Red Queen model of host-parasite coevolution. a Schematic of oscillations through time in the frequency of a host genotype (black) and its matching parasite genotype (gray). Host genotypes decline when common and increase when rare due to negative frequency-dependent selection exerted by the coevolving parasite population. The time-lag in the parasite genotype frequency reflects the period required for the parasite population to adapt to the changing host population. b Diagram of the matching-alleles model, as implemented in the game. Successful infection (+) results when the parasite genotype, represented here by a single allele or card suit, matches the host genotype. The host resists infection (−) when the host and parasite allele are mismatched.