Spandrels

From Wikipedia:

Spandrel is a term used in evolutionary biology describing a phenotypic characteristic that is considered to have developed during evolution as a side-effect of an adaptation, rather than arising from natural selection.

That this term exists is a huge relief to me, since I've so often found myself trying to describe a spandrel to someone without knowing the word for it. An example of a spandrel is the navel: The adult navel does not have a clear function, but instead appears to be a byproduct of the umbilical cord. In contrast, the umbilical cord evolved for a specific purpose, and is therefor not a spandrel.

The word spandrel has great potential to leak into more general usages in everyday language. Instead of calling someone a scoundrel or a sphincter, you could say something like, "You spandrel! Who needs you around?"

Corrected: Thanks to Dr. Perfection for pointing out that "ambilical" is not a word.