Science as Process or Dogma? The Case of
the Peppered Moth
This article appeared in Elemente der Naturwissenschaft 70: 39-51,
1999. Reprinted with Permission
The Story of the Peppered Moth
The peppered moth is used in high school and college biology courses, as
well as in many textbooks, to illustrate evolution via natural selection.
The story goes like this:
The "peppered moth," Biston betularia, occurs in light and
dark (melanic) forms, both of which are shown in Figure 1. The normal
("original") form is a light, peppered color. A specimen of the dark type
was first captured in 1848, near Manchester, England, just 11 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?
This text was published in 1966. Newer accounts give a similar version of
this classic story. Sometimes textbooks update the story: Since the 1960s
clean air acts in Great Britain and the United States have led to markedly
improved air quality around industrial centers and the numbers of dark moths
have fallen significantly in the forests near such centers in Great Britain,
while the light moth is again becoming more prevalent. The evolutionary
trend is reversing. A striking case of parallel evolution has been found
in the forests near Detroit, which had over 90% dark moths in 1960, but
in 1994 only 20%.
The answer is almost self-evident from the photographs shown in Figures
1 and 2. In Figure 1 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 (Figure 2) 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. (Hardin 1966, p. 183)
Light and dark forms of the peppered moth were photographed against
the lichen-covered trunk of a tree in an unpolluted area of England.
The light form is hard to see; the dark form is very conspicuous (from
The same two forms as in Figure 1, but photographed against the trunk
of an oak tree blackened by the polluted air of Birmingham, England.
In this case the dark form is hard to see, while the light form is conspicuous
(from Kettlewell, 1959).
This type of evolution 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. "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" (Kettlewell 1959,
In the early 1980s I began teaching about peppered moth evolution in a university
preparatory, high school biology course in Germany. Using this example 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 was only teaching the peppered moth as an example to make certain
concepts clear, and could spend only a short time with this theme, I used
textbook descriptions and other secondary sources. Essentially, I taught
the story described above, including some of the results of H.B.D. Kettlewell's
experiments, which I will discuss below.
In 1986 I came across a short report on new research concerning the
peppered moth. The report gave me an awakening jolt. The last sentence
stated that Cyril Clarke, a British scientist, had investigated the peppered
moth for 25 years and found only two specimens during daylight in their
natural habitat. What is going on here, I asked myself. I have been showing
students photographs of the moths on tree trunks, telling them about birds
selectively picking off the conspicuous moths, etc. And now someone who
has researched the moth for 25 years reports having seen only two moths.
I immediately ordered Clarke's article (Clarke et al. 1985) and my study
of the primary literature began. In recent years an increasing number
of scientists have expressed their doubts about the classic story. A much
richer, riddle-laden picture has emerged. (For critical reviews of peppered
moth research see Lambert et al. 1986; Majerus 1998; Sargent et al. 1998;
Where is the Peppered Moth?
As strange as it may seem, no one knows where the peppered moth lives during
the day. Clarke's sighting of two moths in 25 years is more than other authors
can claim. How, then, have the moths been studied? Researchers enter the
forests at night and turn on bright lamps that attract nocturnal insects.
In this way they capture the moths. They also set up so-called assembling
traps housing virgin females that release pheromones into the air, attracting
males into the traps. The males only fly into the assembling traps at night;
they are never caught during the day. Since one rarely, if ever, sees these
moths during the day, it is assumed they are resting somewhere in the forest,
becoming active at night.
If the moths aren't observed during the day, where do the 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 Lees and Creed (1975). They describe how the moths
are killed, glued to the tree surfaces, and then photographed. Most photos
in textbooks are reprints from Kettlewell's work (like the ones shown
above); he does not state how they were made. Since the light and dark
forms are so ingeniously placed to show camouflage or lack of it, I suspect
he might have used dead specimens and or at least arranged the moths accordingly.
Readers will normally (and perhaps naively) assume, unless otherwise informed,
that they are looking at a natural phenomenon. The impressive image of
camouflage in the peppered moth sticks in the mind, especially when the
image is accompanied by a text like the one quoted above, which gives
no hint that we are looking at an artificially constructed situation.
And as the textbook states, the explanation of industrial melanism appears
in view of such images almost "self-evident." This self-evident explanation
dissolves when we learn that researchers don't find the moth during the
day and that the pictures are composed by the researchers themselves.
