Figure 1.—

Coefficients of selection for (a) the one-species model and (b) the two-species coevolutionary model. The values in each case give the fitness of the genotypes of the focal species i (indicated to the left). In the one-species model, selection varies through time because α(t) fluctuates (extrinsically imposed). In contrast, in the coevolutionary model, the α_i remain constant but selection fluctuates because of the temporal variation in the genotype frequencies of the coevolving species (b, top).