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Revenge of the Peppered Moths?

peppered moth.jpg

The peppered moth story is familiar — even overly familiar — to most readers of ENV, so I will summarize it only briefly here. Before the industrial revolution, most peppered moths in England were light-colored; but after tree trunks around cities were darkened by pollution, a dark-colored (“melanic”) variety became much more common (a phenomenon known as “industrial melanism”). In the 1950s, British physician Bernard Kettlewell performed some experiments that seemed to show that the proportion of melanic moths had increased because they were better camouflaged on darkened tree trunks and thus less likely to be eaten by predatory birds.

Kettlewell’s evidence soon became the classic textbook demonstration of natural selection in action — commonly illustrated with photos of peppered moths resting on light and dark-colored tree trunks.

By the 1990s, however, biologists had discovered several discrepancies in the classic story — not the least of which was that peppered moths in the wild do not usually rest on tree trunks. Most of the textbook photos had been staged.

In the 2000s the story began disappearing from the textbooks. British biologist Michael Majerus then did some studies that he felt supported the camouflage-predation explanation. But before he died of cancer in 2009, he only managed to publish a report of his study in the Darwin lobby’s in-house magazine Evolution: Education and Outreach. Now four other British biologists have presented his results posthumously in the Royal Society’s peer-reviewed Biology Letters. In an accompanying supplement, the authors presented their version of what they call “the peppered moth debacle.” And a debacle it certainly is, but not in the way they think.

According to Charles Darwin, natural selection has been “the most important” factor in the descent with modification of all living things from one or a few common ancestors, yet he had no actual evidence for it. All he could offer in The Origin of Species were “one or two imaginary illustrations.” It wasn’t until almost a century later that Kettlewell seemed to provide “Darwin’s missing evidence” by marking and releasing light- and dark-colored moths in polluted and unpolluted woodlands and recovering some of them the next day. Consistent with the camouflage-predation explanation, the proportion of better-camouflaged moths increased between their release and recapture.

Throughout the 1970s and 1980s, however, researchers reported various problems with the camouflage-predation explanation, and in 1998 University of Massachusetts biologist Theodore Sargent and two colleagues published an article in volume 30 of Evolutionary Biology concluding “there is little persuasive evidence, in the form of rigorous and replicated observations and experiments, to support this explanation at the present time.” (p. 318)

The same year, Michael Majerus published a book in which he concluded that evidence gathered in the forty years since Kettlewell’s work showed that “the basic peppered moth story is wrong, inaccurate, or incomplete, with respect to most of the story’s component parts.” (p. 116) In a review of Majerus’s book published in Nature, University of Chicago evolutionary biologist Jerry Coyne wrote: “From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong.” According to Coyne, the fact that peppered moths in the wild rarely rest on tree trunks “alone invalidates Kettlewell’s release-and-recapture experiments, as moths were released by placing them directly onto tree trunks.”

In 1999, I published an article in The Scientist summarizing these and other criticisms of the peppered moth story, and in 2000 I included a chapter on peppered moths in my book Icons of Evolution. Then, in 2002, journalist Judith Hooper published a book about the controversy titled Of Moths and Men. Hooper accused Kettlewell of fraud, though I never did; my criticism was directed primarily at textbook writers who ignored problems with the story and continued to use staged photos even after they were known to misrepresent natural conditions.

By then, what had previously been a fairly limited scientific dispute over the cause(s) of industrial melanism had become a debacle. Sargent and I were demonized, and Majerus and Coyne were persuaded to reaffirm the peppered moth story as the prime example of Darwinian evolution in action. Majerus also embarked on the study that was just recently reported in Biology Letters.

In that study, conducted over a seven-year period from 2001 to 2007, Majerus performed release-and-recapture experiments in an unpolluted woodland near his home with 4,522 light-colored and 342 dark-colored moths, using methods he considered superior to Kettlewell’s. He found that dark-colored moths (which were less camouflaged in this situation) had only a 91% survival rate compared with light-colored moths. He also observed 135 moths in resting positions, of which 35.6% were on tree trunks.

