but nobody had ever tested how well camouflaged the moths were to the vision of their key predators — birds — and how their camouflage directly influenced survival.

    Now scientists at the University of Exeter have shown that, to the vision of birds, pale moths are indeed more camouflaged against lichen-covered trees than dark moths — making pale moths less likely to be eaten by birds in unpolluted woodland and giving them an evolutionary advantage.

    “This is one of the most iconic examples of evolution, used in biology textbooks around the world, yet fiercely attacked by creationists seeking to discredit evolution,” said Professor Martin Stevens, of the Centre for Ecology and Conservation on the University of Exeter’s Penryn Campus in Cornwall.

    “Remarkably, no previous study has quantified the camouflage of peppered moths, or related this to survival against predators in controlled experiments.

    “Using digital image analysis to simulate bird vision and field experiments in British woodland, we compared how easily birds can see pale and darker moths, and ultimately determine their predation risk.

    “Our findings confirm the conventional story put forward by early evolutionary biologists — that changes in the frequency of dark and pale peppered moths were driven by changes in pollution and camouflage.”

    Most birds can perceive ultraviolet light — invisible to human eyes — and see a greater range of colours than humans, and the Exeter scientists analysed how well pale and dark moths matched lichen-covered and plain tree bark, as seen by birds.

    To do this, they used museum specimens including some from the collections of Bernard Kettlewell, who conducted famous research on the evolution of the species in the 1950s.

    The researchers also created artificial moths, baited them with food and observed predation rates in UK woodland, mostly in Cornwall.

    “Through a bird’s eyes, the pale peppered moths more closely match lichen-covered bark, whereas darker individuals more closely match plain bark,” said first author Olivia Walton, who conducted the research as part of her master’s degree at Exeter.

    “Crucially, this translates into a strong survival advantage; the lighter moths are much less likely to be seen by wild birds when on lichen-covered backgrounds, in comparison to dark moths.”

    In the experiment using artificial moths, lighter models had a 21% higher chance of “surviving” (not being eaten by birds).

    “We provide strong direct evidence that the frequency of the peppered moth forms stems from differences in camouflage and avian predation, providing key support for this iconic example of natural selection,” Professor Stevens said.

    The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).

    The paper, published in the journal Communications Biology, is entitled: “Avian vision models and field experiments determine the survival value of peppered moth camouflage.”

    The birds that most commonly eat peppered moths include sparrows, great tits, blue tits, robins and blackbirds.

    Story Source:

    Materials provided by University of Exeter. Note: Content may be edited for style and length.

    Journal Reference:

    Olivia C. Walton, Martin Stevens. Avian vision models and field experiments determine the survival value of peppered moth camouflage. Communications Biology, 2018; 1 (1) DOI: 10.1038/s42003-018-0126-3

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