Various aspects of the change in frequencies of the varieties of peppered
moth and their geographical distribution are inconsistent with its
proposed explanation in terms of natural selection resulting from
differential predation.
The claim is based on the naive assumption that local observations of
differential predation rates are sufficient to make accurate
predictions of the relative frequencies of the different varieties of
peppered moth. It also falsely insinuates that early attempts to
explain these phenomena relied exclusively on visually selective
predation to the exclusion of all other factors.
Differential predation was originally proposed as a highly plausible,
and probably the most important, factor influencing the observed
changes in relative frequencies of the varieties of peppered moth
(Tutt 1896, as reported by Berry 1990, 317). However, even before
Kettlewell's famous experiments, Ford (1937, 484-498) had already
noted that visually selective predation alone could not account for all
the facts associated with the spread of industrial melanism, and
suggested that non-visual selection (i.e. selection resulting from
something other than visually selective predation) could also have been
an important factor. In fact, I am not aware of any evolutionary
biologist who has ever suggested that natural selection resulting from
differential predation would by itself account completely for
all
details of the geographical distributions of the peppered moth's
varieties, or of their change in relative frequencies.
Predictions based on the naive assumption mentioned above cannot be
expected to match the observed data with anything more than a very
rough degree of approximation, so it would not have been much of a
surprise to anyone when some discrepancies besides those already
pointed out by Ford were noted between such predictions and
observational data (Haldane 1956). Almost all these discrepancies
have quite natural and obvious possible explanations that were
immediately pointed out whenever the matter was discussed in the
scientific literature.
Besides non-visual selection, already proposed by Ford, factors which
were suggested as possible explanations for the discrepancies include
gene flow, heterozygous advantage (Haldane 1956), and frequency
dependent selection (Bishop et al. 1978, 505). Mani (1990) described
the results of incorporating the effects of gene flow and non-visual
selection into a more comprehensive mathematical model than the one
Haldane had used. Though still crude, the model was nevertheless found
to account for most of the discrepancies left unexplained by Haldane's,
and to provide a "consistent and reasonable picture of the gross
pattern of changes in the melanic frequencies." Thus, the conclusion
from earlier investigations that differential predation is likely to be
the most important single factor influencing the relative frequencies
of the varieties of the peppered moth in Britain has by no means been
called into question by later ones. On the contrary, it has been
complemented and strengthened by them.
References:
Berry, R. J., 1990. Industrial melanism and peppered moths (Biston
betularia (L.)), Biological Journal of the Linnean Society, 39:
301-322.
Bishop, J. A., L. M. Cook and J. Muggleton, 1978. The response of two
species of moths to industrialization in northwest England, I
Polymorphisms for melanism, Phil. Trans. R. Soc. Lond. (B), 281:
489-515
Ford, E. B., 1937. Problems of heredity in the lepidoptera,
Biol. Rev., 12: 461-503.
Haldane, J. B. S., 1956. The theory of selection for melanism in
Lepidoptera, Proceedings of the Royal Society of London, Series B
145: 303-306.
Mani, G. S., 1990. Theoretical models of melanism in Biston betularia
-- a review, Biological Journal of the Linnean Society, 39: 355-371.