How are rRNA genes used to deduce evolutionary relationships?

All genes experience mutations (changes in their nucleotide sequences) over time. Ribosomal RNA (rRNA) genes mutate slowly and at a fairly constant and predictable rate. By comparing the nucleotide sequence in rRNA genes of any two species, scientists can deduce how long ago the two species shared a common ancestor. Only after the common ancestor split into separate lineages will rRNA mutations accumulate independently in different lineages. Consequently, many differences in the rRNA genes between two species indicate that they diverged long ago. Fewer differences indicate a more recent common ancestor, meaning that the organisms being compared are more closely related.