13.5Chapter 13: Variation and Natural Selection

Investigating the peppered moth: past and present — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

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13.5 Investigating the peppered moth: past and present — Alhussan International Academy — Biology (Stages 7–9) notes: Explain, using the peppered moth, how a change in the environment altered the proportions of light and dark forms through natural selection.Peppered moth (Biston …

Peppered moth (Biston betularia)
Inheritance
Before about 1800
Camouflage
Selection pressure
Individuals

Part 1 of 6The moth and its variation

1Peppered moth (Biston betularia)

Peppered moth (Biston betularia) it is a night-flying insect that rests on tree trunks by day and lives from China, Russia and Kazakhstan to Europe and North America.

2Forms

Forms moths vary in colour from almost white to almost black, with a pale speckled form and a dark (melanic) form.

3Inheritance

Inheritance the colour is controlled by a gene.

4Before about 1800

Before about 1800 nearly all moths in Britain were pale,
- which suits birch trees and lichen-covered bark,
- and dark moths were rare, appearing by mutation.

Part 2 of 6Industrial melanism

Pollution

Pollution during the Industrial Revolution factories burned coal and released soot and sulfur dioxide.
Soot blackened tree trunks in industrial regions.
Sulfur dioxide killed pale lichens.

Camouflage

Camouflage on dark trunks pale moths were easy for birds to see and eat, while dark moths were camouflaged.

Result

Result the first dark moth was recorded near Manchester in 1848, and by 1895 about 98% of the city's moths were dark.

Part 3 of 6Natural selection in the moth

There is variation: pale and dark moths.

The environment changes: soot darkens trees.

Dark moths are better camouflaged, so fewer are eaten by birds.

More dark moths survive to breed and pass the allele for dark colour to their offspring.

Over generations the proportion of dark moths increases.

Selection pressure

Selection pressure predation by birds.

Individuals

Individuals a single moth does not change colour, because the proportion in the population changes through differences in survival and reproduction.

Part 4 of 6Kettlewell's investigation

Theory

Theory in clean woodlands pale moths survive better,
but where trees were blackened by pollution dark moths would outnumber light ones through natural selection.

Evidence gathered

Evidence gathered he analysed specimens from collections made before the Industrial Revolution and observations by amateur collectors across the country.

Field experiments

Field experiments in the 1950s he placed light and dark moths on different coloured tree trunks, released marked moths in polluted and clean woods and recaptured them.
In the polluted wood he recaptured more dark moths, and in the clean wood more pale ones.
Birds were seen to take the more visible moths.

Importance

Importance scientists had thought natural selection took place over long periods, and this showed it in action, fast.

Part 5 of 6Criticism and verification

Criticisms

Criticisms some people said moths did not rest on tree trunks, doubted that birds ate them, and criticised placing moths at unnatural densities.

Majerus (2001 to 2008)

Majerus (2001 to 2008) he released 4864 moths over six years at an unpolluted site and recorded where they rested and what ate them.
Few moths rested on tree trunks, and most rested on branches.
Nine bird species ate them, including robins, great tits, blue tits, blackbirds, starlings and wrens.
Over six years a greater percentage of the dark moths than light moths were seen and eaten by birds.
His results showed that the original work was correct.

Part 6 of 6Pollution control and the reversal

Clean air

Clean air from 1970 UK factories became cleaner, after the Clean Air Acts of the 1950s and the decline of coal burning,
- so air and trees became cleaner.
Lichens returned and trees became paler.

Result

Result birds could see dark moths easily again,
- so the proportion of dark moths fell and most peppered moths in the UK and northern Europe are now pale.

Monitoring

Monitoring moth traps and counts measure the change, shown in graphs of percentage dark moths against year.

Environment	Advantage	Result
Sooty, polluted	Dark form camouflaged	Dark moths increase
Clean air, pale lichen	Pale form camouflaged	Pale moths increase

Lesson
- natural selection continues, and the environment decides which variation is advantageous.

Key terms

Term	Meaning
Natural selection	The survival and breeding of the best adapted organisms.
Camouflage	A colour or pattern that makes an organism hard to see.
Melanic	Dark coloured because of high levels of pigment.
Selection pressure	A factor, such as predation, that affects survival.
Industrial melanism	The increase of dark forms in polluted industrial areas.
Predation	Feeding by predators on prey.

Avoid these mistakes

Saying individual moths changed colour to survive.

Writing that the moths needed to become dark.

Forgetting that variation and inheritance are required.

Ignoring the role of predators.

Last updated: 13 October 2026