EVOLUTION: ON THE EVOLUTION OF HUMANS AND CHIMPANZEES: "What does the chimpanzee genome sequence tell us about the role of natural selection in human evolution? Purifying selection is clearly evidenced by the fact that mutations that alter the amino acid sequence, which in many cases presumably have a deleterious effect, have gone to fixation at a much lower rate than those that do not. Traditionally, this is expressed in terms of the ratio of non-synonymous (dN) to synonymous (dS) substitutions, dN/dS, where dS is here used as an index of the rate of unconstrained, neutral evolution. When dN/dS is less than 1, the usual interpretation is that negative selection has taken place on non-synonymous substitutions. When dN/dS is greater than 1, positive selection is likely to have accelerated the rate of fixation of non-synonymous substitutions. Note that purifying selection is the conservative force in molecular evolution, whereas positive selection is the diversifying force that drives molecular adaptation. dN/dS is estimated to be ~0.25, on average, for the human chimp comparison. In other words, about 75% of all amino acid replacements seem to be removed by purifying selection."


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