The Neutral Theory and Reading a Molecular Clock
Most substitutions between species change nothing about fitness, so they accumulate at a rate set by the mutation rate rather than by selection. That regularity is what lets sequence differences be read as elapsed time, once the clock is calibrated.
What a learner can do afterwards
- Explains why neutral substitutions accumulate at a steady rate.
- Estimates a divergence time from a difference count and a calibrated rate.
- Says why the clock runs at different rates in different genes and in different lineages.
1 · Read
Picture tiny typos raining on DNA each generation. Most neither help nor harm, so selection ignores them and chance alone decides their fate. Chance fixes them at the typo rate itself, which stays roughly steady. Those silent gaps pile up like ticks, independent of progress or plans.
Now turn ticks into dates yourself. Count the neutral gaps between two lines and divide by the anchored rate per year. A dated remain fixes at least one split to real years, converting relative distance into absolute time. No anchor, no years: only order.
Next, learn why ticks vary and how you tame them. Guarded regions weed out most edits and tick slowly, while free stretches accept more hits. Fast breeders copy DNA far more often per century, so they pile slips faster. Serious work anchors each gene and each branch alone and averages over long reads to smooth chance noise.
A final window shows the power of the method. Mother line DNA passes down one line without shuffling, so its gaps record clean splits and journeys. Any steady anchored stretch can date events that left no remains. The molecule you pick sets the sharpness, from recent trips to deep splits.
You count neutral gaps, anchor the rate, and read the split date.
2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
Nothing builds on it yet.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.