Determine similarity in percentages between species A and B, A and C, and B and C

Question

The chart above is a graphic that shows the amino acid sequence differences between different organisms for a protein keratin.

The question I am required to answer begins as, "Keratin is made up of about 450 amino acids. Determine the similarity in percentages between species A and B, A and C, and B and C. Explain how amino acid sequences can be used in determining how closely related two species are."

My initial solution to finding the percentages is dividing the respective differences in the chart above by 450. Then, subtract that number by 100, and hence, find the similarity percentage. For example, for species A and B, I would divide 4 by 450, resulting in 0.89%. I would next subtract this percentage from 100%, resulting in 99.11% similarity. Is this the correct procedure?

My initial solution to explaining how amino acid sequences can be used to determine how closely related two species are is as follows: The larger the difference in amino acid sequences between two species, the less closely related two species are. Is this explanation correct as well?

Answer 1

Step number one is correct, but note that there are several similarity (or alternatively distance) measures. What you choose is the simplest and most intuitive one and it a good one without any other assumption or explanation given.

Step number two is also correct, if the similarity is a good proxy for when the last common ancestor between two species is in time, which in this case should be.