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Generation
1
Average colour
Medium
Match to background
Good
How to use: Beetles come in many shades of green. Change the
background colour so it's lighter or darker, and turn up the
predator pressure. Beetles that stand out get eaten more easily;
ones that blend in survive and reproduce. The top graph shows the spread of
colours, the bottom one the average colour over generations.
Notice: individuals don't change colour — but the population does.
Over time the average colour moves toward the background. The variation is already
there; the predator just picks which variants survive. That's natural selection:
it doesn't invent traits, it sorts the ones that already exist.