Natural selection

What it does

Peppered moths rest on tree trunks by day. Most used to be pale and speckled, hidden against pale bark covered in lichen. When factory smoke blackened the trees, a dark form became common, because birds found the pale moths more easily on the dark bark. When the air was cleaned, pale moths came back. This simulator lets you replay that story and change it:

• Tree bark: always light, always dark, soot comes (bark turns dark at generation 10) or clean air (bark turns light at generation 10).
• Birds: the share of visible moths eaten each generation.
• Dark allele at the start: the dark form comes from one dominant allele, C. Light moths are cc.
• Moths: 50, 500 or 5000 moths (the default), or no chance (the average path). In small populations luck matters too: that is genetic drift.
• Mutation: the chance, each generation, that a light allele c is miscopied into a dark allele C (off, 1 in 100,000, 1 in 10,000 as the default, or 1 in 1,000). It keeps a trickle of new dark moths appearing, as happened in real moth populations.

While the bark is still light, birds eat most of the few dark moths. In a small population the last dark allele can then be lost by pure chance before the soot arrives; that is genetic drift, and the page says so when it happens. With mutation off, a lost allele never comes back, so selection has nothing to work on. With 5000 moths and a little mutation this almost never happens. To see drift on its own, with many populations side by side, use the wright-Fisher simulator.

The trunk picture shows the moths of one generation, with the eaten ones crossed out; slide Generation or press Play to move through time. The graph shows the share of dark moths over 60 generations. It fits Class 10 Science: heredity and evolution (variation, natural selection) and Class 12 Biology: evolution (natural selection, the Hardy-Weinberg principle, gene flow and genetic drift). The moth colour gene works like Mendel’s tall and short peas in mendel's pea garden; check real genotype counts with hardy-Weinberg tester, work out p and q from data with allele frequency calculator, and see how populations grow in population growth models.

How to use it

1. Pick the level. Class 10 shows the moths, the eaten ones and the % dark. Class 12 adds genotypes CC, Cc and cc, the allele frequencies p and q, and a Hardy-Weinberg check of the newborns.
2. Pick the bark and set how hard the birds hunt.
3. Set the dark allele at the start. The label shows what share of moths that makes dark (1 − q²).
4. Pick the number of moths and the mutation rate. Press New run to see a fresh run with the same settings; with 50 moths no two runs look alike.
5. Slide the generation or Play. Download every generation as a CSV file, or the picture and graph as an image, PDF or Word file.
6. Check yourself gives fresh questions each time.

Worked example

Open the page: soot comes at generation 10, birds catch 30 % of the moths they can see, and the dark allele starts at 5 % (so about 9.8 % of moths are dark, because CC and Cc moths are both dark: 1 − 0.95² = 0.0975). With no chance and the default mutation rate of 1 in 10,000, dark moths fall while the bark is still light (to 0.4 % by generation 10), then rise once the bark turns dark: about 74 % of moths are dark by generation 30 and 98.9 % by generation 60. With 5000 moths a run follows much the same path. At Class 12 level with 500 moths, the facts show p = (2 × CC + Cc) ÷ (2 × 500) for the newborns and the Hardy-Weinberg expectation p² : 2pq : q², which the newborns fit because they come from random mating.

Tips

• Selection only works on variation that is already there. If the dark allele starts at 0 %, nothing can happen: try 1 %.
• Light moths come back slowly after clean air because the c allele hides in dark Cc moths, where birds cannot “see” it.
• Set the birds to 0 % and use 50 moths to see drift on its own.
• Try soot comes with 500 or 50 moths and mutation off, and press New run a few times: often the dark allele is lost before the soot arrives, and the dark moths never come back.

FAQ

Is the dark colour caused by the soot? No. The moths do not change colour. Dark moths are born dark because of the allele they inherit; soot only changes which moths the birds find.

Why do the newborns fit Hardy-Weinberg if selection is happening? Random mating rebuilds p², 2pq, q² every generation. Selection changes p between generations, so the population is not in balance over time.

Are these real survival numbers? No, they are teaching values you can change.

Last updated 2026-11-09.