Kettlewell (1955, p. 323) stated: "Yet after more than twenty-five years
of observation and constant enquiry, I have found no single instance in
this country [Great Britain] in which anyone has witnessed a bird detecting
and eating a moth belonging to a protectively coloured (or cryptic) species
while sitting motionless on its correct background." Kettlewell knew Great
Britain was a land of good observers, with many bird watchers and ornithologists.
What he doesn't state is that some moth species -- like the peppered moth
-- are almost never seen at all during the day. If one glosses over this
fact, it is much easier to have a simple explanation, but what one is
explaining is not the natural situation itself. Cyril Clarke summarizes:
"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" (quoted in Kaesuk Yoon
In the 1950s Kettlewell, who was a biologist at Oxford, undertook a series
of impressive experiments to see if he could observe experimentally what
nature might be doing in a more hidden way (see Kettlewell 1955, 1956, 1959,
1973). Kettlewell bred moths in the laboratory in order to have large enough
numbers for experiments, especially females, which rarely ever flew into
the light traps at night. He then marked the moths on the underside of the
wings for later identification. The light and dark forms of the moths were
then released early in the morning into unpolluted and polluted forests.
He later recaptured some of the moths in the light and assembling traps.
Kettlewell summarizes the results of two such mark-release-recapture experiments
(1959, p. 29):
In an unpolluted forest we released 984 moths: 488 dark and
496 light. We recaptured 34 dark and 62 light, indicating that in these
woods the light form had a clear advantage over the dark. We then repeated
the experiment in the polluted Birmingham woods, releasing 630 moths:
493 dark and 137 light. The result of the first experiment was completely
reversed; we recaptured proportionately twice as many of the dark form
as of the light.
There is a clear correlation: In polluted forests more dark moths are recaptured
and in unpolluted forests more light moths are recaptured. 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 the moths remained stationary (Kettlewell
1956, p. 294). 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
taken first, and after all the conspicuous moths were eaten, their numbers
were replenished. Camouflaged moths were also eaten, but not as many.
In an aviary similar observations were made (Kettlewell 1955, pp. 328
ff.): Kettlewell observed that the birds -- a pair of Great Tits -- took
no moths within the first two hours, but then within an hour they had
eaten most of the conspicuous moths and a few of the camouflaged ones.
The second time the experiment was performed all the moths were taken
much more quickly, within one half of an hour of being released. "It was
suggestive that the tits were becoming specialists on betularia [the peppered
moth], and subsequently they were seen to be searching each tree trunk
eagerly one at a time immediately after admission, thereby defeating the
purpose of the experiment."
Kettlewell brings results of the mark-release-recapture experiments
together with those of the bird predation experiments and concludes: "the
effects of natural selection on industrial melanics for crypsis in such
areas can no longer be disputed," and "birds act as selective agents as
postulated by evolutionary theory" (Kettlewell 1956, pp. 341f.).
Kettlewell believed his experiments prove that the evolution of the
peppered moth is caused by selective predation by birds. But how compelling
is this conclusion? Consider, for example, the results of his aviary experiments.
He observed that the two birds were much quicker at taking moths after
they had had experience in doing so. They found the camouflaged moths
as well. If one takes this experimental evidence and imagines it transferred
into a natural habitat, wouldn't it be reasonable to think that as the
dark peppered moth began to initially spread (for some unknown reason),
the birds might have begun to recognize them as well? This conclusion
is just as sound, but also just as speculative, as Kettlewell's, which
states that since the birds feed on the conspicuous (light) form first,
its numbers have decreased while the dark form has increased its numbers.
More to Melanism than Meets the Eye
This phrase is taken from the title of a review article on peppered moth
research written in 1982 (Jones 1982). As the title indicates -- and further
research has shown -- the picture of peppered moth evolution among researchers
in the field has become much less straightforward.
