The case of the peppered moth illusion - science teaching needs to challenge traditional views of science

Whole Earth,  Spring, 1999  by Craig Holdrege

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How Programmatic Scientists Lost Touch with the Richness of Nature

In high schools and college biology courses, the peppered moth has been a dramatic story of evolution via natural selection. The story goes like this:

   The "peppered moth," Biston betularia, occurs in light and dark ... forms,
   both of which are shown [at left, and highlighted in the circles above].
   The normal ... form is a light, peppered color. A specimen of the dark type
   was first captured in 1848, near Manchester, England, just eleven years
   before the publication of The Origin of Species. In the years thereafter,
   in various parts of England, the relative frequency of the dark form was
   observed to increase until today, in some regions, only dark forms are
   found. Why the change?

   The answer is almost self-evident from the photographs.... In [them] we see
   a tree trunk of the sort found in rural England far from industrial
   centers: lichens covering the oak tree give it a variegated surface against
   which the lightly peppered moth is hard to see; the black form stands out
   prominently. By contrast, on trees growing in industrial areas, the lichens
   are killed and the trunk is blackened by soot; on such a tree it is the
   black moth that is protectively colored, the light moth standing out "like
   a sore thumb." Birds that prey on the moths have been observed and
   photographed catching moths, and it has been proved that they bring about
   differential mortality favoring the survival of the light forms in
   unpolluted woods and the dark forms in industrially blackened woods.

   The type of evolution represented in this story has been called industrial
   melanism. (Melanin is the pigment that makes the wings dark.) It
   exemplifies the Darwinian view of evolution: a species displays phenotypic
   variation (light and dark forms) on which natural selection can operate. In
   this case, birds selectively feed on conspicuous moths, and because the
   background coloring changes, the moth population evolves to remain
   inconspicuous. Oxford biologist H.B.D. Kettlewell wrote, "Had Darwin
   observed industrial melanism he would have seen evolution occurring not in
   thousands of years but in thousands of days--well within his lifetime. He
   would have witnessed the consummation and confirmation of his life's
   work.--Garrett Hardin. Biology: Its Principles and Implications, 1966

waking up

In the early 1980s I began teaching about peppered moth evolution in a university-prep, high-school biology course in Germany. Using this moth I could clearly develop the concepts of mutation and directed natural selection as factors of evolution--concepts required in the state-regulated curriculum. Since I could spend only a short time with this theme, I used textbook descriptions and other secondary sources; essentially, I taught the story described above.

In 1986 I came across a short report on new research concerning the peppered moth ... and was floored. The report stated that Cyril Clarke, a British scientist, had investigated the peppered moth for twenty-five years and found only two specimens during daylight in their natural habitat. What was going on here, I asked myself. I'd been dutifully showing students photographs of the moths on tree trunks, telling them how birds selectively pick off the conspicuous moths. Now someone who'd researched the moth for twenty-five years reported having seen no more than two moths in all that time. I immediately ordered Clarke's article, and my study of the primary literature began.

WHERE IS THE PEPPERED MOTH?

Strangely, no one knows where the peppered moth lives during the day; Clarke's sighting of two moths in twenty-five years is more than anyone else can claim. How, then, have the moths been studied? Researchers enter forests at night, using bright lamps and virgin, pheromone-releasing females to attract and capture the males. (Females rarely enter the traps.) The moths only fly into the traps at night; they have never been caught during the day. Since one rarely, if ever, sees these moths in daylight, it is assumed they are resting somewhere in the forest. Where are they? Cyril Clarke states, "They might be resting anywhere. The latest story is that they rest on the leaves in the top of trees, but it's not really known. The answer is that, either way, they're very good at hiding."

If the moths aren't observable during the day, where do those beautiful photographs of the moths on trees come from? In general, authors don't report the conditions under which the photos were made. I have found references only in an article by D.R. Lees and E.R. Creed, in the Journal of Animal Ecology (1975). They describe how the moths are killed, glued to the tree surfaces, and then photographed. Readers of other accounts will normally assume they are looking at a natural phenomenon, when in fact the pictures are composed by the researchers themselves.

KETTLEWELL'S EXPERIMENTS

In the 1950s, H.B.D. Kettlewell, the Oxford biologist, undertook an impressive series of experiments to see if he could observe under controlled circumstances what nature might be doing covertly. Kettlewell bred moths in the laboratory in order to have large enough numbers for experiments--especially of females, so hard to trap at night--and marked the underside of their wings for later identification. Light and dark forms of the moth were then released early in the morning into both unpolluted and polluted forests.