a. The nucleotide sequences of S. typhimurium and E. coli are 87% identical and the protein in both are 100% identical. It is because of the redundant triplet code. The changes in the Deoxyribonucleic acid (DNA) nucleotide sequence may not change the encoded protein. Especially changes in nucleotides coding for the third position of codon will not cause change in its encoded protein.
b. Protein sequences can be expected to evolve and diverge more slowly than the genes that encode them.
Due to the flexibility in the third positions of most codons, the DNA sequences can cause many changes without affecting the structure of the protein. As the amino acid changes cause the change in structure and function of the protein, the protein sequences will evolve more slowly.
In natural selection, many deleterious changes in amino acids get eliminated as the amino acid sequence is functionally constrained. So, the rate of amino acid sequence change is slow and the sequence is conserved greatly.