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D1.3.5—Randomness in mutation

Mutations occur randomly with respect to need or advantage, although mutation rate can vary with mutagens, repair and DNA context.

Syllabus
First assessment 2025
Objective
D1.3.5
Level
HL

Mutation Is Random Relative to Need, but Its Probability Is Not Uniform

A mutation does not arise because an organism needs a useful trait. A change can occur before an environmental challenge and later prove harmful, neutral or advantageous.

Mutation probability can still vary with:

  • base identity and local sequence context
  • how often a region is copied or transcribed
  • access to proofreading and repair systems
  • dose and type of mutagen exposure

Random relative to need is not the same as equally likely at every base. Natural selection is the non-random filter; it does not direct the earlier mutation.

Different DNA variants arise before an environmental filter increases the representation of one favoured variant.

Mutation Creates Alleles; Natural Selection Changes Their Frequencies

Mutation is the original source of new alleles. Recombination can reshuffle existing alleles, but it does not create a new DNA sequence at a locus.

new mutation → new allele → inheritance places the allele in a population → environment affects survival and reproduction → allele frequency may change over generations

Most new mutations are neutral or harmful in their current context; a smaller number can improve reproductive success under particular conditions. The same allele can have different effects in different environments.

Populations evolve because heritable allele frequencies change. An individual does not mutate adaptively and then evolve during its lifetime.

Predict a Mutation's Consequence by Following Four Decisions

Ask What the answer predicts
What changed in the DNA? substitution, insertion, deletion or duplication
Where is the change? coding, regulatory or other sequence; possible molecular consequence
Which cell lineage carries it? local somatic clone or possible inheritance through gametes
Does it alter reproductive success? whether selection may change its frequency

The causal chain can stop at any stage: a DNA change can be repaired, lie in a tolerant region, preserve the amino acid, or make no detectable phenotypic difference.

Mutation generates variants without foresight. Cell lineage controls their reach, and natural selection later filters heritable variation according to environmental conditions.

Conservation Can Reflect Selection, Mutation Rate—or Both

HL only
Hypothesis Mechanism Prediction
functional constraint with purifying selection harmful variants reduce reproductive success and are removed disrupting the sequence is likely to impair function
lower mutation rate fewer sequence changes arise, perhaps because repair is especially effective conservation can persist even before selection acts

The explanations are not mutually exclusive. An essential sequence may both acquire mutations slowly and lose harmful variants rapidly through purifying selection.

Highly transcribed genes and their transcribed strands can show lower mutation rates, consistent with enhanced access to repair machinery.

Conservation suggests importance, but sequence comparison alone does not reveal the exact function. Knockout or targeted editing can test the hypothesis experimentally.

Randomness in mutation

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What is a feature of mutations?

Concept essentials

  • Mutations are not directed by an organism’s needs.
  • External factors can increase mutation frequency without making mutations purposeful.
  • Mutation rate may vary across sequences and conditions.
  • Natural selection acts on mutation-generated variation after it arises.