The figure shows the proportion of

-strategists in two distinct 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 2×2 game. The two possible actions (or pure strategies) in the game are labeled

and

. Thus, each individual (regardless of the population to which it belongs) is either an

-strategist or a

-strategist. The payoffs of the game are

,

,

, and

(parameters), where, for instance,

denotes the payoff obtained by an

-strategist when he plays with a

-strategist.

At the end of each time step, after all individuals have played the game, one randomly selected player from each population revises her strategy—

or

—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". Thus, the game is played between individuals of different populations, but imitation takes place within each population.