Moths, Lichen and a Changing Forest
In a woodland, a species of moth has two forms: a pale form and a dark (melanic) form. Before industrialisation, tree trunks were covered in pale lichen and the pale form was common (~95%). After decades of soot pollution killed the lichen and darkened the bark, the dark form rose to ~90%. When clean-air legislation reduced soot, the pale form began increasing again. A visiting student says: 'The moths turned dark because they needed to hide - the pollution made them change colour.' A second student says: 'It's just random mutation; the environment had nothing to do with it.' The local record also shows the moth population crashed during the soot years.
1. Evaluate both students' explanations and give the correct mechanism.
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Both are wrong, in different ways.
- Student 1 (Lamarckian): individual moths do not change colour in response to need. Colour is determined by inherited alleles present before the environmental change. The environment does not instruct the genome.
- Student 2 (random mutation only): mutation is random and does generate both forms, but mutation alone cannot explain the directional shift in frequency from 95% pale to 90% dark and back. That requires selection.
Correct mechanism - natural selection on standing variation:
- Both colour forms already exist (genetic variation from mutation/recombination).
- On soot-darkened bark, dark moths are better camouflaged and are eaten less by visual predators (e.g. birds); pale moths are more visible and eaten more.
- Dark moths therefore survive and reproduce more, so the dark allele frequency rises.
- With clean air, lichen returns, pale moths are favoured again, and the frequency reverses.
This is directional selection acting on existing heritable variation - not individual change and not mutation alone.
2. Explain how the population crash during the soot years could itself have accelerated the change in colour frequency.
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A population bottleneck interacts with selection:
- When the population crashes, genetic drift becomes strong because the surviving sample is small; allele frequencies can shift rapidly by chance as well as by selection.
- If survivors are disproportionately dark (because predation removed pale moths), the bottleneck reinforces the selective shift.
- After the crash, the population recovers from a reduced gene pool, so the dark allele may dominate simply because it was over-represented among survivors.
So the crash is not just a side effect: it can amplify the frequency change through drift, and it reduces genetic diversity, which matters for the population's future adaptability. The clean-air reversal then shows selection can act again on whatever variation remains.
3. Design an observation that would let you test whether predation (rather than drift) is driving the colour change.
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Test the predation hypothesis directly:
- Mark–recapture / survival assay: mark equal numbers of pale and dark moths, release them on bark of known colour (dark soot vs pale lichen), and recapture after a set time. If pale moths survive worse on dark bark and better on pale bark, predation is supported.
- Predator observation: record bird attacks on artificial moth models pinned to bark of each type; compare attack rates by colour and background.
- Control for drift: compare multiple replicate woodlands - if colour shifts track bark colour consistently across sites, selection is favoured; if shifts are random and site-specific, drift is favoured.
Prediction supporting selection: survival of each form is highest when it matches the background, and the direction reverses when the background changes. Drift would not produce such a consistent, background-dependent pattern.