Final answer:
The observation relates to the neutral theory of evolution, which asserts that most genetic changes are neutral and not subject to selection. In pseudogenes, both silent and replacement mutations accumulate at the same rate as these genes are not under selection pressure, supporting the neutral theory.
Step-by-step explanation:
The observation that the rate of silent and replacement changes is identical in pseudogenes can be explained by the neutral theory of evolution. According to this theory, most evolutionary changes at the molecular level are not driven by natural selection, but by genetic drift of neutral mutations. A pseudogene, which is a gene that has lost its function due to mutations, is not subject to natural selection, so any changes, whether they are silent or replacement substitutions, accumulate at the same rate, which is typically the background rate of mutation.
A silent mutation does not alter the amino acid sequence of a protein because the new codon still codes for the same amino acid due to the redundancy of the genetic code. Contrastingly, a substitution mutation, also known as a missense mutation, results in an altered amino acid sequence, which can affect the protein's function. However, in the case of pseudogenes, since the gene is not functional and not under selection pressure, both types of mutations accumulate at a similar rate.