A Simple Test for the Possible Simultaneous Evolutionary Divergence of Two Amino Acid Positions

Summary A taxonomic character for a group of organisms under study is said to be divergent or uniquely derived for that group if, during the evolution of that group, each character state arose only once. Homologous amino acid positions in homologous proteins can be thought of as taxonomic characters whose states are amino acids. In a similar way, DNA nucleotide positions can also be considered taxonomic characters. In the exposition below, we describe a simple test to determine when it is not possible for two amino acid (or nucleotide) positions to be divergent at the same time.