Methane-producing archaeons in the rumens of cows and bioluminescent bacteria within the bodies of squid are both examples of _____, where organisms have "evolved together."

Coevolution is a form of evolution, which involves two or more species interacting together in which each species’ evolution causes a reciprocal effect on the evolution of the other through natural selection. Both species evolved together, thereby forming a kind of symbiotic relationship.

Methane-producing archaeons in the rumens of cows and the bioluminescent bacteria within the bodies of squid are examples of mutualistic relationships in coevolution that are formed as different organism species evolved together.