When two kinds of organisms exert natural selection on each other so they influence each other's evolution, they are undergoing coevolution. Any two organisms may exert selective pressure on each other. Herbivores exert selection on plants favoring the evolution of defenses, and plant defenses exert selection on herbivores to overcome them. Competitors exert selection on each other favoring superior competitive ability. Pollinating insects exert selection on flowering plants to provide attractants and rewards, and plants exert selection on pollinators for superior pollination service. This reciprocal natural selection is the core concept in coevolution. It may produce ongoing evolutionary "warfare," in which the participants constantly change their weapons or tools, or it may produce a relationship that benefits both participants. When the outcome is beneficial to both, it is called mutualism.

In 1964 entomologist Paul Ehrlich and botanist Peter Raven suggested that these reciprocal changes in physical, chemical, or behavioral...