Molecular clock

Definition

A method that uses the rate of molecular change—typically nucleotide substitutions in DNA or RNA, or amino acid replacements in proteins—to estimate the time elapsed since two or more lineages diverged from a common ancestor. The technique assumes that genetic mutations accumulate at a relatively constant rate over evolutionary time, allowing researchers to convert genetic distance into absolute or relative time. In arthropod research, molecular clocks help date the origins of major clades, such as the radiation of beetles or the diversification of spiders, and calibrate phylogenetic trees when fossil evidence is sparse or ambiguous.

Etymology

From 'molecular' (relating to molecules, especially DNA, RNA, or proteins) and 'clock' (suggesting a regular, measurable rate of change analogous to timekeeping).

Example

Using a molecular clock calibrated with known fossil beetles, researchers estimated that the major lineages of Carabidae diverged during the Jurassic, predating the oldest definitive carabid fossils by tens of millions of years.

Synonyms

• gene clock
• evolutionary clock

Related Terms

• Phylogenetics
• divergence time estimation
• calibration
• substitution rate
• coalescent theory
• fossil record
• molecular phylogeny
• neutral theory

Usage Notes

Molecular clocks require calibration—often using fossil or geological dates—to convert relative genetic distances into absolute time. Rates vary among genes, lineages, and over time (violating the strict 'clock' assumption), so relaxed-clock models are now standard. In insects and arachnids, clock estimates frequently conflict with fossil-based dates due to sparse preservation or convergent morphology; combining multiple genes and fossil constraints improves accuracy. The term is sometimes used loosely for any time-resolved phylogeny, but specialists reserve it for rate-based dating methods.