The Evolution of Molecular Clocks: Concepts, Calibrations, and Challenges.

Molecular clocks use the accumulation of genetic changes to estimate the timing of evolutionary and demographic events. Since their formulation in the 1960s, molecular clocks have evolved from relatively simple strict-rate assumptions into a diverse set of statistical frameworks that include strict, relaxed, local and time-dependent models. In this review, we examine the conceptual basis of molecular-clock inference and the biological, demographic and methodological factors that influence clock estimates. We distinguish mutation rates from substitution rates and discuss how molecular, genomic, life-history, environmental and population-level processes can affect estimated evolutionary rates. We then review the main analytical approaches used to estimate branch lengths and divergence times, including maximum likelihood, Bayesian inference, parsimony-based methods and regression-based techniques; we also focus on the statistical evaluation of clock-like behaviour. Particular attention is given to calibration, as molecular clocks only become informative for absolute time estimation when genetic distances are anchored to an external temporal framework. We review calibration strategies based on fossils, geological and biogeographic events, founder events, ancient sequences, pedigrees, direct mutation-rate estimates, longitudinal sampling, secondary calibrations and externally estimated substitution rates. Finally, we discuss cases in which molecular-clock estimates show concordance or discordance with independent archaeological, palaeontological or epidemiological evidence, as well as cases where apparent agreement may reflect circular reasoning. We argue that molecular clocks remain essential tools for evolutionary biology, but that their reliability depends on explicit model testing, transparent calibration choices and careful interpretation of the results.