Natural selection sim

What the sim shows

Natural selection in peppered moths. The moths vary: a dark form (carbonaria, allele C, dominant) and a light form (typica, allele c, recessive). The dark allele first arose by a random mutation; the bark does not cause it. Birds eat the moths they see, so on bark that hides one form better, the better-camouflaged moths survive and reproduce more, and pass on their alleles. Over many generations those alleles become more common: no moth changes, the population does. In small populations chance (drift) also moves the allele frequency a lot. In industrial Britain in the 1800s, soot from coal darkened the bark of trees near cities, and the dark form became common there (in Manchester it went from rare, first recorded in 1848, to most of the moths by the end of the century). After the clean-air laws of the 1950s the bark grew paler again, and the light form became common again. Simplified on purpose: illustrative numbers: a teaching model of peppered moths, not real data; real values differ; one gene with two alleles, C (dark, carbonaria) and c (light, typica); real peppered moths also have in-between forms (insularia); complete dominance: a Cc moth is as dark as a CC moth; only bird predation acts: a moth's chance of being eaten depends only on how much it stands out from the bark (15 % when perfectly hidden, 45 % when it stands out fully); the selection here is strong, so the change takes tens of generations; in the wild it was weaker, and the moths have one generation a year; a seeded model: a fixed list of random numbers, so the same steps always give the same moths; a new run starts with a dark-allele frequency of 0.05 (every dark allele in a Cc moth) or 0.50 (CC, Cc and cc moths in the ratio 1 : 2 : 1); population capped: the tree has food and space for a fixed number of moths (200 or 20), and the survivors' offspring refill it each generation; generations do not overlap; the survivors breed at random, and every offspring gets one allele from each of two parents; mutation: each allele copied changes with a chance of 1 in 10 000 (illustrative), the same at every soot level; each tree trunk is one shade; real bark is patchy, and real moths rest in many places; the lichen on the bench trunk fades out by 60 % soot (lichens die in polluted air); it is drawn, not part of the model; 1 drawn moth = 1 moth in the close-up: the drawn moths are the model's population; the 8 models pinned on the bench trunk show the share of dark moths, rounded; the chance-drift spread is about two standard deviations (a normal approximation), so a change outside it happens by chance alone about 1 time in 20 or less; below 3 generations on one bark it is too short to judge.

• Slider block: sets the soot on the bark (also in the close-up)
• Close-up: the last hunt and the moths now, one drawn moth per moth
• Ring: a moth found and eaten by a bird in the last hunt

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