The reduction in the lichen covering of trees, due to air pollution,
in forests around industrial centers has been viewed as a primary factor
in the evolution of the peppered moth, since fewer lichen would make the
light form more conspicuous and the dark form better camouflaged. In forests
near Liverpool the proportion of dark moths was over 90% in 1959, while
in 1984 there were only 61% dark moths; the population of light moths
has been making a dramatic comeback (Clarke et al. 1985). The air pollution
has decreased in this time, falling steadily from 1962 to 1974 and has
remained since then at a constant low value. Although green species of
lichen have repopulated trees, the light species of lichen, upon which
the light peppered moth is so well camouflaged, is still absent in the
forests. Similarly, in forests near Detroit, the light moths increased
from under 10% of the population in 1960 to over 80% in 1994, even though
the lichen flora has not changed perceptibly in this period (Grant et
al. 1996). Grant et al. therefore "suggest that the role of lichens has
been inappropriately emphasized in the chronicles about the evolution
of melanism in the peppered moth". Clearly, if the lichen abundance has
not changed, then 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 clear-cut textbook story. Lees and Creed (1975) report on research
they performed in rural eastern England. With certain variations, they
basically repeated Kettlewell's experiments. In these forests there was
little atmospheric pollution and the bark of the trees was "relatively
light." When they glued dark and light moths onto trees, human observers
found the light form better camouflaged than the dark form. They came
back to the trees at regular intervals and counted how many specimens
of each type of moth was still present and how many had disappeared, presumably
having been eaten by birds. The results fit well with the observation
of conspicuousness: more of 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, there were about 80% dark
moths and 20% light moths -- exactly the reverse of what would be expected
on the basis of the experiments. If resting moths are hunted by birds
during the day, then the light form would seem to be at a selective advantage.
Yet the forests seem to be populated by many more dark moths than light
moths. Others have made similar contradictory findings (Bishop 1972).
"We conclude therefore that either the predation experiments and tests
of conspicuousness to humans are misleading, or some factor or factors
in addition to selective predation are responsible for maintaining high
melanic frequencies" (Lees and Creed 1975, p. 76).
Textbooks continue to portray a straightforward picture of peppered
moth evolution (for a felicitous exception see below). But the reality
is much more complex -- and more interesting. If one is looking for solid
"proof," then the peppered moth has turned out to be a poor example.
Seeing What We Believe?
Stephen Jay Gould and Richard Lewontin, two foremost 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" (Gould and Lewontin 1979, p. 581). This "unwillingness"
stems from a pre-formed idea that has the quality of a conviction. The idea
that selective predation by birds is the primary causative factor in the
evolution of the peppered moth becomes a fairly rigid paradigmatic framework
under which the facts are subsumed. If Kettlewell hadn't been so convinced
of the truth of bird predation causing peppered moth evolution, he might
have left more room for alternative explanations.
In this example we can see how strongly a theoretical framework informs
the interpretation of the facts. When scientists have, as Lynn Margulis
puts it, "an uncritical acceptance of the mesmerizing concept of adaptation,"
there is a real danger of seeing what one believes (Margulis and Sagan
1997, p. 272). If this happens, then we get the oversimplified portrayals,
like the textbook description quoted above, that turn science into dogma.
It is not very difficult to show that natural selection is at work when
one tacitly weaves the theory into the description of the phenomena. What
you put in, you can get out again. Steiner saw this as a fundamental danger
within science: "The basic error of many scientific strivings today is
that they believe they are reporting pure experience, while actually they
are only reading out of experience the concepts that they already placed
into it" (Steiner 1988, p. 31, transl. CH). These words were written in
1886; unfortunately they have not ceased to be true.
If we are truly interested in understanding the phenomena and not mirroring
our ideas in them, then we must become more aware of our thinking in order
to make it a more adequate and adaptable instrument of understanding.
A basic, but important realization can be that in performing an experiment
we are creating a simple and relatively transparent situation, which is,
of course, not identical with the more complex system of interactions
involved in any given natural phenomenon. We should be extremely wary
of drawing conclusions that go beyond the experimental situation itself.
Kettlewell's field experiments 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 recognize this
and not simply conclude: in nature birds feed selectively on moths in
the manner Kettlewell has shown. We need to hold back the conclusions
in order to free our thinking to consider other alternative explanations
and also to realize what we do not yet know.
Instead of viewing experiments as a way to prove or disprove an idea,
we come to see them as a way of interacting with phenomena.1 Experiments help us to clarify our ideas, to see new phenomena,
to formulate new questions, and to look with new eyes into nature.
