Assertion (A) : The population of melanized moths increased in industrial areas after Industrial Revolution.
Reason (R) : In Industrial environment lichen covered trees were replaced by soot-covered trees offering better camouflage to dark coloured moths.
Assertion (A) : The population of melanized moths increased in industrial areas after Industrial Revolution.
Reason (R) : In Industrial environment lichen covered trees were replaced by soot-covered trees offering better camouflage to dark coloured moths.
Options
Correct option: (A) — Both A and R are true, and R correctly explains A.
Why:
- Before the Industrial Revolution, tree trunks were covered with pale lichens, so light-coloured (white-winged) moths were camouflaged and dark (melanic) moths were picked off by predators.
- After industrialisation, soot killed the lichens and blackened the tree bark. Now the dark (melanized) moths were better camouflaged, while light moths became conspicuous and were eaten.
- Predators (birds) therefore acted as the selective force, and the melanic form was naturally selected, so its population rose — exactly what the Reason states.
Marking Scheme
- 11 mark: correct code (A) — both A and R true and R is the correct explanation of A.
- 2No partial marks; naming Kettlewell / Biston betularia is not required but strengthens reasoning.
Hint
Ask whether the change in tree background (lichen → soot) is the actual cause of the melanic moths increasing.
Quick Oral Answer
Both are true and the Reason explains the Assertion: after industrialisation soot killed lichens and blackened tree bark, so dark peppered moths were now camouflaged while pale ones were eaten by birds, and natural selection increased the melanic population — so the correct code is A.
Analysis & Explanation
Concept — Industrial Melanism:
This is the classic textbook example of natural selection observed by H.B.D. Kettlewell in the peppered moth Biston betularia in England.
- Pre-industrial background: clean, lichen-covered trees favoured the pale morph (camouflage); the dark morph stood out and was eaten by birds.
- Post-industrial background: soot from factories destroyed lichens and darkened bark, reversing the advantage in favour of the melanic morph.
Why R explains A:
The rise of melanic moths is a direct consequence of the environment changing (loss of lichen, soot deposition) altering which colour is camouflaged. The change in background is the mechanism behind the change in population — so Reason is the correct explanation of Assertion.
Exam trap:
Students often mark (B) thinking the statements are unrelated facts. They are causally linked — the environmental change described in R is the very reason the melanic population increased in A.
Real-world:
After the UK Clean Air Acts reduced pollution, lichens returned and the frequency of the pale form rose again — showing selection is reversible when the environment reverses.
Common Mistakes
- 1Choosing (B) — treating A and R as true but unrelated, when the soot/lichen change is precisely the cause of the population shift.
- 2Assuming the moths 'turned dark' due to pollution (Lamarckian). Colour is genetically determined; pollution only changed which colour survived predation.
- 3Confusing this with mutation as the cause — mutation supplies variation, but selection (predation on the exposed morph) drives the frequency change.
Interesting Facts
H.B.D. Kettlewell's 1950s mark-recapture experiments in polluted Birmingham vs unpolluted Dorset provided direct field evidence for natural selection acting in real time.
The dark colour is due to a single dominant allele (carbonaria); its frequency in Manchester rose from under 1% in 1848 to about 98% by 1895.
After Britain's Clean Air Act (1956) reduced soot, pale peppered moths made a strong comeback, demonstrating that selection pressure reversed with the environment.
Spotted a mistake or something unclear?
Tell us — we fix reported answers fast.
Frequently Asked Questions
Did the moths change colour because of the pollution?
No. Colour is genetically fixed. Pollution did not turn moths dark; it changed the background so that the already-existing dark (melanic) form was now better camouflaged and survived predation more often, raising its frequency by natural selection.
Why is the answer (A) and not (B)?
Because A and R are causally linked, not independent facts. The replacement of lichen-covered trees by soot-covered trees is the actual reason dark moths became camouflaged and increased — so R is the correct explanation of A.