Fig 1. A simple case of Constructive Neutral Evolution.

As Lambowitz and colleagues have shown [18], some strains of Neurospora crassa sport, in their mitochondrial rRNA genes, a group I intron which, because of its structure, is able to carry out a necessary interaction (self-splicing) without assistance of any protein. But in other strains, an unrelated protein fortuitously binds to and stabilizes the intron RNA. Destabilizing mutations in the RNA’s structure that would render it incapable of self-splicing without the bound protein are permitted ("pre-suppressed"). Such mutations are occasionally fixed by drift, and when more than one such mutation is possible, it is rare to reverse them all. Dependence on an assisting protein, initially a "pre-suppressor" of such mutations, is effectively locked-in.