The different grades of co-evolution

Ehrlich and Raven first introduced the term co-evolution in 1964 in a paper on the relationship between butterflies and the host food plants of the larval stage. They were concerned in particular with the ‘patterns of interaction between two major groups of organism with a close and evident ecological relationship’ (p. 586). Within the paper they also introduced the term community evolution: ‘evolutionary interactions found among different kinds of organisms where exchange of genetic information among the kinds is assumed to be minimal or absent’ (p. 586). Their definition of community evolution is broad, and they seem to consider co-evolution as a smaller subset of community evolution, but their application of the term has been interpreted in a number of ways by ecologists since then.

The most restricted use of the term co-evolution is applied to the evolutionary relationships between two species which are, ecologically, very closely associated. This is often called pairwise or one-on-one co-evolution. Usually this implies that as one species evolves particular characteristics, the other species also evolves in response to this; the first species then evolves in response to the response of the second, and so on. Examples of pairwise co-evolution include ant species living on acacia trees (see Section 19.2.2), butterfly caterpillars and their host plants, and bees with the plants they pollinate (see Section 19.4.3).