From time to time, evolutionists re-examine a classic experimental study and find, to their horror, that it is flawed or downright wrong. We no
longer use chromosomal polymorphism in Drosophila pseudoobscura to
demonstrate heterozygous advantage, flower-colour variation in Linanthus
parryae to illustrate random genetic drift, or the viceroy and monarch
butterflies to exemplify Batesian mimicry. Until now, however, the prize
horse in our stable of examples has been the evolution of 'industrial
melanism' in the peppered moth, Biston betularia, presented by most
teachers and textbooks as the paradigm of natural selection and evolution
occurring within a human lifetime. The re-examination of this tale is the
centrepiece of Michael Majerus's book, Melanism: Evolution in Action.
Depressingly, Majerus shows that this classic example is in bad shape, and,
while not yet ready for the glue factory, needs serious attention.
[..]
Criticisms of this story have circulated in samizdat for several years, but
Majerus summarizes them for the first time in print in an absorbing
two-chapter critique (coincidentally, a similar analysis [Sargent et al.,
Evol. Biol. 30, 299-322; 1998] has just appeared). Majerus notes that the
most serious problem is that B. betularia probably does not rest on tree
trunks -- exactly two moths have been seen in such a position in more than 40 years of intensive search. The natural resting spots are, in fact, a mystery. This alone invalidates Kettlewell's release-recapture experiments, as moths were released by placing them directly onto tree trunks, where they are highly visible to bird predators. (Kettlewell also released his moths during the day, while they normally choose resting places at night.) The story is further eroded by noting that the resurgence of typica occurred well before lichens recolonized the polluted trees, and that a parallel increase and decrease of the melanic form also occurred in industrial areas of the United States, where there was no change in the abundance of the lichens that supposedly play such an important role.
Finally, the results of Kettlewell's behavioural experiments were not
replicated in later studies: moths have no tendency to choose matching
backgrounds. Majerus finds many other flaws in the work, but they are too
numerous to list here. I unearthed additional problems when, embarrassed at
having taught the standard Biston story for years, I read Kettlewell's
papers for the first time.
Majerus concludes, reasonably, that all we can deduce from this story is
that it is a case of rapid evolution, probably involving pollution and bird
predation. I would, however, replace "probably" with "perhaps". B.
betularia shows the footprint of natural selection, but we have not yet
seen the feet. Majerus finds some solace in his analysis, claiming that the
true story is
likely to be more complex and therefore more interesting, but one senses
that he is making a virtue of necessity. My own reaction resembles the
dismay attending my discovery, at the age of six, that it was my father and
not Santa who brought the presents on Christmas Eve.
[..] Unfortunately, most of the work described is inconclusive; despite the widespread occurrence of melanism, its evolutionary significance is nearly always unknown.
[..]
It is clear that, as with most other work in evolutionary biology,
understanding selection in Biston will require much more information about
the animal's habits. Evolutionists may bridle at such a conclusion, because
ecological data are very hard to gather. Nevertheless, there is no
other way to unravel the forces changing a character. We must stop
pretending that we understand the course of natural selection as soon as we
have calculated the relative fitness of different traits.