An adaptation is a feature that arose and was favored by natural selection for its current function. Adaptations help an organism survive and/or reproduce in its current environment.

Adaptations can take many forms: a behavior that allows better evasion of predators, a protein that functions better at body temperature, or an anatomical feature that allows the organism to access a valuable new resource — all of these might be adaptations. Many of the things that impress us most in nature are thought to be adaptations.

Mimicry of leaves by insects is an adaptation for evading predators. This example is a katydid from Costa Rica.

The creosote bush is a desert-dwelling plant that produces toxins that prevent other plants from growing nearby, thus reducing competition for nutrients and water.

Echolocation in bats is an adaptation for catching insects.

So what’s not an adaptation? The answer: a lot of things. One example is vestigial structures. A vestigial structure is a feature that was adaptive for the organism’s ancestor, but that evolved to be non-functional because the organism’s environment changed.

Fish species that live in completely dark caves have vestigial, non-functional eyes. When their sighted ancestors ended up living in caves, there was no longer any natural selection that maintained the function of the fishes’ eyes. So fish with better sight no longer out-competed fish with worse sight. Today, these fish still have eyes — but they are not functional and are not an adaptation; they are just the by-products of the fishes’ evolutionary history.