RATE OF FIXATION OF NUCLEOTIDE SUBSTITUTIONS IN EVOLUTION

Nucleotide sequences of rodents and of artiodactyls have been compared by DNA/DNA duplex formation. The rate of evolution in terms of fixation of nucleotide substitutions inferred from non-repeated artiodactyl DNAs is three times greater than that inferred from comparative amino-acid sequence of the α and β chains of haemoglobin from these same organisms. The initial rate of nucleotide sequence variation among rodents is ten-fold higher than that among artiodactyls when divergence time is estimated in years. This difference diminishes if generations, rather than years, represent the appropriate interval of evolutionary divergence. © 1969 Nature Publishing Group.