Molecular clock

A method for estimating when evolutionary lineages diverged by relating genetic differences to elapsed time, using calibration evidence such as fossils or geological events.

Molecular evolution: Changes in the genetic composition of populations across generations. The clock measures accumulated genetic change produced by evolution.
DNA sequence: The ordered series of nucleotide bases in a DNA molecule. Molecular clocks commonly measure differences between homologous DNA sequences.
Molecular phylogenetics: The study of evolutionary relationships using molecular sequence data. Dating adds a temporal dimension to relationships inferred from sequences.
Strict molecular clock: A model that assumes the same molecular substitution rate across all lineages and times. It contrasts with relaxed clocks, which permit rates to vary among branches.
Substitution rate: The rate at which one DNA or protein sequence state replaces another in a population over time. A clock converts estimated substitutions into elapsed time using this rate.
Homology (biology): Similarity between biological features due to shared ancestry. Sequence differences are informative about time when the compared sequences share ancestry.
Phylogeography: The study of how geographic distributions of genetic lineages change through time. Divergence dates help connect lineage history with past geographic change.
Fossil record: The preserved remains and traces of past organisms found in geological deposits. Fossils can date evolutionary history directly, while molecular clocks infer dates from genetic change.
Molecular clock calibration: The process of anchoring molecular rate estimates or divergence times to independent temporal evidence. Fossils and geological events supply dates that give sequence differences a timescale.
Common ancestor: An ancestral organism or population from which two or more lineages descend. The clock estimates when descendant lineages separated from their shared ancestor.

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