Concepts of coevolution

The word coevolution has become a standard part of the lexicon of evolutionary biology. More than 1000 papers during the past decade have used coevolution in the title or abstract. Hundreds more have used the word in passing in the body of the paper. A half dozen books now include coevolution in the title. As usage of coevolution has increased, so have the views on the processes and mechanisms of coevolutionary change. The problem now is to understand the ecological and genetic conditions that favor different modes and outcomes of coevolution.