Revision as of 13:35, 15 September 2018

Citation

Kimura, M. (1968). Evolutionary rate at the molecular level. Nature, 217(5129), 624-626.

Links

• https://www.nature.com/articles/217624a0
• https://scholar.google.com/scholar?cluster=10145631598213248063

Notes

Paragraph Four

Kimura constructs an estimate of the rate of genome-wide nucleotide substitutions based on observed amino acid differences between species.

• Average years between amino acid substitutions in 100 amino acids [math]=28\times10^6[/math] yr (today this would be "a", Latin annus, symbolizing year).
• Genome size estimate [math]=4\times10^9[/math] bp (base pairs).
• Gene size coreesponding to 100 amino acids [math]=300[/math] bp.
• Adjustment to also include an estimated additional 20% synonymous mutations that do not change the amino acid [math]1 + 0.2 = 1.2[/math]

[math]28\times10^6 \div \left( \frac{4\times10^9}{300} \right) \div 1.2 \doteqdot 1.8 {yr}[/math]

The publication (I don't know if this comes from Kimura or the typesetter) uses an obscure symbol, [math]\doteqdot[/math], for approximately equal to, [math]\approx[/math].

Kimura argues that a fixation event every 1.8 years is too high of a rate to be explained by selection. Haldane (1957) referenced here is a useful publication to understand this argument. Essentially, this requires many overlapping simultaneous selective sweeps across the genome (the time from the occurrence of a new mutation to its fixation in the population is many generations). The action of selection is limited by an organisms fecundity. If all of the offspring with the most fit genotype survive and reproduce (as the most extreme example) the action of selection is seen in the reduction of survival and/or reproduction of organisms with other genotypes.

This appears to assume that the entire genome is protein coding DNA sequence.