peppered moths – another ‘icon’?

You’ve probably heard about ‘peppered moths’ in class. They’re an example of the ability of natural selection to shape a population in a relatively quick time. But Jonathan Wells asks:

Q: PEPPERED MOTHS. Why do textbooks use pictures of peppered moths camouflaged on tree trunks as evidence for natural selection — when biologists have known since the 1980s that the moths don’t normally rest on tree trunks, and all the pictures have been staged? 

Does he have a point?

Well, no. He’s implying that because the images were staged, the experimental data don’t stack up, & he’s completely wrong. Again.

Peppered moths (Biston betularia) are found in the UK. There are two ‘morphs’, or forms, of this moth: a paler, ‘peppered’-looking moth, and a darker form, & they both perch on trees during the day. Prior to the industrial revolution the pale form was the more common of the two, but in areas where trees became covered in soot from factories, the dark form came to dominate. Scientists hypothesised that this was evidence of natural selection in action: the pale morph was more obvious against the darkened trunks, & more likely to be picked off by birds. More of the dark moths survived and reproduced, and their genes spread through the population.

In the 1950s this hypothesis was tested experimentally by Bernard Kettlewell. He found that birds did indeed seem to prey selectively on the pale morph in sooty, polluted forests, and that the reverse was true in non-polluted areas. Most textbook accounts of this are accompanied by images of the two morphs of betularia, sitting side-by-side on tree trunks – where they were placed in order to obtain the photo.

So Wells’ point is completely spurious. Yes, the pictures are ‘staged’, in that moths were placed in position to be photographed. But this in no way invalidates the experimental data that demonstrated that natural selection was operating on the moths. (No photographer was going to wait around until a moth happened to land just where s/he wanted it.) And in fact, despite the fact that Wells goes on to claim that the moths never rest on tree trunks in the wild, they do this 25% of the time. The remaining time is spent on branches (25%) or the place where branch and trunk join (50%) (See Alan Gishlick’s NCSE article for more details). Biston betularia remains an example of the power of natural selection to bring about evolutionary change in a population.