• Coevolution occurs when two or more lineages reciprocally influence each other's evolution. Coadaptation between species, such as in any example of mutualism, is probably but not necessarily the result of coevolution.

• Insects and plants influence each other's evolution, by the evolution of insecticides in plants and of detoxification and avoidance mechanisms in insects. It has been argued that the evolutionary relations between insects and plants less often take the form of coevolution than of sequential evolution, in which plant evolution influences insect evolution but not vice versa.

• The level of virulence of parasites can evolutionarily decrease or increase. It can be understood in terms of the parasite-host relationship: two factors that influence it are kin selection and the mode of transmission of the parasite between hosts.

• Coevolutionary “arms races” between predators and prey produce escalatory long-term evolutionary trends; they can be seen in the evolution of brain sizes in mammals and of armour and weapons in molluscs and their predators.

• Coevolution within groups of competing species need not have the Red Queen mode. Rates of change, and relative competitive abilities, need not be constant.