Samuel Arbesman from the Harvard Institute for Quantitative Social Science has a fascinating look at the parallels between the way that text mutates over the years (say, because sloppy scholars copy a misquote, rather than referring to the original text -- something I admit to doing myself) and the way that genes change due to conceptually similar transcription errors. And just as mutations provide clues in unravelling the histories of species, so too can textual drift teach us about the histories of our ideas.
It's clear what a mutation is in genetics: a strand of DNA gets hit by a cosmic ray, or copied incorrectly, and some error gets introduced into the sequence. For example, an 'A' gets turned into a 'G', although they can be much larger in effect. These errors can range from causing no problem whatsoever (don't worry - the majority are like this), to causing large-scale issues due to the change in a single letter of DNA, such as in the case of sickle-cell anemia.
Well, there are also systematic errors in copying a text. Whether it's skipping a word or duplicating it, there is order to the ways in which a scribe's mind wanders during his transcription. Many of the errors can be grouped into categories of error, just like the different types of genetic mutations. And not only are there regularities to how both DNA and ancient manuscripts are copied, but it gets even better: despite the differences in terms, these types of errors are often identical.
For example, there is a scribal error known by the Greek term homeoteleuton. This refers to a type of deletion, where there are two similarly ending passages and the scribe skips to the second ending without transcribing the first intervening portion. For example, if a section read, "And you should do the following things because I am the Lord. Here's what you should do, because I am the Lord. Amen." and it was instead copied as "And you should do the following things because I am the Lord. Amen." that would be a homeoteleuton.