The best of science, the worst of creationism
|IN 2000, a popular school textbook called Biology
reluctantly dropped its prime example of evolution in action - industrial
melanism in the peppered moth. Nothing in evolutionary biology had forced
the change. The decision was entirely political, made in response to creationist
The loss of the peppered moth was a blow to science education in the US,
as it is one of the easiest to understand examples of
evolution by natural selection. So it is heartening
to hear that biologists are fighting back (page 46). Thanks to their efforts,
evidence that the moth is an example of evolution in action is more robust
This tawdry tale reveals much of what is good about science - and rotten
about creationism. Creationists went gunning for the moth after a scientific
disagreement over the fine detail of a seminal experiment done in the 1950s.
They used the debate to portray the science behind industrial melanism as
hopelessly flawed, if not fraudulent.
In response, one scientist patiently redid the experiment - it took him seven
years. It is hard to think of another system of thought that is so stringently
self-critical and self-correcting. In science, everything is provisional
.There are no preordained answers and fresh ideas are always welcome,
so long as their proponents are happy for them to be tested.
That is not how creationists work. They already know the answers. They
seek only evidence that confirms their conclusion, and distort or ignore
an unreasoned approach is worthless. Creationists will keep trying to
undermine the theory of evolution. All science can do is continue, with dignity,
to stick to its guns. As with the peppered moth, the best testable explanation
will win out.
8 December 2007 News Scentist
AT FIRST sight the peppered moth is a rather unprepossessing creature. Surrounded
by its more brightly coloured relatives in a moth collection, drab old
Biston betulria scarcely catches the eye. Yet the peppered moth is
more famous - and notorious - than any of its prettier counterparts can ever
hope to be.
For decades, the peppered moth was the textbook example of evolution in action,
unassailable proof that Darwin got it right. Generations of students learned
how, during the industrial revolution in England, a dark-coloured mutant
appeared and in polluted areas quickly replaced the normal light-coloured
form because it was better camouflaged against bird predation. "It is the
most simple to understand, visually attractive story of evolution in action,"
says Michael Majerus, a geneticist at the University of Cambridge.
Recently, though, the peppered moth's status as an icon of evolution has
been under threat. Emboldened by legitimate scientific debate over the fine
details of the peppered moth story, creationists and other
anti-evolutionists have orchestrated a decade-long campaign to discredit
it - and with it the entire edifice of evolution. These days you're less
likely to hear about the peppered moth as proof of evolution than as proof
that biologists cannot get their story straight.
The peppered moth now counts among the anti-evolutionists' most potent weapons.
In the past few years it has helped them get material critical of evolution
added to high-school science lessons in Ohio and Kansas, although the material
has now been removed. In 2000, the authors of the widely used school textbook
Biology reluctantly dropped the peppered moth indirect response to
creationist attacks. "It would be really easy to say it is a fraud, and it
would take a long explanation to respond to that," says co-author Kenneth
Miller of Brown University in Providence, Rhode Island. The latest edition
beaks of Galapagos finches instead.
Now, though, biologists are fighting back. Majerus recently finished an
exhaustive experiment designed to repair the peppered moth's tattered reputation
and reverse the creationists' advances. The preliminary results are out,
and Majerus says they are enough to fully reinstate the moth as the prime
example of Darwinian evolution inaction.
The textbook version of the peppered moth story is simple enough. Before
the mid-l9th century, all peppered moths in England were cream coloured with
dark spots. In 1848, however, a "melanic" form was caught and pinned by a
moth collector in Manchester. By the turn of the 20th century melanic moths
had all but replaced the light form in Manchester and other industrial regions
of England. The cause of the change was industrial pollution: as soot and
other pollutants filled the air, trees used by peppered moths as daytime
resting places were stripped of their lichens then stained black with soot.
Light-coloured moths that were well camouflaged on lichen-coated trees were
highly conspicuous on blackened trees. Melanic moths, in contrast, were less
easily spotted by predatory birds and so survived longer, leaving more offspring
than the light forms. As melanism is heritable, over time the proportion
of black moths increased.
As with all textbook examples, however, this is a simplified account of decades
of field work, genetic studies and
carried out by dozens of researchers. It also draws disproportionately
on the flawed work of one biologist, Bernard Kettlewell of the University
In the 1950s Kettlewell carried out a series of classic experiments that
cemented the peppered moth's iconic status. These were designed to test a
hypothesis first proposed by lepidopterist James Tutt that the rise in melanism
was a result of natural selection caused by differential bird predation.
