Some people have trouble understanding the fact that eukaryotic genomes are made up primarily of transposable elements — genetic parasites that are there (at least initially) because they are good at being there. Some take on functions, but others remain causes of disease or simply become inactivated and remain as molecular fossils.

Here is one of those pictures that circulates around in emails that I find amusing, but which also conveys something about the concept of transposable elements in the genome (click for larger image):

Basically, the sign appears to exist largely for the sake of existing. But note that a small portion of it at the bottom has a critical function (“Also, the bridge is out ahead”).

It’s a weak analogy, maybe, but that is a pretty funny sign.