Figure 1. Gene-for-gene models of host–parasite coevolution. The haploid host has two alleles, A₁ and A₂, with frequencies of p and 1 – p, and the haploid parasite has two alleles, B₁ and B₂, with frequencies r and 1 – r. A₁ hosts are resistant to B₂, but susceptible to B₁ parasites, whereas A₂ hosts are resistant to B₁ parasites and susceptible to B₂ parasites. The arrows show the directions of evolutionary changes of host and parasite allelic frequencies. (a) When the parasite's genotypes are not highly specific, the result is a stable polymorphism. (b) When the parasite's genotypes are more specific, then the result is a cycle of fluctuating allelic frequencies. (c) When parasites do not evolve within their hosts, then this increases the amplitude of the fluctuations, evolving to a succession of states (moving from genetic fixation from one corner of the state space to the next). Adapted from Clarke (1976) and Maynard Smith (1989).

Figure 2. A computer simulation of host–parasite coevolution. (a) The diploid host has three resistance alleles at a locus (a₁, a₂, and a₃), and (b) the haploid parasite has six alleles (b₁ – b₆). Each parasite strain is able to evade the defences of one of the host's genotypes (e.g. b₁ can infect host a₁a₂). From Seger (1992).

Figure 3. The myxoma virus evolved from high to intermediate virulence over time (the range of virulence is the shaded area).

Figure 4. Serial passage experiments typically show that virulence evolves and increases over time when a parasite is artificially transferred from one host to the next. From Ebert.(1998)

Figure 5. Brood parasites, such as the cuckoo (Cuculus canorus), (a) eject one of their host's eggs from the nest and (b) replace it with an egg (bottom) that mimics the appearance of the host's eggs (top). (c) The parasitic young matures rapidly and ejects the host's eggs, and exploits its host's parental behaviour with vocal trickery using calls that sound remarkably like the whole brood of the host's chicks. (d) In a species in which the parasitic chick (left) does not eject its nestmates (right), it has evolved mouthparts that mimic the young of the host. From Krebs and Davies (1987).

Figure 6. The coevolutionary diversification of pocket gopher species and their parasitic chewing lice. Coexisting hosts and lice parasites (connected by dotted lines) are often congruent, but not always. The evolutionary congruencies indicate that cospeciation has occurred, whereas incongruencies indicate host switching. C, Cratogeomys; G, Geomys; O, Orthogeomys; P, Pappogeomys; T, Thomomys and Z, Zygogeomys.