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