Definition
Coevolution is the process in which two closely entangled living things evolve in turn, each adapting to the other's changes. A change in one becomes a new survival pressure on the other, and the other's answer then pushes back on the first. Over time, their bodies and habits are shaped to fit together.
The Nectar Tube Got Deeper, So the Tongue Got Longer
Like a key in a lock, some living things in nature have bodies that fit together as a pair. One orchid in Madagascar has a nectar tube deeper than a hand is long. The nectar sits only at the very bottom.
When Darwin saw this orchid, he predicted that somewhere there must be an insect with an equally long tongue. Sure enough, a moth with an extremely long tongue was found later.
How did such a pair come about? A flower with a slightly deeper nectar tube pulls an insect's body in farther, so pollen is sure to stick to it. Then a moth with a slightly longer tongue can reach that nectar and get more food.
So a change on one side shifts the standard for the other side, and this repeats generation after generation. Each one has been carving the other, bit by bit.
An Arms Race of Chase and Escape
Coevolution doesn't only happen between friendly partners. It can happen even more fiercely between predator and prey.
Some amphibians carry a powerful poison in their skin. Yet some snakes living in the same area have developed bodies that can tolerate it. As resistant snakes become more common, individuals with stronger poison survive. Then even more resistant snakes become common again.
Biologists compare this to the Red Queen. In the story, the Red Queen had to keep running just to stay in the same place. Since the other side keeps getting faster too, the only way to get ahead is to run even faster.
Both creatures keep changing busily, but the balance between them usually doesn't tip very far. Each is running just to hold its ground. Still, in some places the snakes' resistance has gotten far ahead of the poison, and one side pulls well ahead.
A Little More Precisely
To be more precise, two living things changing at the same time is not automatically coevolution. If the climate changed and both species shifted side by side, they didn't put pressure on each other. A change in one has to actually affect the other's survival and reproduction.
Another thing to watch out for is explaining things by purpose. The moth did not stretch its tongue in order to drink nectar. From the start, tongue lengths varied a little. Longer-tongued moths left more offspring, so the group's average moved over time.
Remember the order: variation comes first, and selection follows. That helps you avoid the misunderstanding. Coevolution also happens not just between two species, but across whole groups of many species tangled together. In relationships like flowers and bees, where many partners have adjusted to one another, it often doesn't boil down to a neat pair.
Common misconceptions
Coevolution is the result of a moth stretching its own tongue to reach nectar.
An individual doesn't change its body to fit its needs. Among the many variations already present, those with longer tongues left more offspring, and over generations the group's average shifted.
Coevolution only happens between partners who help each other.
It happens actively between predators and prey, and between hosts and freeloaders too. Any pair that puts survival pressure on each other can coevolve, whatever the nature of their relationship.
Where you meet it
An orchid with a deep nectar tube and a moth whose tongue is nearly as long as that depth.
The yucca plant and the yucca moth, which lays eggs only in the yucca flower while carrying its pollen. The two fit together so tightly that it is hard for either to live without the other.
In one sentence
Coevolution is the process in which two closely linked living things are shaped in turn by each other's changes, until they fit together like a key and a lock.