Question
Explain Darwin’s theory of natural selection using the peppered moth (Biston betularia) as an example. What does this case study demonstrate about the mechanism of evolution?
Solution — Step by Step
Darwin’s theory of natural selection rests on four observable facts:
- Variation: Individuals in a population differ from one another
- Inheritance: Traits are passed from parents to offspring
- Overproduction: More offspring are produced than can survive
- Differential survival: Individuals with favourable traits survive and reproduce more (“survival of the fittest”)
Over many generations, favourable traits accumulate in the population — this is evolution by natural selection.
Before the Industrial Revolution in Britain (pre-1850), the trunks of trees were covered with pale grey lichens. The peppered moth population consisted mostly of light-coloured (typica) moths, which were well camouflaged against the lichen-covered bark.
Melanic (dark, carbonaria) moths existed in the population as a rare variant due to random mutation, but they stood out against the pale bark and were quickly spotted and eaten by birds.
As factories spread across industrial England, soot and pollution killed the lichens and coated tree bark in black. Now the environment changed dramatically:
- Light moths became visible against the darkened bark → birds ate them preferentially
- Dark moths were now camouflaged against the black bark → they survived and reproduced more
The selection pressure reversed completely. What was a disadvantage (dark colouring) became an advantage.
By the late 1800s, dark moths dominated the population in industrial areas — up to 98% in some regions near Manchester. Light moths remained common only in rural, unpolluted areas. This rapid, observable shift in population phenotype is called industrial melanism.
In the 20th century, as clean air legislation reduced pollution and lichens returned, light moths again became more common. The population “evolved back” — a beautiful demonstration that natural selection is directional and reversible.
The peppered moth case proves:
- Natural selection is observable — it happened within decades, not millions of years
- Environment drives selection — the same trait can be advantageous or disadvantageous depending on context
- Selection acts on existing variation — it doesn’t create new traits, it filters existing ones
- Evolution is not “progressive” — there is no “goal,” only fitness to the current environment
Why This Works
Darwin couldn’t see genes or DNA — he observed phenotypes and inferred the mechanism. The peppered moth case beautifully validates his logic: variation exists (light vs dark), it’s heritable (offspring resemble parents), the environment selects (birds eat the visible ones), and the fitter variant spreads (frequency changes over generations).
The reversibility — dark moths declining as pollution fell — is particularly powerful. It shows evolution by natural selection is a mechanical process responding to environmental change, not a one-way march toward complexity.
NEET 2023 and CBSE Class 12 board exams both asked about industrial melanism as an example of natural selection. Be specific: name the moth (Biston betularia), name the location (industrial England), describe the before-and-after frequencies, and explain the mechanism (predation as selection pressure).
Alternative Method
You can also frame the peppered moth case using the concept of allele frequency. The gene for melanism exists as two alleles. Before industrialisation, the melanic allele was rare (low frequency) because carriers had lower fitness. After industrialisation, the melanic allele increased in frequency because carriers had higher fitness. Evolution, in modern terms, is a change in allele frequencies in a population — and this case shows exactly that.
Common Mistake
Students often write “moths developed dark colour because of pollution” — this implies Lamarckian inheritance (acquired characteristics). This is wrong. The dark allele already existed in the population as a rare mutation. Pollution didn’t create dark moths; it changed which colour had higher survival, causing dark moths to increase in frequency. Natural selection selects from existing variation — it does not direct it.