Yet during the seven years of Majerus’s study, thousands of peppered moths must have passed through the woodland near his house, so 135 moths were a tiny fraction of the total. Furthermore, as he himself acknowledged in a 2007 lecture in Sweden, his results might have been “somewhat biased towards the lower parts of the tree, due to sampling technique.”

Indeed. If peppered moths normally rest high in the upper branches, as several researchers concluded in the 1980s, then doing statistics on those visible to an observer on the ground (even one who climbs part-way up some trees, as Majerus did), is bound to suffer from sampling bias. Imagine someone looking over the side of a boat and concluding that most fish in the sea live within ten feet of the surface.

The correct question to ask is not whether peppered moths ever rest on tree trunks, but whether peppered moths normally rest on tree trunks. It’s possible they do, but finding 48 moths resting on tree trunks over the course of seven years does not answer that question — especially when tree trunks are the primary location where you are looking for moths.

Majerus titled his 2007 lecture “The Peppered Moth: The Proof of Darwinian Evolution.” He summarized the results of his seven-year study, but he explained that the real proof of Darwinian evolution is the following:

Darwinian evolution is a logical fact, and had to be even in 1859. Consider Darwin’s four observations and three deductions, upon which selection theory is based.

  • Organisms produce far more offspring than give rise to mature individuals.
  • Yet, population sizes remain more or less constant.
  • Therefore, there must be a high rate of mortality.
  • The individuals in a species show variation.
  • Therefore, some variants will succeed better than others, and those with beneficial characteristics will be naturally selected to produce the next generation.
  • There is a hereditary resemblance between parents and offspring.
  • Therefore, beneficial traits will be passed to future generations.

Given these four observed facts and three simple, logical deductions, selection cannot NOT happen.

So the evidence Majerus presented was ultimately irrelevant. Though consistent with the camouflage-predation hypothesis, Majerus’s results could not “prove” the latter, much less Darwinian evolution. His “proof” was logical, not empirical.

In any case, Darwinian evolution requires much more than the selection of beneficial traits, and much more than a shift in the proportions of light- and dark-colored moths. It requires the descent with modification of all living things from one or a few common ancestors. Darwin did not write a book titled How the Proportions of Two Pre-existing Moth Varieties Can Change Through Natural Selection; he wrote a book titled The Origin of Species by Means of Natural Selection.

Majerus went on to say that “there are a tremendous number of examples of Darwinian selection in action.” And indeed there are: beak changes in Galápagos finches, for one. Natural selection happens; I’ve never met anyone who doubts it. The question is whether natural selection can produce new species, organs and body plans. This question is not answered by shifts in the proportions of pre-existing varieties of the same species. Even if the camouflage-predation explanation for industrial melanism were undisputed, it would not get us any closer to “proving” Darwinian evolution.

Since there are other, better examples of natural selection, why do Darwinists go to such lengths to defend the peppered moth story? And why do they practically bite themselves in two vilifying its critics?
The answer, I think, can be found in the conclusion of Majerus’s 2007 lecture. “The rise and fall of the peppered moth,” he said, “is one of the most visually impacting and easily understood examples of Darwinian evolution in action, [so] it should be taught. It provides after all: The Proof of Evolution.” It doesn’t matter that the camouflage-predation story is scientifically disputed. It doesn’t matter that the story doesn’t come close to demonstrating the origin of a new species, much less the descent of all species from a common ancestor. What matters is that the peppered myth is a useful tool for indoctrinating students in Darwinian evolution.

In 1999 Canadian textbook-writer Bob Ritter, who knew that peppered moth pictures were staged but used them anyway, defended his practice on the same grounds. “You have to look at the audience,” he was quoted as saying in the April 5, 1999, Alberta Report Newsmagazine. “How convoluted do you want to make it for a first time learner?” High school students “are still very concrete in the way they learn,” said Ritter. “The advantage of this example of natural selection is that it is extremely visual.”

It’s no wonder that science education is in trouble.