Evolutionary biologyRelativity for molecular clocks : Nature:
The relative constancy of the rate at which DNA sequences evolve has been a treasured icon of molecular evolution for nearly 40 years. The occurrence of such a stochastic 'molecular clock' was initially quite unexpected, and was explained by Motoo Kimura1 by assuming that most changes to amino-acid and nucleotide sequences were neutral — "neither beneficial nor injurious", in Charles Darwin's prescient phrase.
However, there have been several inklings2, 3, 4 that the rate of molecular evolution accelerates when measured over evolutionarily short timescales. As they report in Molecular Biology and Evolution, Ho and colleagues5 have now put the evidence together. Their analyses of primate and bird data sets reveal that there is indeed a decided acceleration of molecular evolution on short timescales. This is an effect that demands explanation; moreover, estimates for the timing of recent events in population biology will need to be reconsidered.
Much more remains to be done. There is the challenge of formulating a single theory that operates smoothly over disparate timescales, from current heterozygosity to the long-term rate of evolution. In addition, a single mutation rate (mu) does not really exist. Even for nucleotides there are many 'mutation rates', at least one between each pair of nucleotides, and these can be estimated separately using three-dimensional matrices12. The J-shaped curve cannot rest until a single theory holds for it: we live in interesting times.
An unsolved problem in molecular biology identified through phylogenetic analyses. What implications this has for evolution is unclear until a mechanism is developed. Is this variation an effect of evolution and natural selection on long time scales? Or a strange statistical artifact? We don't yet know, and that's what makes science fascinating.