When two separate species evolve independently but in step with each other, it is known as coevolution. Coevolution occurs when two species have a close relationship, such as a predator-prey relationship or a symbiotic relationship. As one species evolves, the other species also evolves in response, in order to maintain their relationship. For example, if a predator species evolves to become faster, the prey species may also evolve to become faster in order to avoid being caught. Over time, both species will evolve in step with each other in order to maintain their relationship.