Hawk-Dove Game
John Maynard Smith's model of a population competing repeatedly for a resource, where 'hawks' always escalate to costly combat and 'doves' only display and retreat. Neither pure strategy is evolutionarily stable — a population of all doves can be invaded by hawks, and a population of all hawks pays such heavy injury costs that a rare dove can out-reproduce it — so the population settles into a stable mixture instead, explaining why species with lethal natural weapons often evolve ritualized rather than fatal combat.