Turing (1952) showed that a fast-diffusing inhibitor can destabilize a homogeneous steady state,
producing spatial patterns. The Gierer-Meinhardt model f(u,v) = a − bu + u²/v, g(u,v) = u² − cv
generates spots when D_v/D_u is large, stripes at intermediate ratios.
Increasing D_v/D_u lengthens the characteristic wavelength.