Relaxed molecular clock dating
But a molecular clock does tell time -- on an epoch scale. Blair Hedges, is a tool used to calculate the timing of evolutionary events.
Instead of measuring seconds, minutes and hours, said Hedges, Penn State professor of biology, the molecular clock measures the number of changes, or mutations, which accumulate in the gene sequences of different species over time.
They generalized this observation to assert that the rate of evolutionary change of any specified protein was approximately constant over time and over different lineages (based on the molecular clock hypothesis (MCH)).
The genetic equidistance phenomenon was first noted in 1963 by Emanuel Margoliash, who wrote: "It appears that the number of residue differences between cytochrome c of any two species is mostly conditioned by the time elapsed since the lines of evolution leading to these two species originally diverged.
In particular, models that take into account rate variation across lineages have been proposed in order to obtain better estimates of divergence times.
These models are called relaxed molecular clocks and are made possible through MCMC techniques that explore a weighted range of tree topologies and simultaneously estimate parameters of the chosen substitution model.
Sometimes only a single divergence date can be estimated from fossils, with all other dates inferred from that.
Other sets of species have abundant fossils available, allowing the MCH of constant divergence rates to be tested.
developed the neutral theory of molecular evolution, which predicted a molecular clock. mutations with no effect on fitness) in a new individual be .
The molecular clock technique is an important tool in molecular systematics, the use of molecular genetics information to determine the correct scientific classification of organisms or to study variation in selective forces.