THE divergence of amino acid sequences which has accompanied evolution of species may be largely due to random fixation of selectively neutral or nearly neutral mutations. This was proposed by Kimura upon consideration of an inferred rapid rate of molecular evolution, the theory of the cost of natural selection, and observed high levels of polymorphism in natural populations1. Among the observations considered to favour the neutral mutation-random fixation theory1-4, the apparent uniformity of the rate of fixation of amino acid substitutions in homologous proteins along several phyletic lines is outstanding. Kimura proposed this test of the neutral mutation theory by examining the amino acid sequences of proteins from so-called 'living fossils'4. If, for a very long slowly evolving line, a rate of molecular evolution equal to that of more rapidly evolving lines is inferred (and it is assumed that the rate of evolution of physiological adaptations generally parallels the morphological evolutionary rate) then a case is made for the fixation of neutral mutations. The amino acid sequence proposed here 5 (Fig. 1) for the α chain of haemoglobin from a 'living fossil'6, Didelphis marsupialis, the Virginia opossum may be taken for such a test of the neutral mutation theory.
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