A two-species reaction-diffusion system that spontaneously breaks spatial symmetry into stripes and spots
Activator A (blue=low, yellow=high)
Inhibitor B (blue=low, red=high)
The Brusselator: ∂A/∂t = Du∇²A + a − (b+1)A + A²B, ∂B/∂t = Dv∇²B + bA − A²B.
Steady state: A* = a, B* = b/a. Turing instability occurs when Dv/Du is large enough.
The key condition: the inhibitor must diffuse faster than the activator.
Stripes vs. spots depend on the ratio b/a and diffusion constants.