Host-parasite coevolution

Host-parasite coevolution is the canonical form of antagonistic coevolution, in which parasites evolve to exploit host vulnerabilities and hosts evolve to resist infection. The dynamic is asymmetric: parasites typically have shorter generation times, larger population sizes, and higher mutation rates, giving them an evolutionary advantage that hosts must offset through sexual reproduction, immune diversity, and behavioral defense. The system never reaches equilibrium because the parasite's fitness depends on the host's vulnerability, and the host's fitness depends on the parasite's virulence — a reciprocal trap that W.D. Hamilton identified as the primary engine maintaining genetic recombination in sexual populations. The coevolutionary pressure extends to the molecular level, producing the evolutionary arms race signatures visible in rapid sequence evolution at host-pathogen interfaces.