Coevolution

Coevolution occurs when two or more species reciprocally affect each other's evolution through natural selection, each exerting selective pressure that shapes how the other changes over time. Charles Darwin first raised the idea in On the Origin of Species in 1859, suggesting how flowering plants and insects might have shaped each other, and the field grew substantially from the 1960s onward, including Daniel Janzen's work on the coevolution of acacias and the ants that defend them. Insect-pollinated flowers are coadapted with their pollinators, rewarding bees, butterflies, moths, and beetles with nectar and pollen in exchange for pollination, a relationship that developed alongside flowering plants during the Cretaceous; hummingbirds and the flowers that depend on them show a similar match of nectar, color, and shape to the birds' vision and bills, and each of roughly 800 fig species depends on its own single species of fig wasp for pollination. This reciprocal pressure often produces an evolutionary arms race, in which an adaptation in one species prompts a counter-adaptation in the other in an ongoing cycle.