Coevolution describes two interacting species evolving in response to each other’s adaptations.
The interaction can involve mutual benefit, competition, predation, or parasitism. For example, flowering plants and their pollinators may influence each other’s traits: plants can evolve floral shapes that favor particular pollinators, while pollinators can evolve body structures or behaviors that improve access to nectar.
Predator and prey species can also engage in an evolutionary arms race. Better defenses in prey may favor improved hunting abilities in predators, which can then favor still stronger defenses. These changes are not necessarily perfectly matched, and coevolution does not mean that every interacting species evolves in lockstep.
The term was introduced into evolutionary biology in the twentieth century and is applied at both species and gene-by-gene levels. Evidence requires a reciprocal evolutionary influence, not merely interaction.