The three components of the geographic mosaic of coevolution. (a) Populations of interacting species are distributed in a spatial mosaic, with the strength of coevolutionary selection exerted by each partner on the other varying between populations. In cold spots (here represented by light-colored tiles), the traits of each species evolve independently, whereas in hot spots (dark tiles) coevolutionary selection is intense. (b) As well as varying in strength, the direction of selection varies spatially (there is a selection mosaic; represented here as different colored tiles), depending on the interactions between the genotypes of both interacting species and the local environment. (c) There is some mixing of genes due to the dispersal of individuals between populations (represented as the individual dots making up the shaded areas). The level of mixing must be sufficient to allow the occasional introduction of new genotypes into populations, but low enough that adaptations are not swamped by gene flow from populations experiencing different selection pressures or strengths. (d) The combination of all three elements leads to a system in which coevolution is a continuous dynamic process that, at the same time, retains ample genetic variation to allow long-term coevolution.