Synonymous substitutions do not change the amino acid sequence of a protein and usually have no phenotypic effect. Thus, not subjected to natural selection and are estimated to mount up freely. But in the case of nonsynonymous substitutions the amino sequence does change and possibly have a broad range of influence on phenotype and hence, subjected to natural selection.
Pseudogenes are DNA (deoxyribonucleic acid) sequences that look like functional genes, but are usually considered to have no function. Actually the pseudogenes are not anything more than genetic fossils of a previous period when the genes used to have some kind of essential function. In general, a pseudogene is a nonfunctional copy of an active gene. As nonfunctional, modifications in the pseudogene are not focus to natural selection and hence all amino acid substitutions are neutral. So, there must be no variation in the rate of synonymous and nonsynonymous substitutions in a globin pseudogene or in any pseudogene. Both mutations are predictable to occur at the same rate.