Whole Earth, Spring 1999 p66(1)
The case of the peppered moth illusion. (science teaching needs to challenge traditional views of science) Craig Holdrege.
Abstract: The peppered moth is often used as an example of evolution, or natural selection. Research, however, shows that using the pepper moth as an example of natural selection may be misleading. Science teachers have simply come to accept natural selection as dogma, and so never questioned the veracity of the claim.
Full Text: COPYRIGHT 1999 Whole Earth
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.
Of the 984 moths released into an unpolluted forest, Kettlewell's team recaptured almost twice as many light-form moths as dark-form; the light moths clearly had the advantage in a clean forest. From the polluted woods of Birmingham, out of 630 released moths, twice as many dark moths were recovered as light ones. There was a clear correlation: in polluted forests more dark moths were recaptured and in unpolluted forests more light moths made it through.
But the experiments do not reveal whether birds are feeding on the moths. Kettlewell investigated this question by performing other experiments. In collaboration with the well-known Dutch ethologist, Niko Tinbergen, Kettlewell released moths (not for recapture) onto tree trunks, where they remained stationary. The scientists hid and observed birds feeding on the moths; Tinbergen even filmed the process. Generally, the more conspicuous moths--those on the "wrong" background according to our human standard--were indeed taken first. Camouflaged moths were also eaten, but not as many. Kettlewell concluded, "the effects of natural selection on industrial melanics for crypsis [camouflage] in such areas can no longer be disputed," and, "birds act as selective agents as postulated by evolutionary theory."
MORE TO MELANISM THAN MEETS THE EYE
Air pollution's decimation of the lichen covering trees around industrial centers has been viewed as a primary factor in the evolution of the peppered moth, since fewer lichens would make the light form more conspicuous (against dark bark) and the dark form better camouflaged. Where air pollution has decreased over time, in forests near Liverpool, the proportion of dark moths has dropped by 30 percent while the population of light moths has made a dramatic comeback. The kicker, though, is that although green species of lichen have repopulated trees, the light species of lichen, against which the light peppered moth is so well camouflaged, is still absent in the forests. Similarly, in forests near Detroit, where the quantity of lichen has not changed perceptibly in the last thirty years or so, the light moths have octupled their numbers. Clearly, if the abundance or colors of lichen have not changed, it is very difficult to understand how selective predation by birds could be the primary factor in the evolution of the moth forms.
This is not the only feature that contributes to the dissolution of the dear-cut textbook story. Lees and Creed report the following research, performed with killed, lab-bred moths pasted on tree trunks in rural eastern England. These forests had suffered little atmospheric pollution and the bark of the trees was "relatively light." Having glued moths of both color forms on the bark of trees each morning, Lees and Creed came back to the trees at regular intervals and counted how many specimens of each type of moth were still present and how many had disappeared, presumably having been eaten by birds. The results fit conveniently with the observation of conspicuousness: the better-camouflaged (light) moths remained longer on the trees than the more conspicuous (dark) moths. When, however, Lees and Creed captured wild moths in traps (at night, when moths were naturally active) there were four times as many dark as light moths--exactly the reverse of what we would expect on the basis of the diurnal experiments. If birds hunt moths during the day, then the light, better-camouflaged form should have the advantage; yet the forests seem to be populated by many more dark than light moths.
Even more baffling, birds do not fly or feed at night and, as we don't know which kind of moths prefer light traps (more females than males? more darks than lights?), there is no correlation between contrived, daytime results and the conclusions drawn from equally contrived observations made at night.
SEEING WHAT WE BELIEVE?
Stephen Jay Gould and Richard Lewontin, highly regarded contemporary evolutionary scientists, are highly critical of the "adaptationist programme," as they call it; and one of their reasons is "its unwillingness to consider alternatives to adaptive stories." If Kettlewell hadn't been so convinced of the truth of bird predation affecting peppered moth evolution, he might have left more room for alternative explanations.
In the moth/bird-predation example we see how strongly a theoretical framework informs the interpretation of the facts. When scientists have, as Lynn Margulis puts it in Slanted Truths, "an uncritical acceptance of the mesmerizing concept of adaptation," there is a real danger of seeing what one believes. We get oversimplified portrayals that turn science into dogma.
If we are truly interested in understanding phenomena, and not in seeing our own preconceptions in them, we must become more aware of our thinking--make it a more adequate and adaptable instrument of understanding. In performing an experiment, we are creating a simple and relatively transparent situation which is, of course, not identical to a natural phenomenon. We should be extremely wary of drawing conclusions that go beyond the experiment itself. Kettlewell's field experiments seem to show that birds feed on moths released onto trees in the early morning. But since the moths are not normally found on lower tree trunks during the day, Kettlewell has created (as all experiments do) an artificial situation. We need to hold back conclusions and consider alternative explanations--the guessable and the unknown. Instead of viewing experiments as a way to prove or disprove an idea, we should come to see them as a way of interacting with phenomena. Experiments help us clarify our ideas, discover phenomena, formulate new questions, and look with new eyes into nature. We can use hypotheses as a way to get started, well knowing that they need to be left behind when we confront the true phenomena, and begin to practice a flexibility of thought. The peppered moth becomes more and more like a deep question, rather than an instance of general theory.
