Cooperators (green) and defectors (red) compete on a lattice. Each cell copies the strategy of its highest-scoring neighbor. Spatial structure enables cooperator clusters to persist.
Generation: 0
Cooperators: —
Defectors: —
Payoff matrix (weak PD):
C vs C: 1 | C vs D: 0
D vs C: b | D vs D: 0
Nowak & May 1992: spatial structure sustains cooperation for b ≲ 1.8.