Host–parasite coevolution

Host–parasite coevolution is reciprocal evolutionary change in which adaptations in hosts alter selection on parasites, and parasite adaptations alter selection on hosts.

Negative frequency-dependent selection: Selection that favors a type when it is rare, reducing its advantage as it becomes common. Rare host genotypes can escape common parasites, while parasite genotypes gain from targeting common hosts.
Natural selection: A change in inherited trait frequencies caused by differences in survival or reproduction. Host and parasite adaptations arise when heritable variants differ in reproductive success.
New Zealand snail–trematode system: A freshwater snail–trematode system used to study local adaptation and reciprocal evolutionary change. Its geographic comparisons test whether parasites perform best on local host populations.
Balancing selection: Selection that maintains multiple genetic variants in a population over time. Changing parasite pressures can preserve host alleles that would otherwise be lost.
Evolutionary arms race: Reciprocal adaptation in which each interacting lineage evolves traits that counter the other's defenses or attacks. It remains difficult to distinguish escalating arms races from cycles or one-sided adaptation.
Gene-for-gene relationship: A genetic model in which host resistance alleles correspond to parasite alleles that determine whether infection succeeds. This model makes reciprocal host resistance and parasite infectivity explicit at the genetic level.
Parasite: An organism that lives on or in another organism and gains resources at its expense. The coevolutionary relationship depends on parasites exploiting hosts and affecting their fitness.
Daphnia–Pasteuria system: A freshwater crustacean and bacterial parasite system studied for genotype-specific infection and resistance. Experiments reveal changing infection success among host and parasite genotypes.
Local adaptation: A population's greater fitness in its home environment than populations from elsewhere achieve there. Host and parasite populations may adapt differently across geographic locations.
Coevolutionary lag: A delay between evolutionary change in one interacting population and the corresponding response in another. The magnitude of lag can determine whether parasites track host defenses or hosts escape them.

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