Textbook examples need to be accurate. Biston betularia is probably a bad choice as an example of ‘evolution in action’.
The classic story of the rise of the melanic form of Biston betularia in response to industrial pollution is the example given to thousands of students to illustrate how Darwin’s theory of evolution by natural selection works. The story is a good one, and it illustrates the point very clearly. Unfortunately it might well be wrong!
I am not suggesting that anyone has set out to mislead – but some of the original research done into the camouflage of the moth is suspect. Moths were released or placed by hand on the trunks of trees in the daylight, but in normal conditions they hide under branches as dawn approaches. The visibility of the moths was considered from a human perspective, ignoring the fact that birds can see UV (what looked like good camouflage to humans turns out to be less good for birds!).
It was also assumed that records made in the Manchester region (UK) were typical of population changes worldwide. This turns out to be untrue. In some places the relative numbers of dark and light forms of the moth are the ‘wrong’ way round!
If the dark form was selected for by the destruction of lichens due to industrial pollution in the first place, then a return to the light form ought to follow the return of lichens to trees when air quality improved. Unfortunately (for the story) studies in Liverpool (UK) have shown that the numbers of light moths increased before the lichens were able to return to the trees.
Something that worries me (personally) is that there seems to be no reference that I can find to the caterpillars of the light and dark forms of the moth. Surely this is crucial if we are dealing with camouflage and predation by birds?
With all these criticisms of this classic story (and more that I have not mentioned) I think it is time to remove the Peppered Moth from Biology Textbooks. It should be replaced with a more ‘robust’ example of species change over time and its causes, maybe Flores Man?
