near neutrality & compensatory evolution

Ohta T 1973 Slightly deleterious mutant substitutions in evolution. Nature 252:351-354.

  • the concept of neutral mutant substitution in the population by random genetic drift can be extended to include random fixation of very slightly deleterious mutations which have more chance of being selected against than of being selected for3,4
  • if this class of mutant substitution is important, we can predict that the evolution is rapid in small populations or at the time of speciation5
  • Fitch and Margoliash6 have shown that the frequency of double fixation of mutants in a leg of their cytochrome c phylogenetic tree is higher than predicted from chance
  • the most plausible interpretation is that the first mutant is slightly deleterious and that the second compensates for the first
  • this interpretation makes Fitch's concept of the covarion7 most easily understood
  • if the first mutant disturbs very slightly the normal function of the molecule, there are probably many ways of compensating, and so the first mutant provides a new possibility for evolutionary change
  • this is considered to correspond to Fitch's observation of unstable covarions7
  • the evolution of tRNA follows the same pattern
  • Yamazaki and Maruyama14 made an extensive compilation and statistical analysis of published data on enzyme polymorphisms
  • the average ratio of the effective to the actual number of alleles (ne / na) from the observed enzyme polymorphisms agrees well with the neutral hypothesis according to Kimura and Crow's infinite allele model
  • Maynard Smith16 and Kimura and Ohta17 have suggested that the model of stepwise allele production is more appropriate
  • (ne / na) is larger in the step allele model, particularly when na is large
  • then the agreement between the observation and Kimura and Crow's model shows that there exist more rare alleles than expected from the strict neutral hypothesis
  • I conclude that rare alleles are on the average selected against
  • the alleles, which can increase their frequency up to several per cent in very large populations like Drosophila species, can replace the previous allele in small populations by random genetic drift
  • King and Jukes2 noticed that lack of vitamin C is always associated with the kind of food
  • they consider that, when selective constraints are reduced, the genes for vitamin C have deteriorated
  • I suggest that very slight genetic deterioration might play an important role in molecular evolution
  • organization at a much lower level in each molecule or in several interacting molecules could provide a clue to understanding the evolutionary change at the molecular level
  • rather than cohesive interaction of the pool often postulated by orthodox neo-Darwinism21
  • there is no evidence that alleles at different loci constitute such interaction systems or "super genes"