Molecular Evolution and the Genetic Code of History
Evolution in Genomes
Substitutions and rates
Molecular evolution studies changes in DNA and protein sequences over time. Substitutions may be synonymous or nonsynonymous, and rate differences can reflect mutation, selection, and functional constraint.
Selective constraints
Protein-coding regions under strong functional constraint accumulate fewer amino acid-changing substitutions. Noncoding regions can also be constrained when they regulate gene expression or chromatin structure.
Neutral vs selected change
A sequence difference does not automatically imply adaptation. Many substitutions are neutral or nearly neutral.
Sequence change categories
| Type | Effect on protein | Typical interpretation |
|---|---|---|
| Synonymous substitution | No amino acid change | Often closer to neutral, but not always |
| Nonsynonymous substitution | Amino acid change | May be neutral, deleterious, or beneficial |
| Regulatory mutation | Changes expression | Can have large phenotype effects |
Which mutation is least likely to change the amino acid sequence of a protein?
A synonymous mutation changes the codon without changing the encoded amino acid.
Correct answer: Synonymous mutation
Why do some genomic regions evolve more slowly than others?
Evolutionary rate reflects both mutation input and the strength of purifying selection.
Correct answer: Because functional constraints and selection remove many changes in important regions, while less constrained regions accumulate substitutions more freely.
Molecular clocks
If substitutions accumulate at an approximately steady rate, sequence differences can be used to estimate divergence times. In practice, rates vary among genes and lineages, so clocks must be calibrated with fossils or known dates.
What is a major limitation of a simple molecular clock?
Clock estimates need calibration because mutation and substitution rates are not perfectly constant.
Correct answer: It requires calibrated time points because rates vary
Interpreting sequence data
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1
Identify whether changes are coding or noncoding.
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2
Distinguish synonymous from nonsynonymous substitutions.
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3
Evaluate whether the region is under strong constraint.
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4
Consider whether a clock model is appropriate and calibrated.