Preponderance of synonymous changes as evidence for the neutral theory of molecular evolution
National Institute of Genetics, Mishima, 411, Japan
ACCORDING to the neutral mutation–random drift hypothesis of molecular evolution and polymorphism1,2, most mutant substitutions detected through comparative studies of homologous proteins (and the nucleotide sequences) are the results of random fixation of selectively neutral or nearly neutral mutations. This is in sharp contrast to the orthodox neo-Darwinian view that practically all mutant substitutions occurring within species in the course of evolution are caused by positive Darwinian selection3–5. This paper shows that by comparative studies of messenger RNA (mRNA) sequences reliable estimates can be obtained of the evolutionary rates (in terms of mutant substitutions) at the third positions of the codon, and that the estimates conform remarkably well with the framework of the neutral theory.
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