MORE ON THE PEPPERED MOTH
Don Frack's strongly worded disagreement with my assessment of the peppered
moth story highlights a number of issues.
Before addressing them, however, I would like to comment briefly on Frack's
accusation that I am guilty of poor and/or dishonest scholarship.
I quoted Majerus (from his 1998 book) to the effect that "...the basic
peppered moth story is wrong, inaccurate, or incomplete, with respect to
most of the story's component parts." Frack responded (based on an email
message from Majerus): "Majerus quotes himself in full to contradict Wells.
Only looking at the two juxtaposed did I notice that Wells changed
'**precised** pepper moth story' to '**basic** pepper moth story'."
Frack makes a big deal of this alleged word change ("I frequently hit
myself in the head and remind myself 'never to trust a creationist
quote'.") The fact is, however, that I quoted Majerus accurately (though I
did not quote the entire sentence, as indicated by the ellipsis); Majerus
merely omits the word "basic" in his email message. Don't take my word for
this; consult the book directly (Michael Majerus, MELANISM: EVOLUTION IN
ACTION, Oxford University Press, 1998). Obviously, Frack did not bother to
check the original.
It is true that I incorrectly cited p. 117 instead of p. 116 in my posting,
and Frack takes me to task for this; is that the level at which we want to
communicate here? Wouldn't it be better to focus on the evidence?
It is clear that there is considerable disagreement among authorities on
the subject of industrial melanism. Frack sides with Michael Majerus, who
first published on the subject in 1978, and discounts the opinions of
Theodore Sargent, who first published on the subject in 1966. Sargent
knows at least as much about peppered moths as Majerus; although this does
not mean that Sargent is right and Majerus is wrong, it clearly
demonstrates that well-informed biologists disagree over some central
issues in the peppered moth story.
Such disagreement is evident from Jerry Coyne's review of Majerus's book,
in NATURE 396 (1998), 35-36. Coyne, a Darwinian geneticist, is certainly
no creationist, yet Coyne's interpretation of the evidence in Majerus's
book is very different from Majerus's. (Coyne found himself unpersuaded by
Majerus's defense of the standard story in the face of conflicting
evidence; indeed, he compared his disillusionment with "the dismay
attending my discovery, at the age of six, that it was my father and not
Santa who brought the presents on Christmas eve.")
What are non-specialists to do when confronted with such disagreements?
One way to deal with them is to ignore them; but the fact that virtually
all biology textbooks use industrial melanism in peppered moths as evidence
for Darwinian natural selection makes its imperative that we try to get the
story straight. (I have met several people who tell me that ALL they
remember about evolution from their high-school biology days is the
peppered moth story.)
So a better way to deal with such controversies is to go back and look at
the evidence, at least to the extent that this is possible for people not
working in the field. This is the established approach of empirical
science; it's why scientific papers are expected to include a "results"
section which reports the experimental data. Since experts such as Sargent
and Majerus disagree over the interpretation of the data, perhaps we should
go to the original papers.
