"That's not all: the structure of the library [genetic code] makes it easy for evolution to move from one meaningful book [genetic sequence] to another. When Wagner and his colleagues tried browsing adjacent "books" – proteins that differ by a single amino acid – they found that most worked just as well as the original. The same was true when they changed another amino acid, and another. In fact, you could move, step by step, from one end of the library to the other without changing the meaning.
This allows populations to accumulate a lot of genetic variation while still remaining viable. In Wagner's metaphor, readers spread into many different rooms of the library. And that's where the big pay-off comes. By wandering far afield, you come to rooms with very different sorts of books nearby. In real terms, you end up in places where changing just a few more amino acids gives you a protein with a radically different function – an evolutionary breakthrough, close at hand."