Natural Selection in Action – Modelling Evolution with Peppered Moths
Natural selection can feel abstract when students first encounter it in GCSE and A Level Biology. Terms like selective pressure, variation, and allele frequency are easy to memorise but harder to visualise.
The classic peppered moth example brings evolution to life. It shows natural selection happening over a short timescale, driven by environmental change and differential survival — exactly what Darwin described.
The Peppered Moth Story
The peppered moth exists in two main forms:
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light-coloured (typica)
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dark-coloured (carbonaria)
Before the Industrial Revolution, most tree trunks were pale and covered in lichens. Light moths were well camouflaged, while dark moths were easily spotted and eaten by birds.
As industrial pollution increased, soot darkened tree bark and killed lichens. Suddenly, the dark moths were better camouflaged. Birds ate more light moths, and the frequency of the dark form increased dramatically.
When air quality improved later in the 20th century, the trend reversed.
This is evolution by natural selection in action.
Modelling Natural Selection in the Classroom
Students can model this process using a simple practical or simulation.
Equipment:
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Paper moth cut-outs in two colours (light and dark)
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Two backgrounds (light paper and dark paper)
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Timer
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Data recording sheet
Method:
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Scatter equal numbers of light and dark moths onto the background.
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Act as the “predator” and remove moths you can see easily within a fixed time.
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Count the remaining moths of each colour.
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Repeat the process over several “generations”.
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Change the background to represent environmental change.
Students quickly see one colour becoming more common than the other.
Typical Results
Light background (pre-industrial):
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Light moths survive in greater numbers
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Dark moths are removed more quickly
Dark background (industrial):
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Dark moths survive better
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Light moths decline rapidly
Over repeated generations, the proportion of moths changes — not because individuals change, but because survival and reproduction are unequal.
Key Biological Concepts Reinforced
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Variation: moths exist in different forms
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Selective pressure: predation
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Differential survival: better-camouflaged moths survive
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Inheritance: colour is genetically determined
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Change in allele frequency: populations evolve over time
This helps students avoid the misconception that organisms “adapt because they need to”.
Evaluation and Limitations
Students can critically evaluate the model:
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paper moths do not move
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predators are human, not birds
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time scale is compressed
Despite this, the model clearly demonstrates the principle of natural selection and is highly effective for learning.
Why This Works in Teaching
The peppered moth example:
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links biology to history and environmental change
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is backed by real scientific evidence
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allows data collection and graphical analysis
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supports exam questions on evolution and selection
It shows evolution as an ongoing process, not just something that happened millions of years ago.
Skills Highlight
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Modelling biological processes
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Recording and analysing population data
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Interpreting trends over generations
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Applying theory to real-world examples
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Evaluating experimental models
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