Two distinct patterns of amino acid substitutions that produce different spatial arrangements of viruses.

(A) The straight arrangement of viruses. If a series of amino acid substitutions all occur in different positions, then the distance from an ancestor to a mutant should be proportional to the number of substitutions (B). These independent substitutions make a straight arrangement of viruses on the map. (C) The curved arrangement of viruses. If amino acids at particular positions were substituted more than once, the distance from an ancestor to a mutant should be less than the number of substitutions (D). These concentrated substitutions at the same amino acid positions make a curved arrangement of viruses. In both panels, viruses are represented by circles, with illustrative examples of their amino acid sequences and substitutions on them.