Natural Selection Simulator

Natural selection is not a force pushing organisms towards improvement. It is an arithmetic consequence of three facts about a population, and if all three hold then the frequencies must change whether anything intends it or not. This simulation contains exactly those three facts and nothing else.

• Variation. The organisms are not identical. Some are dark, some speckled, some light.
• Inheritance. Offspring resemble their parents, because each parent passes on one of its two alleles.
• Differential survival and reproduction. Some variants leave more offspring than others. Here that happens because a badly camouflaged organism is more likely to be eaten before it breeds.

Remove any one of the three and the effect stops. Set selection strength to zero and survival stops depending on colour; the frequency still moves, but it wanders instead of trending. Remove inheritance and offspring would be random; nothing would accumulate. Remove variation, which happens by itself once an allele reaches 100 percent, and there is nothing left for selection to act on.

Watch the habitat panel while it runs and notice what does not happen: no individual organism ever changes colour. Each one lives and dies with the alleles it was born with. What changes is the makeup of the population. That distinction between individual and population is the single most common place students go wrong when writing about evolution.

The habitat on the left is the population as it exists right now. The chart on the right is its history. Both are needed, because they answer different questions: the habitat shows you what the population looks like, and the chart shows you what has been happening to the genes underneath.

• Dots. One per organism, shaded by its inherited colour, on the background you chose. The harder a dot is to see, the better its chances.
• Crossed-out dots. The individuals that did not survive the most recent generation. Their positions are random, so do not read anything into where they are, only into what colour they were.
• The teal line. The frequency of the dark allele D across the whole population, counting two alleles per individual. When it reaches 0 or 1 the population has lost an allele permanently, unless mutation is switched on.
• The shaded bands. The proportion of the population that is dark, speckled and light. These are the phenotypes, which is what selection can actually see.
• Survival chances. The readout shows what each phenotype is up against under the current background and selection strength. It is the quickest way to check that the pressure is pointing the way you think it is.

Watch the relationship between the line and the bands. The bands move first, because selection acts on appearance within a single generation. The line follows, because allele frequencies only change when the survivors breed. When the two disagree, it is usually because heterozygotes are carrying an allele through a generation that is selecting against the phenotype it produces.