Natural Selection is illustrated in textbooks by the case of the Peppered Moth and changes due to industrial pollution. There are problems with this!
The Peppered Moth (Biston betularia) is the usual ‘textbook’ example of evolution in action. This moth can exist in several forms, from light coloured and speckled through to almost black. Victorian moth collectors in the UK noticed the appearance of the hitherto rare black (melanic) form around 1850. After this date the melanic form became common in certain areas, and its success was explained by the good camouflage the dark colour provided against trees affected by industrial pollution.
Industrial Melanism.
The explanation for the success of the melanic form of the Peppered Moth goes something like this:
Birds are the main predators of the Peppered Moth;
Trees in unpolluted areas have trunks covered with lichens;
Light coloured forms of the moth are well camouflaged against these lichens;
Industrial pollution kills the lichens and leaves the trunk exposed;
Dark coloured forms are now better camouflaged against the trees;
Natural Selection leads to an increase in the numbers of dark individuals, and a decrease in the numbers of the light individuals.
So industry favours melanism.
Natural Selection.
The case of the Peppered Moth is used to explain how Darwin’s theory of evolution works. The environment changes and the better survival of a novel form leads to a change in the species. In this case the better survival rates of the melanic form gives them more breeding success and the moth population changes from the original light form to a better adapted black. Biology textbooks use this example to explain what Darwin was suggesting, and also as a proof of the theory. Unfortunately there are a few problems! (a review of some of the problems with the classical explanation)
As an example.
The explanation for the success of the melanic form in industrial areas fits well with Darwin’s suggestion for how evolution proceeds. It clearly illustrates how changes in species over time might come about, and thus far it is a good example. Unfortunately the real story is, as usual, a lot more complex. Using this example might be justified because it is very clear and well documented, but going on to use it as any sort of proof of Darwin’s theory is problematic.
As a proof.
Many biologists have criticised the original assumptions that were used to explain the changes observed in the Peppered Moth. Two of the most serious problems are given here:
Light moths are better camouflaged against lichens than dark moths – this is true for human observers, but birds can see in UV and the ‘camouflage’ argument breaks down when this is taken into account.
Detailed records of moth populations studied in Manchester region (UK) are typical of changes occurring worldwide – unfortunately studies in other areas show that this is not the case, sometimes the light form is more successful in industrial areas, and sometimes the dark form predominates in unpolluted areas.
Where does that leave us?
Besterman and Baggott la Velle (in Volume 41, number 2 of the Journal of Biological Education) suggest that the Peppered Moth story should be removed from textbooks, to be replaced with the evolution of the recently discovered Flores Man (Homo floriensis).
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