All experiments are guided by ideas. Without the concepts of natural
selection and selective predation, most of the research concerning the
peppered moth may well never have been performed. These ideas have guided
and focused the research, and helped scientists to formulate specific
questions and discover new phenomena. Problems arise when we no longer
handle a concept as an instrument to see more, but as something to be
substantiated by nature. When we begin to selectively view the phenomena,
only seeing what seems to confirm our theory, then concepts that initially
sharpened our attention begin to make us blind. If, in contrast, we can
use hypotheses as a way to get started, well knowing that they need to
be left behind when we confront the phenomena, then we begin to practice
a flexibility of thought that leads us further into the complex richness
of the phenomena, and not into a monolithic theoretical construct. The
peppered moth becomes, in this way, more and more like a deep question,
rather than a mere instance of a general theory.
Implications for Science Education
In recent years I taught the complicated picture of the peppered moth to
high school seniors at the Hawthorne Valley School in upstate New York.
This is an independent Waldorf School and its curriculum is not state regulated.
The students were fascinated by the peppered moth and the contrast between
the simple story and the complex reality. We spent more time on this example
than one usually would, because I wanted them to see how science actually
proceeds and is a process of discovery and transformation.
Teaching in this historical, case study approach demands more classroom
time and also more research on the part of the teacher than providing
general overviews of material. But it brings alive science as a process.
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. Students begin to think of science
as a process occurring in an historical context. What could be a more
appropriate way to learn about the science of life, biology?
By proceeding in this way, students gain knowledge, but their knowledge
is dynamic, not static information. They develop capacities and ways of
approaching phenomena that they can apply in various life situations.
Young people are -- if we have not corrupted them too much -- open-minded
and interested in the world. Certainly it makes sense for them to learn
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,
but also to ever new questions.
A significant problem in the way science is taught, popularized, and
in general filtered down into the minds of children is that students are
filled with scientific dogmas: They "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. One task of
secondary and undergraduate science courses could be to dissolve such
dogmatic "knowledge" -- which in reality is only acquired opinion -- by
showing science to be a process. (I have attempted to present genetics
in this way; see Holdrege 1996.) In a given course one can do this for
only a limited number of examples, but it is much 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 either reducing the use of textbooks
or they would have to become compendia of case studies. In perusing textbook
presentations of the peppered moth, I was delighted to find one book (a
high school Biology text) with a short description of the peppered moth
in the section on evolution, but under the heading "Biology in process"
(Towle 1989, pp. 228f.). The author describes Kettlewell's work briefly
and then goes on to state that recent experiments raise doubts about the
selective predation explanation. He thus calls attention to the unresolved
The American Association for the Advancement of Science has published
Benchmarks for Science Literarcy. It is part of the Project 2061 (the
year in which Halley's comet will return; the project began in 1985, the
last time Halley's was here), which has the purpose "to help transform
the nation's school system so that all students become well educated in
science, mathematics, and technology" (back cover). Concerning scientific
inquiry, the text states that high school students should learn that "no
matter how well one theory fits observations, a new theory might fit them
just as well or better, or might fit a wider range of observations. In
science, the testing, revising, and occasional discarding of theories,
new and old, never ends" (p. 8). Most of the book, however, stands in
contrast to this description of science as a process. In the main body
of the book one finds for all grade levels the "benchmarks" for what should
be known in a given field at that age level. In this way the book emphasizes
content, not process. For example, by the end of twelfth grade students
should know that "the theory of natural selection provides a scientific
explanation for the history of life on earth as depicted in the fossil
record and in the similarities evident within the diversity of existing
organisms" (p. 125).
Once we have learned that one of the most cited examples of natural
selection turns out to be very unclear, doesn't this statement seem dogmatic?
If we are teaching dogma, it is important to be able to know that natural
selection is an explanation; if we are interested in giving a sense for
the nature of the scientific endeavor, then it is much more essential
to know how the concept is used, what it reveals, and what it doesn't
reveal. Without intending it, this book gives a very good picture of a
codified view of the nature of things. The conservative slant is discernible
when the authors say "it is important not to overdo the 'science always
changes' theme, since the main body of scientific knowledge is very stable
and grows by being corrected slowly and having its boundaries extended
gradually" (p. 5). If this "stable" body of knowledge entails the myriad
phenomena scientists discover, then I can agree with this statement. (Changing
theories haven't changed the fact that insects have six legs.) But if
scientific theories and models are meant, then I think we should stimulate
our students to continually question what tends toward a cemented standpoint.
