Coevolution

Coevolution happens when species are so closely tied together that each acts as a source of natural selection on the other. A change in one, such as a new chemical defense in a plant, favors a matching change in the other, such as a way for an insect to tolerate it, which in turn favors a further change in the first. The partners can be predator and prey, parasite and host, competitors, or mutualists that both benefit.

Antagonistic partners often become locked in an evolutionary arms race. Some garter snakes have evolved resistance to the powerful nerve toxin of the rough-skinned newts they eat, and newts in the same areas tend to carry more toxin. Bats and moths are another case: some moths jam bat sonar with ultrasonic clicks, and some bats hunt with calls pitched where moths cannot hear them. Mutualists, by contrast, can become finely matched. Fig trees and their tiny pollinating wasps depend entirely on each other, and many flowers have shapes, colors and scents suited to particular bees, birds or bats.

Darwin wrote about plants and insects shaping each other, but the modern idea grew from a 1964 study of butterflies and their food plants by Paul Ehrlich and Peter Raven. Hosts and the microbes that infect them also coevolve, one reason pathogens keep changing.