near neutrality & compensatory evolution
Ohta T 1995 Synonymous and nonsynonymous substitutions in mammalian genes and the nearly neutral theory. J Mol Evol 40:56-63.
- the slightly deleterious mutation theory predicted a negative correlation between amino acid substitutions and the population size of the species
- under the selection model of Gillespie (1991), the episodic process is caused by environmental changes
- under the nearly neutral model (Ohta 1992), it is caused by the interplay between fluctuation of population size and weak selection
- while the population size is small, slightly deleterious amino acid substitutions, that very slightly disturb the protein structure and function, may occur
- the compensatory substitutions coming from the higher-order structure of proteins follow
- resulting in the episodic pattern of substitution
- the two hypotheses are indistinguishable here
- negative selection has a major effect on amino acid substitutions and that both random genetic drift and selection are important
- i.e., slightly deleterious mutation theory
- this theory never predicts that the gene system constantly deteriorates
- from the moment the theory was proposed, the following features were emphasized (Ohta 1973)
- (1) the species population size fluctuates and the effectiveness of selection accordingly varies
- (2) compensatory mutations may be common at the molecular level if one considers higher-order structures of proteins and nucleic acids
- this aspect seems to be different from the selectionists'
- e.g., Gillespie's view (Gillespie 1991)
