How Gene Pools Change
- Junessa Masaya
- Apr 15
- 4 min read
HSC Biology | Free Study Notes
In this lesson
what a gene pool is
how mutation changes a gene pool
how gene flow changes a gene pool
how genetic drift changes a gene pool
how natural selection changes allele frequencies
What is a gene pool?
A gene pool is the total genetic diversity of a population.
HSC marking guidance describes a gene pool as the total genetic diversity of a population and links changes in the gene pool directly to evolution.
Why gene pools matter
The gene pool of a population:
contains all the alleles present in that population
provides the basis for genetic variation
can change over time
Key idea
When the gene pool of a population changes, evolution has occurred. This idea appears directly in HSC marking guidance.
Mutation
How mutation changes the gene pool
Mutation can:
create a new allele
add genetic variation to a population
change allele frequencies over time if the new allele is passed on
Why mutation matters
Mutation is the original source of new alleles. Without mutation, there would be no completely new genetic variants entering the population.
Important point
Mutation alone may introduce a new allele, but other processes often determine whether that allele becomes common or stays rare.
HSC marking guidance notes that mutation may be the underlying cause of an allele, even if it does not by itself explain large differences in allele frequency between populations.
Gene flow
Gene flow is the movement of alleles into or out of a population.
HSC marking guidance defines gene flow this way and gives migration and reproduction as a clear example.
How gene flow changes the gene pool
Gene flow changes a gene pool when:
individuals move into a population and reproduce
individuals leave a population
alleles are added or removed
Why gene flow matters
Gene flow can:
increase genetic diversity
introduce new alleles
reduce differences between populations if they mix regularly
Example
A migrant animal entering a population and breeding may add new alleles to that gene pool. HSC marking guidance uses this as a direct example.
HSC-style example
The 2023 HSC marking guidance explains that introducing males from one pygmy possum population into another could increase genetic diversity through gene flow, improving variation and the population’s ability to adapt.
Genetic drift
HSC marking guidance defines genetic drift as a change in allele frequency due to random selection of alleles or random events.
How genetic drift changes the gene pool
Genetic drift can:
make some alleles more common by chance
make some alleles less common by chance
remove alleles from a population completely
cause alleles to become fixed
Why genetic drift matters most in small populations
Genetic drift has a bigger effect in small populations because:
the gene pool is smaller
chance events affect a larger proportion of the population
allele frequencies can change more dramatically
A 2025 HSC marking guideline states that small gene pools have decreased genetic diversity, so fluctuations in allele frequency have a greater effect.
Example
A 2023 HSC marking guideline describes a bottleneck effect in pygmy possums after bushfires reduced population size. The remaining individuals formed the new gene pool, so some alleles became more common and others disappeared by chance.
Natural selection
How natural selection changes allele frequencies
If an allele gives a survival or reproductive advantage:
individuals carrying it are more likely to reproduce
that allele is passed to more offspring
its frequency increases over generations
If an allele reduces survival or reproduction:
it may become less common
Why natural selection matters
Natural selection is not random. It is linked to environmental conditions and selective pressures.
Example
A 2022 HSC marking guideline explains that if a population faces a virus, natural selection may reduce the frequency of an allele associated with severe inflammation because individuals with certain genotypes may be less likely to survive and reproduce.
How these mechanisms differ
Mechanism | How it changes the gene pool |
Mutation | Introduces new alleles |
Gene flow | Moves alleles into or out of a population |
Genetic drift | Changes allele frequency by chance |
Natural selection | Changes allele frequency through differential survival and reproduction |
How they work together
These mechanisms often act together rather than separately.
Example
A population may:
gain a new allele by mutation
receive additional alleles through gene flow
lose some alleles through genetic drift
then have allele frequencies further shaped by natural selection
This is why questions about gene pools often ask students to explain more than one process.
Why this topic matters in Module 6
Understanding how gene pools change helps explain:
evolution in populations
differences between populations
why small populations are vulnerable
how biodiversity can increase or decrease over time
This is why the Module 6 syllabus places mutation, gene flow and genetic drift together under genetic change, and why HSC marking guidance repeatedly links gene pool change to evolution.
Worked example
Exam-style question
Explain how genetic drift can change the gene pool of a small population.
Worked answer
Genetic drift changes the gene pool by chance. In a small population, random events can have a large effect on allele frequency because the gene pool is small. This may cause some alleles to become more common, become fixed, or disappear completely from the population.
Why this works
This answer:
defines genetic drift clearly
explains why small populations are affected more strongly
links the process directly to changes in allele frequency
Common mistakes
Saying mutation, gene flow, genetic drift and natural selection are all the same process.
Forgetting that mutation introduces new alleles.
Confusing gene flow with genetic drift.
Saying genetic drift is caused by selection pressure rather than chance.
Describing gene pool change without linking it to allele frequency or evolution.
Quick quiz
What is a gene pool?
Which mechanism introduces new alleles into a population?
What is gene flow?
Why does genetic drift have a stronger effect in small populations?
How does natural selection change allele frequencies?
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