 Molecular-clock calibration and rate-time identifiability

A molecular branch length is expected substitutions per site, the product of evolutionary rate and elapsed time. Without external calibration, simultaneous rescaling of rate and time preserves the sequence likelihood. Fossil or geological bounds, or informative sampling dates, supply time information; relaxed clocks allow rate variation. Calibration uncertainty, saturation and gene/species-tree discordance must be propagated rather than turned into an exact historical date.