We should stimulate continual scientific revolution. Just as in the Middle
Ages it seemed to many a self-evident fact that the earth was the center
of the universe, most certainly many of the "scientific truths" (that
is, theories) of today will become historical belief systems in the eyes
of future humanity.
Back to the Phenomena
Once we break out of the strictures of fixed explanatory patterns, we can
turn more openly to the natural phenomena themselves. 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? Similarly, a greater knowledge
is needed about the egg, larval, and pupal stages.
At the same time alternative interpretations for melanism in the peppered
moth need to be actively pursued. Might melanism have completely other
functions than camouflage, like increasing warmth absorption or structural
stability in the wing? Or is perhaps melanism in the adult a secondary
effect of differences in the larval stages? Some research suggests, for
example, that larvae of differing genetic types may not have the same
viability (Creed et al. 1980). Theodore Sargent of the University of Massachusetts,
working with a different species of nocturnal moth, has found evidence
that plants the larvae feed on may induce or repress the expression of
melanism in adult moths (Sargent et al. 1990).
There are certainly many other possible interpretations of melanism
in the peppered moth. I doubt that any one explanation will turn out to
be the right one, since in the long run all biological phenomena show
themselves to be interconnected with an array of factors. We should probably
also expect that in different localities at different times different
explanations may be necessary. This is certainly not a comfortable situation
if we a looking for the cause of industrial melanism, but why should reality
be concerned about our predilection for monocasuality?
One difficulty in our approach to the peppered moth is that we have
studied it only as an example of evolution. We have not yet set out to
understand the moth in its own right. From the outset we have considered
the moth from a limited perspective. It was interesting that a student
of mine questioned whether the peppered moth really is such a good example
of evolution. He took, for a moment, the standpoint of the species, and
not the framework I had set by introducing the peppered moth as a good
example of evolution. He said: what the peppered moth is really showing
us is how a species, by having different forms, is more flexible and able
to survive as one species; the populations and varieties of the species
fluctuate, but the species as a whole continues to thrive. This student
was implicitly raising the question of the validity of using micro, intraspecific
changes as a model for macroevolution.
In the same course in which I taught about the peppered moth -- entitled
"Zoology and Evolution" -- we spent a good deal of time studying two animals,
the elephant and the sloth. Going into quite a bit of detail, we learned
how these animals are integrated wholes in which all features and functions
hang together and interrelate. Every part of the sloth has "sloth" written
all over it (Holdrege 1998). When we then came to the peppered moth and
studied it in the light of evolution, I realized -- as a contrast -- that
we weren't really giving the moth itself adequate treatment (which I also
haven't done in this essay). The moth had been to a certain degree reduced
to an example, which would be comparable to looking at the sloth only
as an example of adaptation to arboreal life. Certainly, the moth became
more and more of a riddle even within the evolutionary perspective, but
it is important to be aware of the limitations of understanding implicit
in how one formulates a theme. Since limiting is also a way of focusing
and finding an entryway into a theme, we can't just abandon points of
view. But what we can do is to take different approaches in different
contexts to show that there are various avenues of understanding, each
with its strengths, but also limitations. This exercise of mental flexibility
and mobility can bring us nearer to the flexible nature of life itself.
For decades the peppered moth has been a standard classroom and textbook
example of evolution. Millions of students have learned this "living proof"
of natural selection. The story they have been, and are, being told is most
likely false, or to put it more mildly, filled with half-truths. This is
not because teachers and writers are intentionally lying, or hiding and
bending facts, but because the example is only brought to prove a point,
so that complications appear extraneous to the argument (if not to the truth).
Moreover, the idea of natural selection has become so ingrained into the
modern mind that it can become like a pair of spectacles that one doesn't
remove anymore. Concepts then become axiomatic and science ends up being
promulgated in a dogmatic form. As a correlate, the complex and rich phenomena
of nature degenerate, as it were, into mere instances of overriding principles.
Instead of illuminating, the idea becomes, in Goethe's words, a "lethal
generality" (Goethe 1995, p. 61).
This tendency toward solidification is not what keeps science alive.
Vitality in science comes from researchers doubting conclusions, making
new observations and constructing new experiments, from scientists thinking
original ideas that break through the constrictions of dominant paradigms.
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.
1. For a lucid discussion of the nature of experimentation
and the perils involved in making conclusions about experiments, see Goethe's
seminal essay The Experiment as Mediator between Object and Subject (Goethe
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