An adaptation is a feature that is common in a population
because it provides some improved function. Adaptations are well fitted to
their function and are produced by natural selection.
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 an
adaptation 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.
In fact, biologists have a lot to say about what is and is not an