Fig 2. General examples of CNE.

(a) Multifunctionality evolution through CNE. (1) α is a generic noncoding RNA that mediates RNA silencing on its target mRNA (red), and β is an enzyme that participates in cellular metabolism. (2) By chance, α and β may interact in the cellular environment creating an ephemeral complex. (3) Excess capacity in β results in the stabilization of the α–mRNA complex and then exerts a suppressive effect on mutations in α, which are now no longer deleterious [19,114]. (4) At this stage, α is reliant on interaction with β to exert its activity, while β has gained a new function in becoming a chaperone for α. (b) Increasing molecular complexity by CNE. (1) Mutations that inhibit the catalytic function of β (black arrow) or compromise α–β interactions (red arrow) are dangerous for the organism and would be eliminated by purifying selection. (2) In the case of gene duplication of β (represented by β1 and β2), there is now an excess capacity in the system that can exert a presuppressive activity [19,113]. (3) Mutations that compromise the stability of α–β interactions in β1 and the enzymatic activity in β2 are no longer deleterious since that function can be carried out by the other protein. (4) β1 and β2 can now evolve as 2 different proteins, without adaptive evolution. CNE, constructive neutral evolution.