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A short class with Sarah and her students. The other ways are under More.
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Before the Industrial Revolution, almost all peppered moths in Britain were pale and speckled. Why would that be a good colour?
Because they rest on pale bark covered in lichen, so birds can't see them.
Exactly. Then factories filled the air with soot, and the lichens died. What happened to the trunks?
They turned dark, so the pale moths would stand out.
Right. The first dark moth was recorded in Manchester in 1848, and by 1895 about 98% of the moths there were dark.
Did the soot make the moths dark?
No. The dark form came from a random mutation. The soot changed which form survived.
Natural selection needs variation, inheritance, overproduction and differential survival. Where is the variation here?
There were pale moths and dark moths in the same population.
And inheritance?
Wing colour is controlled by a gene, so the offspring get it from their parents.
Good. Females lay hundreds of eggs, which is overproduction. And differential survival?
In polluted woods, birds ate more pale moths, so more dark moths lived to reproduce.
Exactly. So the population changed over generations, even though no single moth changed colour.
How could we test whether birds really were the cause?
Release marked moths of both colours and see which ones survive to be caught again.
That is what Bernard Kettlewell did in the 1950s. In polluted woods more dark moths were recaptured, and in clean woods more pale ones.
Were there any problems with his method?
Some, such as releasing moths in daylight and in large numbers, but a later, more careful study by Michael Majerus found the same result.
And when the air was cleaned up, did the pale moths come back?
Yes. After the clean air laws, the dark form became rare again, which is just what natural selection predicts.
The peppered moth has a pale, speckled form and a dark form; colour is inherited and the dark allele is dominant.
The dark form arose by chance mutation; pollution changed which form survived, not the moths' genes.
On soot-darkened bark, birds ate more pale moths, so the dark form spread (industrial melanism).
Variation, inheritance, overproduction and differential survival all appear in the peppered moth story.
After clean air laws, lighter bark favoured pale moths again and the dark form became rare.
By 1895, about 98% of the peppered moths around Manchester were dark, less than fifty years after the first dark moth was recorded there in 1848.
In a polluted wood, if birds eat 40% of 500 pale moths and 10% of 500 dark moths, the dark form rises from 50% to 60% of the survivors in one generation.
After the Clean Air Act of 1956, cleaner air and returning lichens made pale moths the better-camouflaged form again, and the dark form declined.
Soot on the tree, dark moths go free; clean bark again, pale moths remain.
A family of moths lives in a wood that slowly turns black with soot. The pale cousins are picked off by birds, while the dark cousins hide and raise large families. Years later the wood turns pale again, and the pale cousins take their turn to thrive.
Very Interesting Old Debate: Variation, Inheritance, Overproduction, Differential survival.
M.S.B.R. - Mutation, Soot, Birds, Reversal: the peppered moth story in order.
Term
What are the two main forms of the peppered moth?
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Did soot cause the moths to turn dark?
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What was the selective agent?
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Industrial melanism
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What happened after clean air laws?
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Biston betularia, a night-flying moth with pale and dark forms, studied as an example of natural selection.
Dark-coloured because of a large amount of the pigment melanin.
Colouring or patterns that help an organism blend in with its surroundings.
The increase of dark forms of a species in areas darkened by industrial pollution.
The factor in the environment that causes differential survival; for the peppered moth, birds that hunt by sight.
A method in which marked animals are released and later recaptured to estimate survival or population size.
Natural selection that favours one extreme form, shifting the population in one direction.
The peppered moth shows natural selection in action: when soot darkened tree trunks in industrial Britain, birds ate more of the pale moths, so the dark form became common; when the air was cleaned, the pale form became common again.
This lesson uses the peppered moth, Biston betularia, as a worked example of natural selection. It describes the pale and dark forms, follows the rise of the dark form during the Industrial Revolution and its fall after clean air laws, and maps the story onto the four components of natural selection: variation, inheritance, overproduction and differential survival. It also examines the evidence, from Bernard Kettlewell's release-recapture experiments to later studies, and works through simple calculations of how predation shifts the make-up of a population.
The peppered moth flies at night and rests during the day on trees, where birds that hunt by sight search for it. Its story is one of the best-documented examples of natural selection, because the change happened quickly enough for people to record it.
Wing colour is inherited: it is controlled mainly by one gene, and the allele for the dark form is dominant. The dark form arose by a random mutation; the soot did not cause it. In 2016, scientists traced the dark colour to a piece of DNA inserted into a gene called cortex, a change thought to have first appeared around 1819.
During the Industrial Revolution, coal smoke covered trees near industrial cities with soot, and air pollution killed the pale lichens on their bark. The first dark peppered moth was recorded in Manchester in 1848; by 1895 about 98% of the peppered moths around Manchester were dark. On blackened trunks, dark moths were camouflaged and pale moths were easy for birds to find. This rapid spread of dark forms in polluted areas is called industrial melanism.
Over many generations, the frequency of the dark allele rose. Individual moths did not change colour; the population changed, because one form left more offspring than the other. This is directional selection: the environment favoured one extreme.
Imagine a polluted wood with 500 pale and 500 dark moths. If birds eat 40% of the pale moths but only 10% of the dark ones before they breed, the survivors are 300 pale and 450 dark. The dark form has risen from 50% of the population to 450 ÷ 750 = 60% in a single generation. Repeated over many generations, small advantages like this transform a population.
In the 1950s, Bernard Kettlewell released marked pale and dark moths into woods and later recaptured as many as he could. In polluted woodland near Birmingham, a higher proportion of the dark moths were recaptured; in unpolluted woodland in Dorset, a higher proportion of the pale moths were. Birds were also observed taking the more visible moths from tree trunks. Critics later pointed out weaknesses, such as releasing moths in daylight and in unnaturally large numbers. A long study by Michael Majerus in the early 2000s, designed to avoid these problems, again found that birds ate more of the conspicuous moths, supporting the original explanation.
After clean air laws in Britain, beginning with the Clean Air Act of 1956, the air became cleaner, lichens returned and tree bark grew lighter. Pale moths were camouflaged again, and the dark form became rare in most areas. A similar rise and fall happened in industrial regions of the United States. The reversal is strong evidence that the environment, acting through predation, was driving the change.