Though he put forward the idea in 1896, it was ignored for decades because
few ornithologists or lepidopterists believed birds were major predators
of moths.Kettlewell, spurred on by his Oxford mentor Edmund Brisco Ford,
thought otherwise. In 1953 and 1955 he carried out experiments in polluted
woodland in Rubery, Birmingham, and unspoiled woodland in Dorset. In the
mornings he dropped hundreds of marked moths, both light and melanic, on
tree trunks, where they quickly took up resting positions. In the evenings
he used moth traps to recapture them. In Birmingham, he recaptured twice
as many dark as light moths. In Dorset, he found the opposite, recapturing
more light moths. The obvious conclusion was that light moths were more heavily
predated than dark moths in Birmingham, and vice versa in Dorset.
During these experiments Kettlewell also directly observed robins and hedge
sparrows eating peppered moths. As expected, the birds noticed and ate more
light-coloured moths on soot-covered trees, and more melanic ones on
lichen-covered trees. This was a breakthrough, as hardly anyone
in Kettlewell's time believed that birds ate moths. In case anyone doubted
his observations Kettlewell recruited his Oxford colleague, ethologist
Tinbergen (who would later win a Nobel prize for his work on animal
behaviour), to film birds eating moths.
Kettlewell's experiments were quickly accepted as proof that the rise of
the melanic moth was a case of evolution by natural selection, and that the
agent of selection was bird predation. The peppered moth quickly found its
way into textbooks, often accompanied by striking photographs of light and
dark moths resting on lichen-covered and soot-stained bark.
In truth, however, there were problems with Kettlewell's experiments. Perhaps
the most significant was that he released moths onto tree trunks. Although
moths occasionally choose trunks as a daytime resting place, they prefer
the underside of branches. Kettlewell also let his moths go during the day,
even though they normally choose their resting place at night. And he released
more moths than would naturally be present in an area, which may have made
them more conspicuous and tempted birds to eat them even if they wouldn't
normally. These problems were familiar to evolutionary biologists, many of
whom tried to resolve them with experiments, but were not given a general
airing until 1998, when Majerus pointed out the flaws in Kettlewell's work
in his book Melanism: Evolution in action.
Majerus had unleashed a monster. In November1998, Nature published
a review of his book by evolutionary biologist Jerry Coyne of the University
of Chicago (vol 396, p35). In it, Coyne wrote a sentence that would come
back to haunt him: "For the time being we must discard Biston as a
well-understood example of natural selection in action."
Coyne's motivations were entirely honourable. "I thought I was drawing attention
to some problems and was doing the scientific community a service," he says.
He did not mean to imply that the peppered moth was not an example of evolution
by natural selection, merely that the fine details were still lacking. "I
wasn't very clear. The key was well-understood."
But to anti-evolution organisations such as the Discovery Institute,
Coyne's true intentions were irrelevant. His words were manna from heaven.
By selectively quoting him and Majerus they managed to portray the textbook
version of events as hopelessly flawed, and with it the entire theory of
evolution. In a pincer movement, they also pointed at the textbook pictures
which are often staged with dead specimens - and proclaimed that the science
behind those pictures was staged too.
In 2000, in an attempt to rescue the peppered moth's reputation, Majerus
embarked on a large experiment designed to iron out the problems with
Kettlewell's work. He barely had time to get going, however, before things
took a turn for the worse. In 2002, journalist Judith Hooper published a
popular science book called
Of Moths and Men:
Intrigue, tragedy & the peppered moth in which she openly accused
Kettlewell of manipulating his data to prove his hypothesis. Hooper's book
is a roller-coaster tale and brilliantly written. It is not a creationist
text, but creationists seized on it anyway as evidence that Kettlewell was
No evidence of fraud
He wasn't. As numerous historians and scientists have pointed out, Hooper's
book is littered with factual errors, not least the accusation that Kettlewell
forged his data. There is no evidence he did so. "Hooper went too far," says
Paul Brakefield of Leiden University in the Netherlands. "Anyone who knew
Kettlewell would tell you that the last thing he would do was commit fraud."
Coyne himself wrote a scathing review of Hooper's book in which he accused
her of unfairly smearing Kettlewell and concluded that "industrial melanism
still represents a splendid example of evolution in action" (Nature, vol
418, p19). It is fair to say that this accurately represented the views of
the vast majority of evolutionary biologists at the time, but by then the
damage had been done.
