 This diagram is interpreted as follows: the leftmost point represents the state of the population where everyone defects, the rightmost point represents the state where everyone cooperates, and intermediate points represent states containing a mix of both cooperators and defectors. (One maps states of the population onto points in the diagram by mapping the state when \(N\)% of the population defects onto the point of the line \(N\)% of the way to the leftmost point.) Arrows on the line indicate the evolutionary trajectory followed by the population over time. The open circle at the rightmost point represents the fact that the state where everybody cooperates is an unstable equilibrium, in that if a small portion (any amount \(\epsilon &gt; 0\)) of the population deviates from the strategy Cooperate, then the evolutionary dynamics will lead the population away from the all-Cooperate state. The solid circle at the leftmost point indicates that the state where everybody Defects is a stable equilibrium, in the sense that if some portion of the population deviates from the strategy Defect, then the evolutionary dynamics will drive the population back to the all-Defect state.