This process can lead to pseudogenes - unused copies of a gene that can mutate and change without effect. Since the main copy is maintaining the function, there is less selection pressure on the pseudogene. Although this might seem to imply that with no pressure to evolve the gene is going nowhere fast, that is to misunderstand evolution. Drift allows 'bad' mutation as well as 'good' - accumulating potential structural or functional differences in the process. If the pseudogene is 'reactivated' by attaching a promoter then those changes that have benefited the protein can be selected for and defects can be selected against.

See: orthologs and paralogs