Q.Explain natural selection using the well-known example of the peppered moth, Biston betularia, in industrial England. How did the relative proportions of the light-coloured and dark-coloured (melanic) forms change before and during the period of industrial soot pollution, and why?
Step 1. Before industrialisation, the pale speckled-grey moth form was common and well camouflaged against pale, lichen-covered tree bark, escaping bird predation.
Step 2. A rare dark (melanic) form existed but stood out against pale bark and suffered heavier predation.
Step 3. Industrial soot pollution blackened tree bark and killed the lichens.
Step 4. This reversed the camouflage advantage: the dark form now blended in, the pale form now stood out.
Step 5. Over subsequent decades, the dark form's frequency rose sharply in polluted regions — and later fell again as pollution controls reduced soot, confirming the selective mechanism.
Camouflage advantage reversed with soot pollution: pale-favoured before industrialisation, dark (melanic)-favoured during it, demonstrating natural selection responding to a changing environment.
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