An exaptation is a trait that resulted from selection for something other than the trait's current function. Think about feathers. Scientists know from the fossil record that the earliest creatures with feathers didn't fly. So what was the purpose of feathers on these early flightless animals? Maybe they served as insulation (as feathers today do), or maybe they served some other function. Regardless of their actual purpose, what scientists do know is that the feathers didn't have anything to do with flight — which they can tell from the skeletal structure of the fossils (these creatures didn't have wings!).
So although feathers subsequently evolved to be used in flight, the benefit of flight wasn't the selective force responsible for their origin. It just so happened that feathers — an adaptation selected by some fitness advantage unrelated to flight — subsequently became something that natural selection shaped into a wing.
Occasionally, you hear the term preadaptation, which means the same thing as exaptation. The problem with preadaptation is that folks tend to misinterpret the word, thinking that it implies premeditation in the process of selection. But selection isn't premeditated, of course, so the preferred term is exaptation. Truth to tell, however, most evolutionary biologists are likely to say preadaptation instead of exaptation when they're sitting around talking science over a few beers. But that's when happens when scientists get sloppy drunk.
One of the beauties of the evolutionary process is the way that existing structures are altered for new uses. For that reason, scientists always need to be cautious with any specific example in which they state that a particular trait evolved for a particular reason. Why? If you're trying to understand the process of natural selection, you need to make sure that the traits you're examining were actually the result of natural selection.
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