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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsA molecular clock estimates when animal lineages diverged by combining genetic sequence data, an evolutionary model, and an external time calibration—often from fossils. It does not read a split date directly from DNA: the result is an estimated divergence time, usually with a range of uncertainty.
What a molecular clock actually estimates
DNA and protein sequences accumulate changes over generations. Comparing sequences can help researchers infer how much evolutionary change separates species and how their lineages are related. But the number of genetic differences is not, on its own, a measure of elapsed time: mutation and substitution rates vary among genes and lineages, and changes at the same sequence position can obscure one another. Researchers therefore need models of sequence evolution and a way to anchor molecular change to time.
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The target is generally the estimated age of a common ancestor, or the time at which descendant lineages began separating. That is not necessarily the same as the instant a new species formed. Gene copies can have histories that differ from the history of the species carrying them, particularly around recent divergences.
How the estimate is built
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Collect comparable sequences
Researchers sample DNA or protein sequences from the animal species relevant to the question, selecting genes or genomic regions that can help infer relationships and branch lengths. Taxon and sequence sampling affect what the data can resolve.
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Infer relationships and molecular change
A phylogenetic analysis estimates the relationships among sampled species and the amount of sequence change along the tree’s branches. Substitution models account for patterns such as different rates of change among sites. Because multiple substitutions can occur at one site, the observed differences may understate the total change; model choice and adequacy matter. See the review of divergence-time estimation in Annual Review of Ecology, Evolution, and Systematics.
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Choose a clock model
A strict clock assumes the same molecular rate across branches. A relaxed clock allows rates to differ among lineages or through time, according to specified statistical assumptions. Local-clock approaches allow different rates in different parts of a tree. None is a universal default: the appropriate model depends on the data and its assumptions, which should be reported explicitly. Method options are reviewed in Rutschmann’s review of molecular dating methods and a 2026 review of molecular clocks.
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Anchor the tree to geological time
Sequence data can inform relative amounts of change, but an absolute date requires an external temporal calibration. Fossils are common calibrations. With node calibration, fossil evidence constrains an internal divergence; with tip calibration, a fossil taxon is included as a dated tip. A fossil generally establishes that a lineage existed by at least the fossil’s age. Its age and taxonomic placement can support a minimum constraint and, when justified, a probability distribution for a node—not an exact date for the split. The challenges of fossil calibration are discussed in The evolution of methods for establishing evolutionary timescales and Dating the Time of Origin of Major Clades.
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Estimate ages and uncertainty
Likelihood and Bayesian methods combine sequence data, the phylogeny, clock model, and calibration information to estimate node ages. Bayesian analyses represent uncertainty as distributions over model parameters and ages. The resulting estimate is conditional on the choices made along the way; it is not a direct observation of an evolutionary event. For an overview of Bayesian dating in the genomics era, see Bayesian molecular clock dating of species divergences in the genomics era.
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Why different studies can give different dates
Two estimates for an apparently similar divergence can differ without either being automatically wrong. The result depends on how the tree is built and calibrated, which sequences and taxa are sampled, and how rate variation is modeled. Rooting and branch-length estimates also influence the placement and age of nodes. Compare the assumptions and uncertainty intervals, not just the headline dates.
- Calibration: Which fossil or other temporal evidence was used? How old is it, how was it identified and placed, and what bounds or probability distribution were assigned?
- Clock assumptions: Does the analysis impose a strict clock, or allow rates to vary? What model specifies that variation?
- Sampling and tree: Which species and sequences were included, and how were relationships, branch lengths, and the root inferred?
- Uncertainty: What interval or age distribution was reported? A narrower interval is not automatically more reliable if its calibrations or model assumptions are not well supported.
How to read a reported divergence date
Look for an estimated age and its uncertainty interval, then check the calibration and clock model used to obtain it. Treat a fossil minimum as evidence that a lineage existed by that time, not as the exact date its descendant lineages split. If a paper dates a gene-tree ancestor, do not assume that date is identical to the species divergence: the two histories can differ, especially for recent splits.
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