Species are said to "coevolve" when their respective levels of fitness depend not only on their own genetic structure and adaptations but also the development of another species as well. The gene pool of one species creates selection pressure for another species. Although the changes are generally reciprocal, they may also be unilateral and still be considered coevolutionary.

The process of coevolution arises from interactions that establish structure in communities. A variety of different types of interactions can occur–symbiotic, where neither member suffers, or parasitic, predatory, and competitive relationships, where one member of a species pair suffers.

Coevolution can result from mutually positive selection pressure. For example, certain plants have in an evolutionary sense created positive situations for insects by providing valuable food sources for them. In return the insects provide a means to distribute pollen that is more efficient than the distribution of pollen by wind...