board · 732 × 732grid · on
Nowak and May's spatial prisoner's dilemma showed that plain space can rescue cooperation. On a well-mixed population defection always wins, but on a grid cooperators survive by clustering: interior cooperators earn enough from each other to outscore the defectors gnawing at their edges.
Each patch plays the dilemma with its eight neighbors — mutual cooperation pays 1, defecting against a cooperator pays the temptation b, everything else pays roughly nothing — then adopts the strategy of its best-scoring neighbor, itself included. The classic four-color view tracks who stayed or switched, and a single central defector at b = 1.65 grows into the famous symmetric kaleidoscope with persistent C/D coexistence.