AP Syllabus focus:
‘Mutation is a random process that introduces new genetic variation into a population’s gene pool.’
Mutations are the ultimate source of new alleles. By altering DNA, they create heritable differences among individuals that selection and other evolutionary processes can later act on, especially when environments change.
Core idea: how mutation generates variation
Mutation creates new genetic information in a population by changing nucleotide sequences or by altering how genetic material is arranged. Most variation already present in a population comes from reshuffling existing alleles, but only mutation produces novel alleles.
Mutation: a random change in genetic material (DNA sequence or chromosome structure) that can create a new allele.
Although mutations occur randomly with respect to “need,” their consequences are not random: different mutations have different effects on phenotype and reproductive success.
Where mutations come from (sources)
DNA replication errors
During S phase, DNA polymerases can:
insert the wrong nucleotide (misincorporation)
slip on repetitive sequences (creating small insertions/deletions)
copy damaged templates inaccurately
Cells reduce but do not eliminate these errors via proofreading and repair, so some become permanent changes.
DNA damage from the environment
Environmental factors (often called mutagens) can damage DNA, increasing the chance that repair or replication produces a mutation, for example:
UV light causing pyrimidine dimers
ionising radiation causing breaks in DNA strands
certain chemicals modifying bases or intercalating between them
Mutagens increase mutation rate, but they do not direct mutations toward beneficial outcomes.
Types of mutations that matter in AP Biology
Small-scale (gene-level) changes
Point mutations (single-base substitutions)
can be silent (no amino acid change), missense (amino acid change), or nonsense (stop codon)
Insertions/deletions (indels)
can cause a frameshift if not in multiples of three, altering downstream codons
This diagram contrasts a normal open reading frame with a frameshift caused by a single-base insertion. The shifted grouping of codons changes the amino acid sequence downstream and can generate an early stop codon, typically producing a truncated, nonfunctional protein. Source
These can change a protein’s structure, abundance, or function, or affect gene regulation (for example, promoter or enhancer changes).
Large-scale (chromosome-level) changes
Mutations can also involve bigger DNA segments, including:
This schematic illustrates several chromosome-scale changes, including deletion, duplication, and translocation, as well as whole-chromosome gain/loss (aneuploidy). It highlights how structural variants can reposition or change the copy number of large DNA segments, which can alter gene dosage and regulation. Source
duplications (extra copies of genes or regions)
deletions (loss of genes or regions)
inversions (segment reversed)
translocations (segment moved to a new location)
Large-scale changes can modify gene dosage, disrupt coding sequences, or alter regulation by repositioning genes relative to regulatory elements.
From genotype to phenotype: effects of mutations
A mutation’s effect depends on where it occurs and what it changes.
Coding region mutations can change amino acid sequence and protein folding.
Regulatory mutations can change when, where, or how much a gene is expressed.
Noncoding mutations may have no detectable effect, or may affect splicing, RNA stability, or chromatin structure.
A useful way to categorise outcomes:
Neutral: no measurable effect on phenotype or fitness in the current environment
Deleterious: reduces function or performance (often the most common outcome for random changes in functional DNA)
Beneficial: increases performance or reproduction in a specific environment (rare but critical for adaptation)
Heritability: which mutations enter the gene pool?
Only mutations in germline cells (cells that produce gametes) can be inherited by offspring and thus add variation to a population’s gene pool.
This figure compares a somatic mutation arising during development (creating a mosaic organism with mutant and non-mutant cell lineages) with a germline mutation originating in a gamete (propagating to essentially all cells of the offspring). It visually reinforces why only germline mutations reliably enter the population’s gene pool across generations. Source
Somatic mutations occur in body cells and can affect the individual (for example, contributing to cancer) but are generally not passed to offspring.
Because inheritance is required for population-level evolutionary change, AP Biology emphasises germline mutations as the key link between molecular change and variation among generations.
Mutation rate and persistence of variation
Mutation rates differ among organisms and genomic regions. Even with repair mechanisms, low but continuous mutation means:
new alleles are steadily introduced over time
variation can reappear after being reduced
populations maintain a reservoir of rare alleles that may matter if conditions shift
Many new alleles remain at low frequency because they are neutral, recessive, or harmful, but their existence increases the range of possible phenotypes in future generations.
Why “random” matters for evolution
The randomness of mutation means:
mutations do not occur because organisms “try” to adapt
adaptation results when a pre-existing or newly arisen mutation happens to improve reproductive success under particular conditions
predicting which mutation will occur is difficult, but describing how mutations generate variation is central to explaining evolutionary potential
FAQ
Transposons can insert into genes or regulatory DNA, disrupting function or changing expression.
They may also promote unequal crossing over by creating repeated sequences, increasing duplications/deletions.
Local sequence features (e.g., repeats) can increase polymerase slippage.
Chromatin state and access of repair enzymes can also influence which regions accumulate mutations.
Yes. Genetic background matters: other alleles can buffer or magnify effects.
Dominance relationships and epistasis can make the phenotype vary across individuals.
They use mutation-accumulation experiments, sequencing across generations, or counting new variants in pedigrees.
Statistical models estimate rates per generation or per nucleotide site.
Most random changes in functional sequences disrupt existing function.
A beneficial outcome requires a specific change that improves performance under particular environmental conditions.
Practice Questions
State what is meant by a mutation and explain why mutations are important for genetic variation in a population. (2 marks)
Defines mutation as a random change in DNA/genetic material that can create a new allele (1)
Explains that mutations introduce new alleles into the gene pool, increasing heritable variation (1)
Describe two different types of mutation and explain how each could lead to a change in phenotype. Include at least one example of how the mutation could be neutral, harmful, or beneficial. (5 marks)
Describes a point mutation/substitution OR insertion/deletion accurately (1)
Links that gene-level change to altered codons/protein sequence or premature stop/frameshift (1)
Describes a large-scale mutation (e.g., duplication, deletion, inversion, translocation) accurately (1)
Links the large-scale change to gene dosage, gene disruption, or altered regulation/expression (1)
Explains that effects can be neutral/harmful/beneficial depending on context and what is altered (1)
