DNA differences are not a stopwatch
A molecular clock estimates when two lineages separated by counting genetic changes that accumulated after their split. The basic logic is simple: if changes build up at a roughly steady rate, more differences usually mean more time apart.
But DNA does not record time directly. A mutation—a change in genetic code—may appear in one generation and never spread, or become common over many generations. The rate at which changes accumulate can differ among branches, so equal genetic distance does not necessarily mean equal elapsed time.
Why finch branches can run at different speeds
Island finches share ancestry, but their lineages have not necessarily experienced identical conditions or evolved at identical rates. Differences in generation time, population history, or biology can affect how quickly genetic changes arise or persist. The island setting does not itself make a lineage’s clock fast or slow; the relevant point is that separate branches can tick at different paces.
Imagine two finch lineages that each show the same amount of DNA difference from a relative. If one accumulated changes quickly, it may have split more recently. The slower lineage could have taken longer to reach the same genetic distance. A clock that assumes a single shared pace would assign them the same date and misplace at least one split.
Dating requires more than counting
Researchers can account for this by allowing rates to vary among branches and by anchoring estimates to independent timing evidence where available. The result is not a perfect timestamp, but a more realistic estimate than treating every lineage as identical.
That distinction matters for reconstructing how island finches diversified: a genetic difference can tell us that lineages changed, but only a rate-aware clock can help distinguish a recent split with rapid change from an older split with slower change.
