Q.(a) Why is the collection of white winged moths and dark winged moths made in England between 1850 – 1920 considered a good example of natural selection ?
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Start your 14-day free trial to unlock the full solution →The peppered moth example (a) demonstrates natural selection because environmental change (industrial pollution) directly led to differential survival and reproduction based on moth coloration, shifting population frequencies. Evolution (b) involves chance through random mutations and unpredictable environmental events, but natural selection acts on these variations in a non-random, directional manner.
(a) Why the collection of white-winged and dark-winged moths in England (1850-1920) is a good example of natural selection
The classic case of the peppered moth, Biston betularia, in industrial England between 1850 and 1920 is a textbook example of natural selection because it vividly illustrates how environmental changes can drive rapid evolutionary shifts in a population. It demonstrates all the core tenets of natural selection: variation, inheritance, differential survival, and differential reproduction.
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Initial Variation and Environment (Pre-1850): Before the Industrial Revolution, most peppered moths in England were light-colored (white-winged with black speckles), blending in perfectly with the light-colored, lichen-covered tree trunks. A rarer, naturally occurring dark-colored (melanic) variant also existed due to a random mutation, but these were easily spotted by predatory birds against the pale background.
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Environmental Change (1850-1920): The Industrial Revolution led to widespread coal burning, releasing massive amounts of soot and pollution into the atmosphere. This pollution had two major effects:
- It killed the light-colored lichens on tree trunks, leaving the bark exposed and darkened by soot.
- It directly darkened the tree trunks themselves.
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Shift in Selective Pressure: With the trees now dark, the situation reversed. The previously well-camouflaged light-colored moths became conspicuous against the sooty bark, making them easy targets for predatory birds. Conversely, the dark-colored moths, once easily seen, now blended in perfectly with the darkened tree trunks. This change in camouflage effectiveness created a strong selective pressure.
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Differential Survival and Reproduction:
- Dark moths: Were better camouflaged, survived predation more often, and thus had a higher chance of reproducing and passing on their genes for dark coloration to the next generation.
- Light moths: Were poorly camouflaged, suffered higher rates of predation, and consequently had lower survival and reproductive success.
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Observed Evolutionary Change: Over a relatively short period (decades), the frequency of the dark-winged moth variant dramatically increased in polluted industrial areas, while the frequency of the light-winged variant declined. In some areas, dark moths came to constitute over 90% of the population. This direct, observable shift in allele frequencies within a population, driven by environmental change and differential fitness, is precisely what natural selection predicts.
This example is powerful because it was a direct, observable demonstration of natural selection occurring in real-time, driven by a clear environmental change (industrial pollution) and a measurable selective agent (bird predation).
(b) ‘‘Evolution is based on chance events in nature and chance mutations in organisms.’’ Justify the statement.
The statement that evolution is based on chance events in nature and chance mutations in organisms is largely true, but it's crucial to understand the specific roles of "chance" and "non-chance" elements in the evolutionary process.
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Chance Mutations in Organisms:
- Randomness of Mutation: Mutations are random changes in the DNA sequence. They occur spontaneously due to errors during DNA replication, exposure to mutagens (like radiation or certain chemicals), or viral infections. Crucially, these mutations arise without any foresight or purpose regarding their potential benefit or harm to the organism. An organism does not "choose" to mutate in a way that would be advantageous in its environment.
- Raw Material for Evolution: These random mutations introduce new genetic variations into a population. Without this constant input of random variation, there would be no new traits for natural selection to act upon, and evolution would grind to a halt.
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Chance Events in Nature:
- Unpredictable Environmental Changes: The environment is constantly changing, and many of these changes are unpredictable and random from the perspective of the organisms living within them. Examples include:
- Natural Disasters: Volcanic eruptions, earthquakes, floods, droughts, or asteroid impacts can drastically alter habitats, wipe out populations, or create new selective pressures. …
- Unpredictable Environmental Changes: The environment is constantly changing, and many of these changes are unpredictable and random from the perspective of the organisms living within them. Examples include:
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