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Q.''Appearance of melanised moths post-industrialisation in England is a classic example of evolution by natural selection.'' Explain.

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The rise of melanised (dark) peppered moths in industrial England is a textbook case of natural selection because a change in the environment (soot-darkened trees) suddenly made the dark colouration advantageous for camouflage, leading to a rapid shift in the population's gene frequency — a direct, observable example of evolution in action.

The Concept: Evolution as a Change in Population, Not an Individual

Evolution is often misunderstood as an individual changing during its lifetime. That's not what happened here. No single moth turned from light to dark. Instead, the proportion of dark moths in the population increased over generations. This is the core of evolution by natural selection: a heritable trait becomes more common in a population because individuals with that trait survive and reproduce more successfully in a given environment.

The peppered moth (Biston betularia) story is powerful because it's simple, fast, and the selective pressure is crystal clear.

Step-by-Step Explanation

1. The original state: a well-camouflaged light form.

Before the Industrial Revolution, tree trunks in England were covered with light-coloured lichens. The typical peppered moth had a light, speckled pattern. This was excellent camouflage against the lichen-covered bark. Birds, the moth's main predator, could not easily spot them. The light form was the wild type — the most common variant.

2. A rare genetic variant: the melanic form.

Occasionally, a genetic mutation produced a dark, almost black form of the moth (called carbonaria). This melanic form was extremely rare because it was highly conspicuous against the light lichen. Birds ate them easily. Natural selection kept the dark allele at a very low frequency in the population.

3. The environmental change: industrial soot.

The Industrial Revolution (mid-1800s) brought coal-burning factories. Soot coated tree trunks, killing the light-coloured lichens and turning the bark dark grey or black. The environment changed dramatically.

4. The reversal of selective advantage.

Now, the light moths were the conspicuous ones. They stood out against the dark bark and were eaten by birds at a much higher rate. The dark moths, previously at a disadvantage, were now camouflaged. They survived better, lived longer, and laid more eggs.

Watch out

A common mistake is to think the moths chose to change colour or that the soot caused the mutation. Neither is true. The mutation for melanism existed before the pollution. The environment simply changed which variant was favoured.

5. The population-level result: a shift in allele frequency.

Because dark moths survived and reproduced more, they passed on the dark allele to more offspring. Over just a few decades, the frequency of the dark allele soared. In heavily polluted areas like Manchester, over 90% of peppered moths were the dark form by the 1890s. This is a measurable change in the genetic makeup of the population — which is the definition of evolution.

6. The reverse experiment: proof of the cause. …

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