How a leopard gets its spots: Turing's two-chemical pattern machine

A leopard's spots and a zebra's stripes start out as a smooth sheet of near-identical cells. In 1952 Alan Turing showed that two interacting chemicals, one that activates and one that inhibits and spreads faster, can break that uniformity all by themselves.

Play with a reaction-diffusion simulation below: nudge two dials and the same equations turn spots into stripes and mazes. Studies of angelfish skin and zebrafish pigment cells support the idea that nature uses it.