Spatial PD (Nowak & May 1992): on a lattice, cooperators can survive by clustering — defectors exploit isolated cooperators but can't invade dense cooperative clusters.
Strategies: Always-C, Always-D, Tit-for-Tat (copy last move), Pavlov (win-stay/lose-shift).
Each cell copies the strategy of its best-scoring neighbor each generation. Adjust Temptation T to shift the balance.