4. How might similarities and differences in genetic codes, or the proteins built as a result of these codes, be used to determine how closely related different species are?
The universality of the genetic code is generally regarded by biologists as definitive evidence in favor of the theory of universal common descent. Proteins with the same three-dimensional structure need not have identical amino acid sequences; any irrelevant similarity between the sequences is evidence for common descent. In certain cases, there are several codons (DNA triplets) that code for the same amino acid. Thus, if two species use the same codon at the same place to specify an amino acid that can be represented by more than one codon, that is evidence for a recent common ancestor.