A duplication event could provide the conditions for the intermediate nonfunctional site to be neutral, which would allow a transition without affecting the fitness of the organism. (a) Without a duplication event, the loss of a negative charge could have deleterious effects if the charge is important for the function of the protein. (b) The redundant paralogous gene copy could serve as a backup and prevent deleterious effects created by the loss of the charge. The backup copy could then be retained or lost. In the latter case, the system would be different from its ancestor.