Jeremy B. Yoder 

Coevolution is widely defined as specific, simultaneous, reciprocal adaptation by pairs of interacting species. This strict-sense definition arose from a desire for conceptual clarity, but it has never reflected the much wider diversity of ways in which interacting species may shape each other's evolution. As a result, much of the literature on the evolutionary consequences of species interactions pays homage to the strict-sense definition while addressing some other form of coevolution. This tension suggests we should re-frame the key question in coevolution research from ''when is it coevolution?'' to, rather, ''how is it coevolution?''. This re-framing shifts our focus from identifying case studies for a single, narrowly defined process to describing the many ways — specific and diffuse, simultaneous and stepwise, adaptive and non-adaptive — in which species evolve together.