Neutralism and selectionism: the molecular clock.

The neutrality theory predicts that the rate of molecular evolution will be constant over time, and thus that there is a molecular clock for timing evolutionary events. It has been observed that the variance of the rate of evolution is generally larger than expected according to the neutrality theory. Several modifications of the theory have been proposed to account for the 'overdispersion' of the molecular clock, by postulating effects attributed to generation-time, population size, slightly deleterious mutations, repair mechanisms, and the like. An extensive investigation of two proteins, glycerol-3-phosphate dehydrogenase (GPDH) and superoxide dismutase (SOD), manifests that none of these modifications can simultaneously account for the disparate patterns observed in both proteins. GPDH evolves very slowly in Drosophila species, but several times faster in mammals, other animals, plants, and fungi. SOD evolves very fast in Drosophila species and also in mammals, but much more slowly in other animals and still slower when plants and fungi are compared to one another, or to animals.