(Image: BBC GCSE Bitesize)
The change in colour of the peppered moth after the Industrial Revolution is a classic example of evolution in action. Almost every biology textbook re-tells the story, and for every student taking GCSE Biology with the OCR syllabus, or Higher Biology in Scotland, it is compulsory learning.
The peppered moth story is simple, easy to illustrate, and makes intuitive sense. When newly industrialised parts of Britain became polluted in the nineteenth century, smoke killed lichens growing on trees and blackened their bark. Pale coloured moths which had been well camouflaged before when they rested on tree trunks became very conspicuous and were eaten by birds. Rare dark moths, which had been conspicuous before, were now well camouflaged in the black background. As birds switched from eating mainly dark moths to mainly pale moths, the most common moth colour changed from pale to dark. Natural selection had caused a change in the British moth population. The moths had evolved.
This hypothesis was proposed by J.W. Tutt in 1896, and tested by Bernard Kettlewell in the 1950s. It then became a classic example of Darwinian evolution in action.
This story has two limitations as an example of evolution. Firstly, it only involves a very small-scale change. Secondly, the experiments which were once thought to have proved the story are now known to have been flawed.
The peppered moth story only involves small scale change
The pale and dark forms of the peppered moth are similar in every way except their superficial colour. The two types are both part of the same species, and can interbreed. Moreover, both types existed before the industrial revolution. It is only the frequency of the different types which has changed.
Since smoke pollution has decreased in the UK, the light coloured moths have started to become more common again. The process which took place after the industrial revolution is beginning to be reversed. By 2019, the dark moths were only expected to make up 1% of the peppered moth population in Britain.
The peppered moth story only provides evidence for fluctuating frequencies of different types of moth. It does not give evidence for large scale evolutionary changes involving new organs or body plans. It does not show us how moths came into existence in the first place.
The experiments behind the peppered moth story are known to be flawed
The evidence that predation by birds has caused the change in frequency of moth colour rests on a series of experiments which are now known to have been flawed. Bernard Kettlewell, who carried out most of the studies, assumed that the moths rested on tree trunks during the day. However, it is very difficult to find wild moths in their natural resting places (most textbook photos are of dead moths glued to tree trunks). Painstaking subsequent observations of wild moths have shown that they prefer to settle in locations higher in the tree than those used by Kettlewell. Another problem is that Kettlewell released his moths at the wrong time of day. This meant that they were not able to settle naturally in their preferred resting site. Futhermore, he released large numbers of moths, which may have created an artificial magnet for predatory birds. The experiments were simply too artificial – the moths were released at the wrong time of day, in the wrong places, and in the wrong numbers.
The problems of Kettlewell’s experiments are so widely acknowledged that new experiments are being carried out at the moment by a scientist in Cambridge to try to come up with a less doubtful proof. This researcher, Dr Michael Majerus, said this in a lecture to the British Humanist Association on 12th February 2004 (see link in bibliography below for full context):
I know the peppered moth, and I know that J.W. Tutt was essentially correct in his explanation of the rise of carbonaria [the dark form]. However, for those who do not ‘know’ the peppered moth, whether they are scientists, teachers or members of the public, this should not, indeed, it must not be enough.
More recently, Dr Majerus presented some of his findings at a scientific conference, with more convincing evidence demonstrating that natural selection can be due to predation by birds. (The text and slides of Dr Majerus’ presentation can be found in these links). Nevertheless, the significance of this work stays the same - the peppered moth remains a peppered moth, even Dr Majerus' later work does not overcome this problem (the links to his 2007 address can be found here). It is true to say that only the relative frequencies of light and dark varieties will alter. When the environmental conditions change again, so do the frequencies. This clearly demonstrates the on-going presence of both light and dark forms within the gene pool of this species.
It is interesting to speculate on an alternative scenario. Let us assume, for example, that the environmental conditions permanently change everywhere to totally favour the dark melanic form of the peppered moth. In this situation, it is conceivable that all light forms of the peppered moth would be permanently eradicated due to predation. The total information content of the gene pool of the peppered moth would be diminished accordingly. If then the conditions were to change to cause the dark form to become more vulnerable to predation, there would be the real possiblity that the peppered moth as an entire species would become extinct. Seen in this light, ongoing variation within the gene pool of a given species allows for the preservation of that species in a changing environment.
Conclusion
School children need to learn that the peppered moth story provides evidence for changes of frequencies of different types within a population, but does not show that large scale evolution can occur. They should also understand that the original experiments behind the peppered moth story have widely acknowledged flaws, and some of these issues have been addressed in more recent experiments.
Bibliography
Proffitt, F. 2004 In defense of Darwin and a Former Icon of Evolution. Science 304:1894-1895
Majerus, M. 2004 The Peppered Moth: decline of a Darwinian Disciple. Lecture to the British Humanist Association, 12th February 2004
Majerus, M. 2007 The Peppered Moth: proof of Darwinian Evolution. Lecture at ESEB Conference, 22nd August 2007