Jeremy Cherfas, Exploding the myth of the melanic moth, "New Scientist",
Vol. 112, No. 1540/1541, 25 December 1986/1 January 1987, p. 25.

Exploding the myth of the melanic moth
Jeremy Cherfas

THE PEPPERED moth, Biston betularia is one of the classic heroes of
evolutionary biology. In the beginning, there were light-coloured moths,
which were well camouflaged on the lichen encrusted tree trunks where
they rested. When the sooty outpourings of the industrial revolution
blackened the tree trunks, dark forms of the moths were better hidden
predators picked off the lighter moths and so natural selection favoured the
dark, melanic forms. Most of the peppered moths in industrial areas were
dark. In rural areas, where the tree trunks remained light, the moths did
too. And now that the air is becoming cleaner, the moths are reverting to
the light form (New Scientist, 2 January, p 11).

Experiments proved the case. Moth specimens pinned to the "wrong"
background were found and eaten more quickly by birds.

That is the classic story, and with knowledge of the rate of increase of the
melanic forms or their ability to evade birds, it is possible to calculate the
selective advantage they enjoyed. Difficulties arise when applying this
information to computer models of the evolution of industrial melanism,
the data do not fit the models. In particular, there often seem to be more
melanic moths than the models would allow. Some additional selection,
apart from the visual powers of birds, must be involved.

Michael Majerus and Rory Howlett, of the genetics department of the
University of Cambridge, believe that so much emphasis is placed on
models that they are "eclipsing careful consideration of what the organism
is actually doing". Majerus and Howlett have investigated selection in
peppered moths afresh. They conclude that the moths do not rest on tree
trunks as the story says they do, and that when this is taken into account
the models may work better.

After 20 years of moth-hunting, the researchers believe that the peppered
moths prefer to rest in shaded sites, for example beneath a branch or where
a branch joins the trunk. Part of the problem is that peppered moths are
elusive in the wild. Majerus has noted the position of each of the 25
peppered moths he has come across since 1964; most (9 of 11 light ones,
six of 14 dark ones) were at the join between branch and trunk. Likewise,
moths attracted to the neighbourhoods of mercury vapour lights were
found largely at the joint between trunk and branches.

These observations throw the original story into some doubt. But they are
nothing new. Bernard Kettlewell, who did so many of the experiments on
birds eating peppered moths, admitted that "whilst undertaking large-scale
releases of both forms in the wild at early dawn, I have on many occasions
been able to watch this species taking up its normal resting position which
is beneath the larger boughs of trees, less commonly on the trunks". so
Kettlewell, despite conducting his experiments with moths on tree trunks,
knew that tree trunks were not the moths' normal resting places.

[...]

Now you see it, now you don't. But dark moths aren't so vulnerable in
clean air

Another element of the story is that moths actively seek out and prefer to
rest on backgrounds that match their own colouring. But the wings of the
light-coloured typical form are partially translucent, so in terms of the light
reflected from them, they are more like black than white. Howlett
measured the reflectance of the wings of 10 specimens of each form.
Reflectance from the dark wing is about per cent, and from the light wing
about 30 per cent, so when given a straight choice both ought to rest on
black. Howlett repeated one of Kettlewell's classic experiments and in his
studies both forms chose to settle on black.

Majerus and Howlett went on to investigate how birds would cope with
moths in their natural positions. They placed light and dark moths in
polluted and unpolluted woods, either on the trunks or below the joint with
a major bough. Three days later they returned to see which moths had been  eaten.

As expected in the polluted woods, birds picked off the fight moths; in
cleaner woods they went for the dark ones. This was true for both types of
resting site although birds found more moths on the trunk than beneath the
join. Most interesting, however was that the dark forms were better hidden
at the branch joint than were light forms. and the effectiveness of the
camouflage was greatest in the unpolluted woods.

Majerus and Howlett are aware of the shortcomings of their experiments.
But they suggest that their field observations might make more sense of
other researchers' results. The dark form, they believe, may have an even
greater advantage over the light form in polluted areas than was previously
thought; more important, they are at less of a disadvantage in cleaner areas
than supposed. This could explain the unexpectedly high numbers of dark
moths still found in some country locales.

The peppered moth remains one of the best examples of evolution in
action. But as in so many other cases, the real story is turning out to be
more complicated than the biologists first thought.
 

POWRÓT