Molecular Evolution and Neutral Theory
The Molecular Record of Evolution
Substitution versus mutation
Mutation creates new variants; substitution is the fixation of a variant in a lineage. Molecular evolution studies both the tempo and mode of substitution across genomes and lineages.
Neutral and nearly neutral theory
Neutral theory argues that much molecular evolution is governed by drift acting on selectively neutral alleles. Nearly neutral theory extends this by emphasizing weakly selected variants whose fate depends on and the distribution of fitness effects.
Core inference
A high rate of molecular change does not automatically imply adaptive evolution; many substitutions may be neutral or nearly neutral.
Molecular clocks
If substitutions accumulate approximately linearly with time, genetic divergence can be used to estimate lineage splits. In practice, rate heterogeneity, time dependency, and calibration uncertainty require relaxed-clock and model-based methods.
What is the key distinction between mutation and substitution?
Substitution is a population-level outcome after a mutation spreads or fixes.
Correct answer: Mutation creates a new variant; substitution is fixation in a population lineage.
Why can weak selection be less effective in small populations?
Nearly neutral variants are especially sensitive to drift.
Correct answer: Because stochastic drift can dominate when selection coefficients are small relative to , preventing consistent deterministic increase or decrease.
Common molecular comparisons
| Statistic | Interpretation |
|---|---|
| Evidence for purifying, neutral, or positive selection at protein-coding loci | |
| Synonymous divergence | Often used as a proxy for neutral evolution |
| Haplotype structure | Can reveal recent sweeps or demographic events |
| Site frequency spectrum | Sensitive to selection and population history |
Inferring selection from sequence data
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1
Estimate divergence and polymorphism at coding and noncoding sites.
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2
Compare nonsynonymous and synonymous change while accounting for codon usage and mutation bias.
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3
Evaluate the site frequency spectrum and haplotype structure.
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4
Control for demography using neutral or nearly neutral models.
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5
Assess whether deviations are best explained by selection or history.
A value of is often interpreted as evidence for what?
An excess of nonsynonymous substitution relative to synonymous substitution can indicate adaptive protein evolution.
Correct answer: Positive selection on amino acid changes
Why is synonymous change not always perfectly neutral?
Neutrality is often approximate rather than absolute.
Correct answer: Because synonymous sites can affect splicing, mRNA stability, translation efficiency, or may be linked to selected loci.