On the right is a population of peppered moths resting on pale, lichen-covered bark. Most moths are light and blend in; only a few are dark (melanic) and stand out. Birds hunt by sight, so the conspicuous dark moths are eaten more often. ▶ Play has not run yet — this is generation 0, before selection acts.
Now factory soot darkens the bark — the background turns from pale to sooty. The situation flips: the dark moths are hidden and the light moths are exposed. ▶ Play and watch birds remove the poorly-camouflaged light moths, while the survivors breed. Generation by generation the population shifts toward dark — this is natural selection caught in the act.
After many generations almost the whole population is dark — the better-camouflaged variant survived and reproduced, so its colour became common. The moths themselves never changed; the proportions in the population changed. ↻ Reset to start over, or use ⏭ Step to advance one generation at a time and read the colour ratio.
Switch to the Allele frequency tab. The graph plots the frequency of the dark allele across generations. The flat green line is Hardy–Weinberg equilibrium — with no selection, the frequency stays constant and the population does not evolve. The rising red curve is the same allele under selection. ▶ Play to watch selection push the dark allele upward.
The bars below the graph show the genotype proportions from p² + 2pq + q² = 1, where p is the dark allele and q the light. Hardy–Weinberg equilibrium holds only with a large population, random mating, no mutation, no migration and no selection. ▶ Play and see how, as p rises under selection, the dark homozygotes (p²) come to dominate.
Open the Evidence tab. Four forelimbs — human, whale, bat and horse — are drawn side by side, each bone colour-coded to the same plan: one upper-arm bone, two forearm bones, the wrist, and the digits. The shapes differ for grasping, swimming, flying and running, but the underlying bone plan is identical. These are homologous organs — proof of a common ancestor (divergent evolution). ▶ Play to highlight each matching bone group in turn.