The main point, it seems to me, is that research conducted in the 1980's
shows quite clearly that peppered moths do not normally rest on tree trunks
in the wild. At the risk of belaboring the point and trying readers'
patience, here is an excerpt from the review I wrote last year:
from SECOND THOUGHTS ABOUT PEPPERED MOTHS
by Jonathan Wells (1998)
....Since 1980, however, it has become clear that peppered moths do
not normally rest on tree trunks. Mikkola observed that "the normal
resting place of the Peppered Moth is beneath small, more or less
horizontal branches (but not on narrow twigs), probably high up in the
canopies, and the species probably only exceptionally rests on tree
trunks." He noted that "night-active moths, released in an illumination
bright enough for the human eye, may well choose their resting sites as
soon as possible and most probably atypically." Thus "the results of
Kettlewell (1955, 1956) fail to demonstrate the qualitative predation of
the morphs of the Peppered Moth by birds or other predators in natural
Mikkola used caged moths, but data on wild moths support his
conclusion. In twenty-five years of field work, Clarke and his colleagues
found only one peppered moth on a tree trunk, and admitted that they knew
primarily "where the moths do not spend the day." (11) When Howlett and
Majerus studied the natural resting sites of peppered moths (Biston
betularia) in various parts of England, they found that Mikkola's
observations on caged moths were valid for wild moths, as well, and
concluded: "...it seems certain that most B. betularia rest where they are
hidden.... [and] that exposed areas of tree trunks are not an important
resting site for any form of B. betularia." (12) In a separate study,
Liebert and Brakefield confirmed Mikkola's observations that "the species
rests predominantly on branches.... Many moths will rest underneath, or on
the side of, narrow branches in the canopy." (13)
In a recent book on melanism, Majerus criticizes the
"artificiality" of much previous work in this area, noting that "in most
predation experiments peppered moths have been positioned on vertical tree
trunks, despite the fact that they rarely chose such surfaces to rest upon
in the wild." (14)
10. K. Mikkola, "On the selective forces acting in the industrial melanism
of Biston and Oligia moths (Lepidoptera: Geometridae and Noctuidae),"
Biological Journal of the Linnean Society 21: 409-421, 1984; pp. 416-418.
11. C. A. Clarke, G. S. Mani, G. Wynne, "Evolution in reverse: clean air
and the peppered moth," Biological Journal of the Linnean Society 26:
189-199, 1985; p. 197.
12. R. J. Howlett, M. E. N. Majerus, "The understanding of industrial
melanism in the peppered moth (Biston betularia) (Lepidoptera:
Geometridae)," Biological Journal of the Linnean Society 30: 31-44, 1987;
13. T. G. Liebert, P. M. Brakefield, "Behavioural studies on the peppered
moth Biston betularia and a discussion of the role of pollution and lichens
in industrial melanism," Biological Journal of the Linnean Society 31:
129-150, 1987; p. 129.
14. M. E. N. Majerus, Melanism: Evolution in Action. Oxford: Oxford
University Press, 1998; p. 116.
[Regarding the above: Clarke and his colleagues actually found TWO
peppered moths resting in the wild in their 25 years of field work. Frack
notes sarcastically that "Wells has lost a moth" because I cite only one;
but if Frack had consulted the original paper by Clarke et al. he would
have learned that one of their two moths was found resting on a fence, not
a tree trunk.]
Frack notes that Majerus's book includes two tables of data on the resting
places of peppered moths (p. 123). Frack himself acknowledges that Table 2
is of dubious relevance, since the data were collected in the vicinity of
artificially constructed mercury vapour moth traps, and I will not consider
it further. Most of the data in Table 1 are from a 1987 paper by Howlett &
Majerus (reference 12, above) which concluded (in addition to the
statements I quote above): "This brings into question the validity of
fitness values of the different forms obtained by putting moths on tree
trunks in relatively exposed positions." (p. 41)
A comparison of Majerus's 1987 table (25 moths) and the 1998 book version
(47 moths) shows that between 1964 and 1996 22 additional moths were found
in the wild, only two of which were found on exposed tree trunks. In other
words, Majerus's tables show that the percentage of moths he found on
exposed tree trunks actually dropped from 16.0% to 12.7% with increased
collecting. And Majerus acknowledges (in his 1987 paper, p. 40) that the
difficulty of finding moths high up in the trees means that most such moths
"would have been missed.") Thus 12.7% is an artificially high percentage,
and of course the total numbers are rather underwhelming (in 40 years of
intensive research, Majerus reports only six moths found on exposed tree
trunks, where they are most easily spotted.)
Although these data add to Clarke et al.'s 1985 report of finding only one
trunk-resting moth in 25 years of field work, they confirm the conclusion
that peppered moths do not normally rest on tree trunks in the wild.
Although a small percentage of moths do so, at best this makes cryptic
coloration and selective bird predation a possible (but minor) contributing
factor in industrial melanism.
