Red Queen Hypothesis

An evolutionary hypothesis that organisms must continually adapt and evolve not merely to gain advantage but to maintain relative fitness because interacting species (predators, parasites, competitors) are also evolving; hence, ongoing evolutionary change is required to avoid decline.

A coevolutionary proposition that, in systems with strong biotic interactions (particularly antagonistic interactions such as host–parasite or predator–prey), species must undergo continual adaptive change merely to maintain relative fitness because interacting species are also evolving.