According to the traditional peppered moth story, cryptic coloration
confers protection to the moths from predators, and as the habitat changed
due to industrial pollution, natural selection caused the frequencies of
different color varieties of the moth to change. As the trees became
darker, the lighter moths stood out more, so the darker ones became more
plentiful, and vice versa as the pollution cleared. That story is no
longer supportable because of flaws found in the experiments, such as
where the moths rested, and the occurrence of
contrary data,
such as unaccountable frequencies of uncamouflaged
moths in
areas.
Although the experiments were not perfect, they were not fatally
flawed. Even though Kettlewell released his moths in daylight when a
night release would have been more true to nature, he used the same
procedure in areas that differed only in the amount of industrial
pollution, showing conclusively that industrial pollution was a factor
responsible for the difference in predation between color varieties.
Similar arguments can be made for all other experiments. Although no
experiment is perfect (nor can be), even imperfect experiments can give
supporting or disconfirming evidence. In the case of peppered moths,
many experiments have been done, and they all support the traditional
story (Grant 1999).
Even without the experiments, the peppered moth story would be well
established. Peppered moth melanism has both risen and fallen with
pollution levels, and they have done so in many sites on two
continents (Cook 2003; Grant 1999).
The peppered moth story is consistent with many other experiments and
observations of crypsis and coloration in other species. For example,
bird predation maintains the colorations of Heliconius cydno, which
has different coloration in different regions, in both regions
mimicking a noxious Heliconius species (Kapan 2001). Natural
selection acting on the peppered moth would be the parsimonious
hypothesis even if there were no evidence to support it.
The peppered moth story is not simple. The full story as it is known
today fills thousands of pages of journal articles. Familiarity with
the literature and with the moths in the field is needed to evaluate
all the articles. But the research and the debates over its
implications have all been done in the open. Charges of fraud and
misconduct stem from neglect and misrepresentation of the research by
the people making the charges (Grant 2000). Of those familiar with the
literature, none doubt that bird predation is of primary importance
in the changing frequencies of melanism in peppered moths (Majerus
1999).
In teaching any subject to beginners, simplifying complex topics is
proper. The peppered moth story is a valuable tool for helping
students understand how nature really works. Teachers would be right
to omit the complexities from the story if they judged that their
students were not yet ready for that higher level of learning (Rudge
2000).
