Fig. 3
A schematic depiction of genetic assimilation through degeneracy for RNA. The vertical lines represent the energy scale in the Boltzmann ensemble of individual sequences. This means that the higher up a structure is, the higher its free energy and therefore the lower its Boltzmann probability. Thus the lowest structure is the MFE structure of the sequence. The label NS denotes the neutral network of S. A nondegenerate RNA sequence (left) which has structure S at MFE level is forced by selective pressure to neutrally (cryptically) evolve into a degenerate sequence folding to two RNA structures S,S′ (center). Further selection causes the sequence to enter the neutral network of S′ and discard its original phenotype S. This way the acquired phenotypic character S′ becomes inherent to the genotype