Buffalo graze on the grasses of the North American Plains, wild Yaks graze on grass and Northern Asia. How do Darwin's categories of species variation apply to this comparison?

You are studying a protein that you call protein x. there is an aspartic acid at a key position in protein x which is important in the folding and stabilization of that protein. if this aspartic acid is changed to a different amino acid, which type of amino acid substitutions is most likely to allow the protein to fold normally?