English: Evolving host-parasite interaction network. Starting from a host phenotype (green node) and a parasite phenotype (red node), a complex network of interactions (arrows) between hosts and parasites emerge from the coevolutionary process. New nodes representing different host and parasite phenotypes appear and disappear over evolutionary time. The abundance of individuals expressing each phenotype changes continuously (indicated by the size of the nodes) altering the pattern of interactions, and thus influencing the subsequent coevolutionary dynamics. Each interaction between a host phenotype and a parasite phenotype is depicted as two arrows pointing in opposite directions: the thickness of the red arrows depict the proportion of infections a particular parasite is responsible for inflicting on the indicated host, while the green arrows depict the proportion of hosts infected by a particular parasite accounted for by the indicated host. Sometimes asymmetry between the thicknesses of arrow-pairs lead to red arrows dominating the picture: most parasites expressing a given phenotype are infecting a small fraction of hosts expressing a given phenotype, while the majority of those hosts are being infected by parasites with other phenotypes.