ABSTRACT
The neutrality theory of molecular evolution predicts that the rate of molecular evolution is constant over time, and thus that there is a molecular clock that can be used for timing evolutionary events. Experimental data have shown that the variance of the rate of evolution is generally larger than expected according to the neutrality theory. This raises the question of how reliable the molecular clock is or, indeed, whether there is a molecular clock. We have carried out an extensive investigation of nine proteins in organisms belonging to the three multicellular kingdoms, namely ADH, AMD, DDC, GPDH, G6PD, PGD, SOD, TPI, and XDH. We observe that the nine proteins evolve erratically through time and across lineages. The observations are inconsistent with the neutrality theory and also with various subsidiary hypotheses proposed to account for the overdispersion of the molecular clock.
