Natural selection – light and dark peppered moths
Integrated ScienceGenetics & EvolutionAges 14–15
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Genetics & Evolution
A moth population made up of light-colored individuals (dd) and dark-colored individuals (DD, Dd) rests on a tree trunk. Each generation, birds eat more of the moths that stand out against the bark; the survivors breed to produce the next generation, and the allele frequency and phenotype ratio shift gradually on the chart. Change the background color (tap the trunk, or let it darken gradually from industrial pollution) and the selection pressure to see the direction of evolution change.
Lesson: Concepts of evolution and modes of selection; Mechanisms of evolution
What it shows
The peppered moth is a classic example of natural selection observed in the wild. Dark wing color is controlled by a dominant allele D, so DD and Dd moths are dark and dd moths are light. In each generation birds eat the moths that contrast most with the bark, the survivors mate at random and the next generation is built from their alleles. The chart tracks the frequency of allele D, the proportion of dark moths and the bark darkness, and the text compares genotype shares with the Hardy–Weinberg expectation. Selection changes which variants survive; individual moths never change color.
How to use
Set Trunk background color to Light → dark over generations and press Auto-run to watch allele D spread as soot darkens the bark. Press Reset, then compare a low and a high Selection pressure to see how fast the frequency changes. Press Change background during a run to reverse the direction of selection, and repeat with a small Population size to see random fluctuations in the curve.
Parameters you can change
- Trunk background color Light (clean forest, with lichen), Dark (industrial soot), Light → dark over generations
- Selection pressure (fraction of conspicuous moths eaten) 0.1–0.9
- Maximum number of generations 5–50 generations
- Initial frequency of the dark allele (D) 0.05–0.95
- Population size 40–400 moths
Questions to explore
- Why can light dd moths reappear even when almost every moth in the population is dark?
- How does a higher selection pressure change the number of generations needed for allele D to spread?
- Why does a population of 40 moths give a more irregular curve than a population of 400?
