When is it coevolution? Daniel H. Janzen Coevolution is an evolutionary change in a trait of individuals in one population in response to a trait of individuals in a second population, followed by an evolutionary response by the second population. Diffuse coevolution occurs when either or both populations are represented by an array of populations that generate a selective pressure as a group. Ehrlich and Raven's (1964) paper on butterfly-plant interactions was the first to focus on coevolution, but they did not define it. The term has been misused in evolutionary thought due to lack of a clear definition, an inappropriate example, and the common occurrence of coevolutionary events in plant-animal interactions. Three major misuses of the term coevolution are: 1) Mutualistic congruence between species is not necessarily coevolution. For example, mammal-dispersed seeds may coevolve with mammal dietary needs, but the mammal may have entered the plant's habitat with established dietary preferences. 2) A herbivore parasitic on a plant is often thought to be coevolved with the plant's defenses, but the parasite may have arrived in a new habitat and adapted to circumvent defenses without evolving in response to the host. 3) When a parasite evolves traits to circumvent a host's defenses, it is often assumed coevolution has occurred. However, the defense trait may not have evolved in response to the parasite. In conclusion, the term 'coevolution' should not be confused with 'interaction,' 'symbiosis,' 'mutualism,' or 'animal-plant interaction.' A bee is not necessarily coevolved with the flower it pollinates, a caterpillar is not necessarily coevolved with its host plant, and a bruchid beetle is not necessarily coevolved with the legume seeds it preys on.When is it coevolution? Daniel H. Janzen Coevolution is an evolutionary change in a trait of individuals in one population in response to a trait of individuals in a second population, followed by an evolutionary response by the second population. Diffuse coevolution occurs when either or both populations are represented by an array of populations that generate a selective pressure as a group. Ehrlich and Raven's (1964) paper on butterfly-plant interactions was the first to focus on coevolution, but they did not define it. The term has been misused in evolutionary thought due to lack of a clear definition, an inappropriate example, and the common occurrence of coevolutionary events in plant-animal interactions. Three major misuses of the term coevolution are: 1) Mutualistic congruence between species is not necessarily coevolution. For example, mammal-dispersed seeds may coevolve with mammal dietary needs, but the mammal may have entered the plant's habitat with established dietary preferences. 2) A herbivore parasitic on a plant is often thought to be coevolved with the plant's defenses, but the parasite may have arrived in a new habitat and adapted to circumvent defenses without evolving in response to the host. 3) When a parasite evolves traits to circumvent a host's defenses, it is often assumed coevolution has occurred. However, the defense trait may not have evolved in response to the parasite. In conclusion, the term 'coevolution' should not be confused with 'interaction,' 'symbiosis,' 'mutualism,' or 'animal-plant interaction.' A bee is not necessarily coevolved with the flower it pollinates, a caterpillar is not necessarily coevolved with its host plant, and a bruchid beetle is not necessarily coevolved with the legume seeds it preys on.