One evolutionary idea is that the number of amino acids differences in a similar protein from different species can be used as an evolutionary clock. The rate of amino acid substitution varies among proteins as shown in the following table:

Protein changes per protein per billion Year
Fibronectin 9.0
Phospholipase A2 1.9
Cytochrome b5 0.45
Lactate Dehidrogenase 0.34
Cytochrome c 0.22
Histone 4 0.01

a) what is the average rate of amino acid substitution among all the proteins shown?

b) on average, how long would it take to make one change in histone H4? would this serve as a good evolutionary clock?

c)In the time it takes for one change in cytochrome b5, how many changes would we expect to find in fibronectin?

d) In the two million years' time that horses have been differentiated from donkeys, how many amino acid substitutions would be found altogether in these six proteins?