Natural selection – changing allele frequency (peppered moth)
BiologyEvolutionAges 17–18
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Evolution
A moth population rests on tree trunks: genotypes AA and Aa give dark wings, aa gives light wings, and each genotype has its own fitness (survival rate to reproduction). Each generation, individuals are removed according to fitness, alleles are passed on, and A ⇄ a mutation occurs at a small rate; a graph tracks allele frequency across generations against the theoretical Δp curve. Students drag the fitness values, switch the bark color to reverse the direction of selection, and watch the favored allele rise over time.
Lesson: The modern synthesis: natural selection and mutation change allele frequency in a population
What it shows
Natural selection changes allele frequencies because genotypes differ in fitness, the chance of surviving to reproduce. Here, dark wings (AA, Aa) or light wings (aa) are better or worse camouflaged depending on bark color, echoing the peppered moth during industrial pollution. Each generation the simulation removes moths according to fitness and builds the next generation from the survivors' alleles. The simulated frequency p of allele A is compared with the deterministic prediction Δp, so students see both the direction set by selection and random drift in small populations.
How to use
Start with the default fitness values and press Next generation a few times, then let it run and watch p climb along the theoretical curve. Press Change bark color to swap the fitness of AA and aa and reverse the direction of selection. Compare runs with a small and a large Population size N, and set all three fitness values equal to see what happens when there is no selection.
Parameters you can change
- Fitness of AA (dark wings) 0–1
- Fitness of Aa (dark wings) 0–1
- Fitness of aa (light wings) 0–1
- Population size N 50–2000 individuals
- Initial frequency of allele A 0.01–0.99
- A ⇄ a mutation rate per generation 0–0.01
Questions to explore
- Why does selection against light aa moths remove allele a only slowly once it has become rare?
- What happens to allele frequencies when the fitness of AA, Aa and aa are all equal?
- Why does a population of 50 moths stray further from the theoretical curve than a population of 2000?
Teaching ideas
All guides- Hardy–Weinberg Virtual Lab: Selection and Genetic DriftHardy–Weinberg virtual lab: model allele frequencies over many generations, calculate p², 2pq and q², then break the rules with selection and drift.
- Selective Breeding Virtual Lab: Artificial SelectionA selective breeding virtual lab: choose the hairiest Fast Plants as parents, measure S and R, calculate realized heritability and watch inbreeding rise.
