The figure shows the proportion of Hawks in two populations ( and ) with the same number of individuals. At each time step, all individuals are randomly matched in pairs made up of one individual from population and one individual from population to play a symmetric Hawk–Dove game (also called snowdrift or chicken). The payoffs in this game are: a Hawk meeting a Dove gets the highest payoff (3), but if he meets another Hawk they both get the lowest payoff (0). A Dove meeting a Hawk gets 1, and when two Doves meet they both get 2. At the end of each time step, after all individuals have played the game, one randomly selected player from each population revises her strategy—Hawk or Dove—according to the following rule: "I look at another (randomly selected) individual in my population; if and only if she got a payoff higher than mine, I adopt her strategy".