The Neutral Theory and Reading a Molecular Clock
Under the neutral model, many substitutions have little effect on fitness and accumulate at an approximately mutation-linked rate. With a suitable calibration, sequence differences can estimate elapsed time since lineages diverged.
What a learner can do afterwards
- Explains why neutral substitutions can accumulate at an approximately steady rate under the neutral model and suitable calibration.
- 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
New neutral mutations arise at some rate. Selection ignores them, so each one spreads or vanishes by chance. A new copy fixes with probability equal to its starting frequency, 1 in 2N in a diploid population of N. A bigger population makes more new mutations but gives each a smaller chance, so the two effects cancel. The neutral substitution rate therefore tracks the neutral mutation rate, which is why neutral differences tick like a clock.
Neutral substitutions build up at the neutral mutation rate.
To estimate a split time, first define the rate. If a calibrated rate is 2 pairwise differences per million years and two lineages differ at 60 neutral sites, divide 60 by 2. The estimate is 30 million years. Without a calibration, the differences give relative order, not years. The rate must fit the gene and lineages being compared; there is no universal rate for every sequence.
A calibrated pairwise rate turns sequence differences into an estimated time.
A dated fossil or other dated event supplies a calibration point. It anchors a node to an estimated time, with uncertainty. It does not make every sequence difference exact.
Functional constraint means that in important regions fewer of the possible changes are neutral; selection removes the harmful ones, so those regions tick more slowly. Less constrained regions accept more neutral substitutions. The rate can also differ among lineages because generation time and mutation processes differ.
Shorter generation time often means more DNA-copying rounds per century, which can produce more substitutions per year. This is a tendency, not a rule that ignores species-specific mutation and repair.
Averaging across a suitable long sequence reduces the effect of random noise in one short region. The sequence still needs a model that separates neutral changes from changes affected by selection.
There is no single clock rate for every gene and lineage. A study should test or calibrate the rate for the gene, lineages, and time span being compared. A rate borrowed from slower-changing lineages can mislead: faster lineages gather more pairwise differences per year, so dividing their count by the slower rate gives a split date that is too old.
Count neutral differences, define and calibrate the rate, then divide to estimate a split time. Rates can differ among genes and lineages.
Original lesson, assessment, and flashcard text authored for this topic in the Open Source Taxonomy project.
Reading adapted from the Khan Academy article Evolutionary time; publisher terms are unknown. The rewritten reading was written for this dataset. Readings on neutral substitution rates and on rate variation were written for this topic in the Open Source Taxonomy project.
2 · Watch
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