 It should be noted that Hilbe et al. defend and employ a “pairwise comparison” model of evolution that is markedly different from the kinds of evolutionary dynamics discussed previously in this entry. At each stage a pair of agents is randomly selected and the first adopts the strategy of the second with a probability that increases with the difference between their payoffs. Under this kind of dynamics, if the rate of mutation is sufficiently low, the population will always move to “fixation,” i.e., a state in which every agent employs the same strategy. The time to reach fixation increases with population size and, if every strategy gets the same payoff against every other, then the odds of strategy \(\bs\) becoming fixed are proportional to the fraction of the population that employs \(\bs\). These features correspond to familiar properties in population genetics but they are not true of, for example, the replicator dynamics. 