I'm not sure how exactly you wanted this question to be answered. You're either talking about the symmetry where animals would have two legs or two arms for example and thus producing a pair of each muscle on both sides of the body, or you're refering to the development of agonistic and antaonistic muscles where each of them served a different purpose; either extending or contracting.
Answer:
I think that the beak of the finches will evolve over time depending on what kinds of food they eat. I made this hypothesis by reaching other birds of that nature.
Explanation:
The Neural Stem Cells. Hope this helped!