Meanwhile, Majerus was steadily working through his experiment in his own
garden in Cambridge. He started by identifying 103 branches that were suitable
resting places for peppered moths, ranging in height from 2 to 26 metres,
many of them covered in lichen. For seven years, every night from May to
August, he placed nets around 12
branches and released a single moth into each net. Around 90 per cent were
light coloured to reflect the natural frequencies the two forms around Cambridge.
The moths took up resting positions overnight, usually on the underside of
the branch. At sunrise the next morning Majerus removed the nets and 4 hours
later checked see which moths were still there. His assumption was that,
as peppered moths spend the whole day in their resting position any that
disappeared between sunrise and mid-morning had almost certainly been spotted
and eaten by birds.
Because he was able to watch some of the branches from his house through
binoculars he also observed the moths being eaten by many species of bird
- including robins, blackbirds, magpies and blue tits. As expected the birds
were better at spotting the dark moths than the camouflaged light ones, he
Majerus says his design addresses all the flaws in Kettlewell's experiments.
He let moths choose their own resting positions, he used low densities, he
released them at night where they were normally active, and he used lo~ moths
at the frequencies found in nature.
This August, Majerus presented his preliminary results at a meeting of
evolutionary biologists at the University of Uppsala in Sweden. He said that
over the 7 years, 29 per cent of his melanic moths were eaten compared with
22 per cent of light ones.This was a statistically significant difference.
As in many parts of the UK, pollution in Cambridge has declined since the
adoption of clean air acts in the 1950s, and melanic moths are becoming
increasingly rare, declining from 12 per cent of the population in 2001 under
2 per cent today. According to Majerus his results show that bird predation
is the agent of this change. Birds were better at spotting dark moths than
light ones, ate more of them and reduced the percentage of black moths over
time. "It provides the proof of evolution," he says. He will soon publish
his results in a peer-reviewed journal.
Still, Majerus's experiment doesn't satisfy all evolutionary biologists.
Even though birds were responsible for differential moth mortality in the
experiment, James Carey of the University of California, Davis, wonders how
important they are in nature. He points out that other animals also eat moths,
and should have different preferences for dark and light forms. "The question
is, does preferential predation by birds matter?" he says. "Unless you know
what moths die of in nature, you don't know how important birds really are."
Majerus counters this by saying that there no evidence that other moth predators
preferentially choose dark or light forms. Bats are a major predator, but
they hunt by echolocation rather than vision and Majerus has good experimental
evidence that they have equal preference for light and dark moths. Even if
birds account for only 1 per cent of moth mortality, as long as the other
99 per cent is not selective, bird predation will result in changes in gene
frequency. Still, there is an unknown factor remaining: no one yet knows
the impact of invertebrate predators such as earwigs and beetles.
Majerus's study also leaves a long-standing problem unsolved. For reasons
that are not clear, the frequencies of dark and light moths do not always
correlate with the level of pollution. In East Anglia, for example, dark
moths have always been relatively common despite low pollution levels. Biologists
feel this can be explained, says Brakefield. Kettlewell himself proposed
that peppered moth larvae, which hatch in the tree canopy and bang on silk
threads, can be blown long distances by the wind. It's possible that the
dark moths of East Anglia arrive on prevailing
south-westerly winds from the industrial
areas around London, though this idea has yet to be properly tested.
These are legitimate problems that require scientific explanations. But they
do not point to a fundamental problem with the peppered moth story, let alone
the theory of evolution. Brakefield, Majerus, Coyne and the rest of mainstream
biology all agree that the peppered moth was and is a well-understood example
of evolution by natural selection. There is no doubt that the peppered moth's
colour is genetically determined, so changes in the frequencies of light
and dark forms demonstrate changes in gene frequencies and that is evolution.
What's more, the direction and speed at which this evolution occurred can
only be explained by natural selection. The agent of selection remains
contentious, but bird predation is the only hypothesis with any experimental
backing. There is no controversy among the people who work on peppered moths,"
says Bruce Grant at the College of William & Mary in Williamsburg, Virginia.
Anti-evolutionists will continue to suggest there is, of course, but as far
as Majerus and others are concerned their claims have been debunked and the
peppered moth should be reinstated as a textbook example of evolution in
action. Not just to teach children either, but also as a direct rebuttal
of anti-evolutionism. As Majerus told the conference in Sweden: "The peppered
moth story is easy to understand because it involves things that we are familiar
with: vision and predation and birds and moths and pollution and camouflage
and lunch and death. That is why the anti-evolution lobby attacks the
peppered moth story. They are frightened that too many people will be able
Jaap de Roode is a biologist and writer at the University of Georgia in
8 December 2007 New Scientist