AP Syllabus focus:
‘Mutations introduce new genetic variants into populations, providing raw material for evolution and contributing to observable phenotypic diversity.’
Mutations create the ultimate source of new alleles. When heritable mutations enter a population’s gene pool, they increase genetic variation, enabling evolutionary change and helping explain differences among individuals and populations.
Core idea: why mutations matter for variation
Mutations are changes in DNA sequence.
Diagram illustrating three common point-mutation classes at the DNA level: base substitution, insertion, and deletion. This makes the idea of “new alleles” tangible by showing exactly how a sequence can differ from an ancestral version, creating alternative genetic variants. Source
At the population level, their key importance is that they generate new alleles, increasing the number of genetic options on which evolutionary processes can act.
Genetic variation and phenotypes
Variation can be described at two linked levels:
Genotypic variation: differences in DNA sequences (alleles) among individuals
Phenotypic variation: observable differences that arise from genotype, environment, and gene-by-environment interactions
Not all new alleles noticeably change phenotype, but even “silent” genetic differences can matter later (e.g., in new environments or in combination with other alleles).
Genetic variation: Differences in DNA sequences (alleles) among individuals in a population.
A population with more genetic variation is more likely to include individuals with traits that allow survival and reproduction under changing conditions.
From mutation to population-level diversity
A mutation contributes to genetic variation only if it becomes part of the population’s gene pool.
Heritable vs non-heritable mutations
Only mutations in cells that contribute DNA to offspring change allele frequencies across generations.
Germline mutations (or mutations in reproductive cell lineages) can be inherited and add to population variation
Somatic mutations affect only the individual and are not passed to offspring (though they can influence that individual’s phenotype)
Gene pool: The total collection of alleles present in a population.
A single new mutation typically appears first in one chromosome of one individual. It then either:
is lost by chance,
persists at low frequency,
or rises in frequency over generations.
Mutations as “raw material” for evolution
The specification emphasizes that mutations “provide raw material for evolution.” This means mutations supply the new alleles, while evolutionary mechanisms determine what happens next.
How new alleles translate into evolutionary change
Once a new allele exists, several outcomes are possible:
Natural selection can increase beneficial alleles (higher reproductive success)
Selection can remove harmful alleles (lower reproductive success)
Neutral changes may persist without strong selection, especially in large stretches of DNA with minimal functional impact
Chance effects can cause allele frequencies to fluctuate, especially when alleles are rare
Mutations therefore do not “try” to meet an organism’s needs; rather, they introduce variation, and the environment influences which variants become more common.
Standing variation vs new mutation
Adaptation often uses standing genetic variation (alleles already present). However, long-term evolutionary novelty depends on continual input of new mutations, especially after environments change or when new challenges arise.
Observable phenotypic diversity: connecting genotype to traits
Mutations contribute to observable phenotypic diversity when they affect:
Protein structure (changing amino acid sequence and protein function)
Protein amount (changing when/where/how much gene product is made)
Noncoding functional sequences (altering gene regulation)
Some traits are influenced by many genes; in such cases, mutation-generated allele differences across multiple loci can produce a wide range of phenotypes within a population.
Environment and context dependence
Whether a mutation contributes to “useful” diversity depends on context:
The same allele can be advantageous, neutral, or disadvantageous in different environments
Effects may depend on genetic background (interactions with other alleles)
This context dependence helps explain why populations can maintain multiple alleles over time, supporting ongoing diversity.
Rates and limits: why variation persists but is not unlimited
Mutation is a continuous source of new alleles, but populations do not accumulate unlimited harmful changes because:
many harmful mutations reduce reproductive success and are removed over time,
many mutations have no detectable effect on phenotype,
only a subset of mutations become common enough to noticeably shape population diversity.
Importantly, even rare alleles can matter: when conditions change, previously uncommon variants may become beneficial, allowing rapid evolutionary responses.
FAQ
Common approaches include comparing parent–offspring genome sequences to count new variants, and using “molecular clock” reasoning across lineages.
Key challenges include sequencing error, detecting mosaicism, and distinguishing truly new mutations from rare parental variants.
Mutation probability can vary with local DNA context and structure.
Examples include repetitive sequences, regions prone to polymerase slippage, and sites where chemical instability is higher, leading to “hotspots” of new variation.
Mutation–selection balance occurs when new harmful alleles are introduced by mutation while selection removes them.
This can maintain low-frequency deleterious alleles in a population, contributing to genetic variation even when alleles reduce fitness.
Yes. Variation can arise when a gene copy gains changes over time and diverges in function.
Gene duplication followed by divergence can create novel or specialised functions, expanding the kinds of phenotypes available for selection to act on.
In small populations, new beneficial alleles may be lost more easily when rare.
Also, reduced overall genetic diversity can limit the combinations of alleles available, making it harder for selection to assemble high-fitness genotypes under changing conditions.
Practice Questions
Explain how mutations act as a source of genetic variation in a population. (2 marks)
New mutations create new alleles / new DNA sequences in individuals. (1)
If the mutation is heritable (in germline), it can be passed to offspring and enter the population gene pool, increasing variation. (1)
A new allele arises by mutation in one individual within a population. Describe how this mutation could contribute to observable phenotypic diversity over several generations. (5 marks)
The mutation creates a new allele at a locus (new genetic variant). (1)
If present in germline/reproductive lineage, it can be inherited by offspring. (1)
The allele can change phenotype by altering protein structure or altering gene expression level/pattern. (1)
The allele’s frequency may increase if it improves reproductive success in that environment (selection). (1)
Alternatively, it may persist at low frequency or be lost when rare due to chance effects, affecting how much diversity is observed. (1)
