Jeremy Cherfas, Clean air revives the peppered moth, "New Scientist", Vol.
109, No. 1489, 2 January 1986, p. 17.

Clean air revives the peppered moth
The white form of the peppered moth is making a comeback

Jeremy Cherfas

THE PEPPERED moth, Biston betularia, is a classic example of natural
selection in action. As the dark satanic mills blackened the English
landscape the predominant form of the peppered moth changed from light
to dark, a case of industrial melanism. Since the Clean Air Acts came into
force industrial pollution has declined. A new study by Sir Cyril Clarke and
his associates shows that the light form of the peppered moth has become
much more common (Biological Journal of the Linnean Society, vol 26, p
189).

The first black specimen of Biston betularia was caught in Manchester in
1848. By 1895, 98 per cent of the moths in the area were dark, an
extremely rapid change given that the peppered moth breeds just once a
year. The usual explanation is that the pollutants of the industrial revolution
changed the environment, thus favouring the initially rare, dark form.
Smoke blackened the tree trunks and sulphur dioxide killed the encrusting
lichens on the branches both of which made the light form more
conspicuous and thus more likely to be found and eaten by birds. Dark
moths were selected for, presumably because they were better
camouflaged.

Cyril Clarke has been trapping moths at his home on Merseyside for the
past 25 years. He uses two sorts of trap. one attracts moths to a mercury-
vapour lamp the other uses virgin females to lure males by their
pheromones. Looking over the collections, Clarke could see a distinct
change in about 1915 Before that date about 90 per cent of the moths were
dark. After 1975 there is a steep decline. and in 1984 only 60 per cent were
dark. Similar results were obtained at a number of other sites in the area.

During the same period the amount of smoke and sulphur dioxide has also
dropped. and detailed consideration of these changes, particularly of
sulphur dioxide, gives some insight into the mechanisms that might underlie
industrial melanism. The mean concentration of sulphur dioxide fell almost
linearly from around 300 micrograms per cubic metre (ug/m^3) in 1970 to
less than 50 ug/m^3 in 1975. It then remained roughly constant. Given that
the proportion of dark peppered moths began to drop once sulphur dioxide
had reached a constant low level it is possible to assess various models of
selection.

The best fit to the data is obtained if one assumes that the ratio of the
fitness of the light form to that of the dark increases as the levels of sulphur
dioxide decrease, and then stays constant. The fitness of the typical light
form is thus correlated with levels of sulphur dioxide but, as Clarke and his
colleagues admit, "the exact mechanism by which cleaner air is operating is
not fully known, and there are several problems to be solved".

One concerns the relative importance of genetic and visual factors. It could
be that the light moths are genetically superior in cleaner air for reasons
that are unconnected with their appearance. If that is so, there would be
some delay while numbers built up, which does square with the
observations.

Alternatively camouflage might be the important factor, as has always been
popularly assumed. As the air grew cleaner the background lightness would
have increased until birds could no longer spot the lighter form. The
matter, however, is difficult to resolve. Clarke and his colleagues do not
think that there has been any great change in the state of the lichens that
are supposed to be hiding the moths. This suggests that bird vision is not
the only important aspect of selection.

The real difficulty is that we do not know where the birds actually spend
the hours of daylight. Clarke thinks that they do not hide among the lichens
on trees and walls, at least in his area. In 95 years he has found only two
specimens in that situation, and none elsewhere, so their normal hiding
place remains a mystery.

Industrial melanism has been reversed as industrial pollution has been
decreased that much is clear. Exactly how the changes in the peppered
moth are brought about will require more work.
 

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