Nucleotide substitution type dependence of generation time effect of molecular evolution

Abstract

Using DNA sequence data of 18 genes from 14 mammals, we analyzed how the average molecular evolution rate per year per site (V_y) depends on the generation time (g). (I) Assuming the relation V_y ∝g^−α, the index of generation time effect, (α), was estimated to be about 0.14 for amino acid replacement substitutions (A), and about 0.32 for synonymous substitutions (S). (II) Assuming the relation V_y=V^m_g g^-1+V^e _y, where V^m_g and V^e _y are constant independent of g, the fraction, r^e ≡V^e_y/V_y, of the mutation rate independent part (V^e_y) in the total evolution rate (V_y) was estimated under the assumptions of the star phylogeny and the constancy of the mutation rate per generation. r^e was smallest for mouse with the shortest generation time among our analyzed species, and it was estimated to be about 0.57 for A and 0.31 for S. Both results do not support the view that V_y is equal to the neutral mutation rate per site both for A and for S. They are in line with the thesis that, at least for A and probably even for S, the molecular evolution rate is influenced by some causes other than the mutation rate, such as changing environment.