[edsheppa]

It is a statistical thing, not a selection thing. Neutral traits can become fixed purely by chance.

[Nebullis]

Also, note that the same kind of neutral mutations can take on larger scales. Proteins have active sites and sites which can change without making a functional difference. As soon as a critical amino acid changes (by chance), the whole structure of the protein can flip around, incorporating the neutral mutations which now contribute to the new structure. Darwin spoke of larger scales. Legs that grow longer, necks that grow taller, all have skin which has grow along with it but is not itself the feature selected for.