IMPLICATIONS FOR SCIENCE EDUCATION
In 1997 I taught the full picture of the peppered moth to high school seniors at a school in upstate New York whose curriculum is not state-regulated. The students were fascinated by the moth and by the contrast between the simple story and the complex reality; by the process of discovery and transformation. This historical, case-study approach demands more classroom time, and more research on the part of the teacher, than does providing general overviews of material. But it brings science alive. We learn how scientists make observations, formulate ideas and questions, and test their hypotheses through experiments. We see how contradictions arise, how concepts become rigid, and then--often in the face of resistance--how they are modified or even dropped. Young people (if we have not corrupted them too much) are open-minded and interested in the world. Certainly it makes sense for them to study science (and of course other disciplines) not as codified knowledge to be memorized, but as a way of interacting with nature that leads to insights, and to ever-new questions. They begin to think of science as a process occurring in a historical context. What could be a more appropriate way to learn about the science of life, of biology?
A significant problem in the way science is taught, popularized, and in general filtered down to children is its portrayal as dogma. Kids "know" that in evolution the fittest survive; they "know" that the brain is a computer; they "know" that the heart is a pump; they "know" that genes determine heredity. Science courses could dissolve such dogmatic "knowledge"--really only an acquired opinion--by showing science to be a process. Even using limited examples, this would be more stimulating for students than imbibing large amounts of non-contextual information which, in the end, can be taken only dogmatically. Teaching science as process would mean reducing the use of textbooks (or at least reducing them to compendia of case studies), replacing maxims with exploration.
BACK TO THE PHENOMENA
If nothing else, the history of peppered moth research shows the need for very basic natural history, without which experiments and theories are anchorless. Many essential questions can only be answered by direct observation--as difficult as that may be in many situations.
Clearly, we need to know more about the life history of the peppered moth. Where does it rest during the day? What are its natural predators? How far can it fly? How long do the moths live? A greater knowledge is needed about the egg, larval, and pupal stages. Alternative interpretations for melanism need to be actively pursued. Might melanism have functions unrelated to camouflage, such as increasing warmth absorption or structural stability in the wing? Or is melanism in the adult a secondary effect of differences in the larval stages and the plants consumed? There are many possible interpretations of melanism in the peppered moth. Not a comfortable situation if we are looking for the cause of industrial melanism, but why should reality be concerned about our predilection for monocausality?
LETHAL GENERALITIES
For decades the peppered moth has been a textbook example of evolution. Millions of students have been inculcated with this "living proof" of natural selection. The story they are being told is most likely false--or at least filled with half-truths. This is not because teachers and writers are intentionally lying, or hiding and bending facts, but because the example is used only to prove a point. Complications appear extraneous to the argument. The idea of natural selection has become ingrained in the modern mind, like a pair of spectacles you never take off. Concepts become axiomatic, and science becomes dogma. As a correlate, the complex and rich phenomena of nature degenerate into instances of overriding principles. Instead of illuminating, the idea becomes, in Goethe's words, a "lethal generality."
This ossification is not what keeps science alive. Vitality in science comes from researchers doubting conclusions, making new observations and constructing new experiments; from scientists thinking with originality. Science teaching need not only serve the codified "body of knowledge." It can also serve ongoing exploration and the continual renewal of ideas. Since there is "more to melanism than meets the eye," peppered moth research can be an excellent teacher of the living scientific process.
SOURCES:
"THE SPANDRELS OF SAN MARCO AND THE PANGLOSSIAN PARADIGM: A CRITIQUE OF THE ADAPTATIONIST PROGRAMME," S.J. GOULD AND R.C. LEWONTIN. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON, 1979.
"THE SLOTH: A STUDY IN WHOLENESS," CRAIG HOLDREDGE. NEWSLETTER OF THE SOCIETY FOR THE EVOLUTION OF SCIENCE, 1998.
THE EVOLUTION OF MELANISM. H.B.D. KETTLEWELL, CLAREDON PRESS, 1973.
INDUSTRIAL MELANISM: GENETIC ADAPTION OF ANIMALS TO AIR POLLUTION, D.R. LEES. ACADEMIC PRESS, 1981.
DETAILED SOURCES AVAILABLE FROM THE NATURE INSTITUTE.
Craig Holdrege is the high school teacher every parent yearns for. He makes his kids ponder and wonder and gives the students great ideas to bring home to the table. He has written the best book on the state of genetics and biotechnology, Genetics and the Manipulation of Life (see Whole Earth No. 93). He is a dedicated student of Rudolph Steiner and the philosophies of the Waldorf Schools. He has just founded The Nature Institute, dedicated to whole-organism biology, an end to genetocentrism, holistic ecology, creating a biological context for technology, and phenomena-centered science and nature education. We thank Lynn Margulis for pointing Craig and his work to Whole Earth. The Nature Institute, 169 Route 21C, Ghent, NY 12075, 518/672-0116, nature@taconic.net.
Mag.Coll.: 98C3793.
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