Frack follows Majerus in discounting the possibility that direct induction,
rather than natural selection, was responsible for industrial melanism in
peppered moths. Heslop Harrison claimed in the 1920's that induction could
account for melanism in other insect species (he didn't work on peppered
moths), but others investigators were unable to replicate his work. Some
biologists (such as Thedore Sargent) continue to maintain that induction is
still a theoretical possibility. Personally, I am not arguing for
induction, but merely pointing out that natural selection is not the only
conceivable possibility (unless one is a dogmatic Darwinist). I remain
skeptical of induction; but recent evidence has also made me skeptical of
selective bird predation. Any scientist interested in discovering the true
cause(s) of industrial melanism would presumably be wise to remain open to
more than one possibility.
What about the ubiquitous photographs of peppered moths in biology
textbooks? To the best of my knowledge, every textbook photograph showing
peppered moths on tree trunks has been staged. Either the moths were
manually placed there for the photographer (one peppered moth expert quoted
by Frack told me personally that this was his method), or the moths in the
photo are dead, and glued or pinned to the trunk (another peppered moth
expert, also quoted by Frack, told me personally that this was his method).
In one famous photograph (Scientific American , 232:1), showing
peppered moths on three different tree trunks, it is obvious to the naked
eye that the moths on the first tree trunk are the same two moths shown on
I agree with Frack that staging is not necessarily deceptive, as long as it
accurately portrays the natural circumstances. According to Frack,
however: "Staging should, of course, never misrepresent the purpose of the
illustration. In the present case, moths must always be placed on
backgrounds that represent their actual environment (unless any deviation
is made perfectly clear in the legend)."
Given our present knowledge, shouldn't the captions of staged photographs
in biology textbooks point out, at the very least, that the photos do NOT
represent the normal resting place of the vast majority of peppered moths?
Shouldn't they include a disclaimer pointing out that the moths were
artificially positioned in order to make the photograph? Shouldn't the
accompanying text elaborate on the complexities of the story instead of
pretending that selective bird predation is "the" demonstrated cause of
On a personal concluding note, I would like to add that my objections to
the textbook peppered myth have nothing to do with creationism, despite
Frack's repeated attempts to discredit me with that label. Even if the
classical peppered moth story were 100% accurate, it would pose no threat
to creationism -- including the young-earth variety -- so why would anyone
oppose it on "creationist" grounds anyway?
No, my objections have to do with the integrity of empirical science. As a
biology student, I accepted the peppered moth story at face value for
years; when I began looking beneath the surface, it struck me as an example
of how the theoretical commitments of otherwise good scientists can color
their use of empirical data. I have encountered such theoretical
commitments before, even in areas which have no bearing on Darwinian
evolution. In the present case, though, I have been surprised by the
intense hostility which has been directed at me for criticizing the
classical story. The heated reaction from peppered myth advocates makes it
clear that there is more at stake here than quibbles over whether
references have been cited correctly or whether one or two or six moths
have been observed resting on tree trunks in the wild.
Part of the heat I have felt appears to be a result of hositilities that
have long been simmering among experts on peppered moths -- probably
comparable to what one would expect from stepping into the ongoing verbal
war between Stephen Jay Gould and Richard Dawkins over the nature of
evolutionary mechanisms. Whatever one says, one is bound to get blasted.
But there are deeper issues here, as well. One issue is whether scientific
theories are accountable to the evidence, or whether experts with
transparent theoretical commitments are to be given sole authority to tell
us what's true. Another issue is whether students of biology are to be
presented with the facts, or with staged photographs which misrepresent the
undisputed facts. And another issue is whether Darwinian evolution is
empirically so shakey that it cannot tolerate a public debate over the
adequacy of its evidence.
Is "creationism" versus "evolutionism" at issue here? If it is, it's not
because I brought it up. If Frack thinks it is, perhaps he should explain
Jonathan Wells, Ph.D.
Department of Molecular & Cell Biology
University of California, Berkeley
Discovery Institute, Seattle