Evolutionary Rate

 In the studies of molecular evolution, the rates of nucleotide or amino acid substitutions (per site per year) are usually used as the evolutionary rate. Compare two nucleotide sequences, A and B, as an example. Denote K_n as the number of nucleotide substitutions between sequences A and B. When the ancestral node, O, of the two sequences A and B is considered, it follows that nucleotide substitutions should have accumulated during the time of lineages OA and OB (Figure 1). If the divergence time between sequences A and B, which is measured by years, is denoted T, K_n is the number of substitutions that have taken place during 2T. This is because the divergence time of OA plus OB should be equal to 2T. Thus, the rate of nucleotide substitution (v_n( can be computed by the formula v_n=K_n /(2T). The rate of amino acid substitution (v_a) can be obtained in a similar way: v_a=K_a /(2T). Note that the number of nucleotide and amino acid substitutions can be estimated by a number of different methods.

Figure 1. Estimating the rate of evolutionary change in nucleotide or amino acid sequences from the difference between two present-day homologous sequences. Rate of nucleotide substitution: v_n=K_n / (2T); rate of amino acid substitution: v_a=K_a /(2T) .