Synonymous and nonsynonymous rates.
Encyclopedia of the Human Genome,
Cooper, D. N. et al., Editors.
Nature Publishing Group Reference, New York, NY, USA, 2003.
When comparing protein-coding DNA sequences between species, it is
useful to distinguish between synonymous and nonsynonymous
substitutions. Because the two types of changes are under different
intensities of selective constraints, comparing their rates provides
insights into the mechanisms of evolution of the gene under study.