Answer: Directional selection occurs when individuals with traits on one side of the mean in their population survive better or reproduce more than those on the other. ... Directional selection does the “heavy lifting” of evolution by tending to move the trait mean toward the optimum for the environment.
Following the requirement of question I have attached a picture of phylogenetic tree.
Here is the source publication PNAS June 26, 2012 109 (26) 10293-10297
Answer:
B. they assume the role od bringing 2 amino acids together