Ken Miller
The Peppered Moth - An Update
August, 1999

For years the story of the peppered moth, Biston betularia, has provided one of the
best-known examples of natural selection in action. The story of the moth was outlined on
pages 297-298 of the Elephant Book, and highlights the field experiments of British
ecologist H. B. D. Kettlewell.

However, a recent book by Michael Majerus (Melanism -Evolution in Action) makes it clear
that the peppered moth story will need to be rewritten. Joe Levine and I will post a revision
of pages 297-298 here just as soon as we can, but in the meantime, here is an update of
what the fuss is all about: 

The Peppered Moth is routinely used as an
example of evolution.

But is this well-known story wrong?

The "typica" form of the moth.

The light-colored form of the moth, known as typica, was the
predominant form in England prior to the beginning of the industrial
revolution. Shown at left, the typica moth's speckled wings are easy
to spot against a dark background, but would be difficult to pick out
against the light-colored bark of many trees common in England. 

Around the middle of the 19th century, however, a new form of the moth began to
appear. The first report of a dark-colored peppered moth was
made in 1848. By 1895, the frequency in Manchester had reached a reported level
of 98% of the moths.

This dark-colored form is known as carbonaria, and (as shown at
right), it is easiesr to see against a light background. As you can
well imagine, carbonaria would be almost invisible against a dark
background, just as typica would be difficult to see against a light
background. The increase in carbonaria moths was so dramatic that
many naturalists made the immediate suggestion that it had to be
the result of the effects of industrial activity on the local landscape.

The "carbonaria" form.

As noted on page 297, coal burned during the early decades of the industrial
revolution produced soot that blanketed the countryside of the
industrial areas of England between London and Manchester. Several naturalists
noted that the typica form was more common in the
countryside, while the carbonaria moth prevailed in the sooty regions. Not
surprisingly, many jumped to the conclusion that the darker moths
had some sort of survival advantage in the newly-darkened landscape.

In recent years, the burning of cleaner fuels and the advent of Clean Air laws
has changed the countryside even in industrial areas, and the
sootiness that prevailed during the 19th century is all but gone from urban
England. Coincidentally, the prevalance of the carbonaria form has
declined dramatically. In fact, some biologists suggest that the dark forms will
be all but extinct within a few decades.

For evolutionary biologists, the question behind the rise and fall of the
carbonaria form is "Why?" Why should the dark phenotype have
appeared so suddenly, come to dominate the population in industrial areas, and
then have declined just as sharply when levels of pollution
declined? To many biologists, the answer seemed obvious. In areas where
pollution had darkened the landscape, the darker moths were better
camoflaged and less like to be eaten by birds. Under less-polluted conditions,
the light-colored moths prevailed for similar reasons.

But was the obvious answer correct? That's what Kettlewell set out to check in a
series of classic studies carried out in the 1950s. As described
in Chapter 14 of the text, his results seemed to confirm that background
camoflage was the key:


BIOLOGY by Miller & Levine, page 298: 

"Kettlewell found that in unpolluted areas, more of his light-colored moths
had survived. In soot-blacked areas, more of the dark-colored moths had
survived. Thus Kettlewell showed that in each environment the moths that
were better camoflaged had the higher survival rate. It was logical to conclude
that when soot darkened the tree trunks in the area, natural selection
caused the dark-colored moths to become more common. Today
Kettlewell's work is considered to be a classic demonstration of natural
selection in action." 

However, in 1998, Michael E. N. Majerus of the Department of
Genetics at the University of Cambridge carefully re-examined
Kettlewell's studies, as well as many others that have since
appeared. What he reported, first of all, was that Kettlewell's
experiments, indicating that moth survival depends upon
color-related camoflage, were generally correct:

" Differential bird predation of the typica and carbonaria forms, in
habitats affected by industrial pollution to different degrees, is the
primary influence on the evolution of melanism in the peppered
moth."

(P. 116, Melanism - Evolution in Action, M. E. N. Majerus, Oxford University Press,
New York, 1998). 

However, Majerus also discovered that many of Kettlewell's experiments didn't
really test the elements of the story as well as they should have.
For example, in testing how likely light and dark moths were to be eaten, he
placed moths on the sides of tree trunks, a place where they rarely
perch in nature. He also records how well comoflaged the moths seemed to be by
visual inspection. This might have seemed like a good idea at
the time, but since his work it has become clear that birds see ultraviolet much
better than we do, and therefore what seems well-camoflaged to
the human eye may not be to a bird. In addition, neither Kettlewell nor those
who checked his work were able to compensate for the degree to
which migration of moths from surrounding areas might have affected the actual
numbers of light and dark moths he counted in various regions
of the countryside.

These criticisms have led some critics of evolution to charge that the peppered
moth story is "faked," or is "known to be wrong."

Neither is true. In fact, the basic elements of the peppered moth story are
quite correct. The population of dark moths rose and fell in parallel to
industrial pollution, and the percentage of dark moths in the population was
clearly highest in regions of the countryside that were most
polluted. As Majerus, the principal scientific critic of Kettlewell's work
wrote, "My view of the rise and fall of the melanic form of the peppered
moth is that differential bird predation in more or less polluted regions,
together with migration, are primarilty responsible, almost to the
exclusion of other factors." (p. 155).

So, what's going on here?

Well, the best way to put it is that what we are seeing is the scientific
process at its best. Majerus and other ecologists have carefully examined
the details of Kettlewell's work and found them to be lacking. As Majerus
explains, to be absolutely certain of exactly how natural selection
produced the rise and fall of the carbonaria form, we need better experiments to
show that birds (in a natural environment) really do respond to
camoflage in the ways we have presumed, that the primary reason the dark moths
did better in polluted areas was because of camoflage (and
not other factors like behavior), and that migration rates of moths from the
surrounding countryside are not so great that they overwhelm the
influence of selection in local regions by birds. Until these studies are done,
the peppered moth story will be incomplete. Not wrong, but
incomplete.

What we do know is that the rise and fall of dark-colored moths, a phenomenon
known as "industrial melanism," remains a striking and
persuasive example of natural selection in action. What we have to be cautious
about is attributing 100% of the work of natural selection in this
case to the camoflage of the moths and their direct visibility to birds.

Would you like to comment on this story?

In a few days we will make it possible to read a digest of student and teacher
comments on the peppered moth, and to send in your own
thoughts on the issue.

Ken Miller
Brown University
August, 1999

Oryginal:
http://biocrs.biomed.brown.edu/Elephant%20stuff/Chapters/Ch%2014/Moths/Moth-Update.html

POWRÓT