Calibration
The molecular clock alone can only say that one time period is twice as long as another: it cannot assign concrete dates. To achieve this, the molecular clock must first be calibrated against independent evidence about dates, such as the fossil record. Alternatively, for viral phylogenetics and ancient DNA studies, two areas of evolutionary biology where it is possible to sample sequences over an evolutionary timescale, the dates of the samples themselves can be used to calibrate the molecular clock.
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