neutrality
Takahata N 2007 Molecular clock: an anti-neo-Darwinian legacy. Genetics 176:1-6.
- the high evolutionary rate estimated from hemoglobin and other proteins was a key to development of the neutral theory by Motoo Kimura
- rather than, as sometimes asserted, the large extent of electrophoretically observed polymorphism
- indeed, Kimura's 1968 article begins with a discussion of the amino acid substitution rate obtained from Zuckerkandl and Pauling (1965)
- although Kimura (1969) was impressed by the remarkable rate constancy in protein clocks, he nevertheless regarded this only as additional support for his neutral theory
- two years later, however, considering the large amount of noncoding DNA in the vertebrate genome and redundancy in the genetic code, he changed his mind and agreed that the constant rate, rather than the high rate, in amino acid or nucleotide substitutions is the strongest evidence for neutrality
- despite all this progress in molecular evolution, Kimura remained skeptical of his theory until the high rate of substitutions at synonymous sites (Kimura 1977) and in pseudogenes (Li et al. 1981; Miyata and Yasunaga 1981) was actually observed
- consistent with their having little or no apparent selective constraint
- Kimura thought that phenotypic evolution is based on neutral variation in coding regions that may be selected for when the environment changes
- unlike neo-Darwinism, which regards mutation as merely raw material and natural selection as the creative power, Nei's mutationism assumes that the most fundamental process for adaptive evolution is the production of functionally more efficient genotypes by mutation (especially birth and death of duplicated genes) and by recombination
- since a sufficiently large amount of genetic variation is presumably not maintained in a population or species, an inevitable fate of organisms under drastically changing environments is extinction unless indispensable mutations arise and are incorporated into the species in time
- in this view, Nei considers the basic process of phenotypic evolution to be essentially the same as that of molecular evolution and the extent of neutral genetic changes of phenotypic characters to be as great as that of protein variation
