Q.Which type of selection explains industrial melanism observed in moth, Biston bitularia:
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Start your 14-day free trial to unlock the full solution →Industrial melanism in the peppered moth Biston betularia is a classic example of directional selection, where one extreme phenotype (the dark form) became more common because it offered better camouflage in a changed environment.
The story of the peppered moth is one of the most vivid demonstrations of natural selection in action. Before the Industrial Revolution in England, the typical peppered moth had a light, speckled appearance. This colouration was an excellent camouflage against the lichen-covered tree trunks where the moths rested. Birds, the moth's main predators, could easily spot and eat any moth that stood out against this background. In this environment, the light-coloured form was the most common.
Then came the Industrial Revolution. Soot and smoke from factories coated the tree trunks, killing the lichen and turning the bark a dark, grimy colour. The light-coloured moths, once so well-hidden, were now starkly visible against the darkened trees. Birds could pick them off with ease. Meanwhile, a rare, dark-coloured (melanic) form of the same moth, which had always existed in the population, suddenly had the advantage. It was now the dark moths that were camouflaged, and the light moths that were conspicuous.
The key is that the environment changed, and with it, the selective pressure. The advantage shifted from one extreme form (light) to the other extreme form (dark). This is the hallmark of directional selection.
This is precisely what directional selection means. It favours individuals at one extreme end of the phenotypic range. In this case, the "dark" extreme was favoured over the "light" extreme. Over several generations, the frequency of the dark allele increased dramatically in industrial areas, while the light form became rare. The population's average phenotype "moved" in one direction—towards darker colouration.
Let's briefly see why the other options do not fit.
- Stabilising selection favours the intermediate or average phenotype and eliminates extremes. For example, human birth weight is under stabilising selection—very small or very large babies have lower survival rates. In the moth story, the intermediate form was not favoured; one extreme replaced the other. …
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