Gene pools and allele frequency change: WACE Year 12 Biology
“Explain how genetic drift, gene flow, the founder effect and bottlenecks change allele frequencies in a gene pool”
A gene pool is all the alleles in a population. Besides natural selection, its allele frequencies change through genetic drift (random change, strongest in small populations), gene flow (alleles entering or leaving through migration), the founder effect (a new population started by a few individuals carries only some of the original alleles) and bottlenecks (a sharp population crash leaves a small, unrepresentative sample of alleles).
What this dot point is asking
SCSA wants you to define a gene pool and explain the non-selective forces that change allele frequencies, distinguishing random processes (drift, founder, bottleneck) from gene flow. A strong answer says why each effect is stronger in small populations.
The gene pool
A gene pool is the total collection of alleles present in all the individuals of a population. Evolution can be described as a change in the allele frequencies of a gene pool over time. Natural selection is one cause of such change, but several other processes also shift allele frequencies, sometimes without any survival advantage being involved.
Genetic drift
Genetic drift is random change in allele frequencies from one generation to the next due to chance alone. Because real populations are finite, not every allele is passed on in exact proportion. By chance, some alleles become more common and others rarer, and rare alleles can be lost entirely.
In a large population, chance fluctuations average out and allele frequencies stay stable. In a small population, chance has a large effect, so drift can rapidly change frequencies and even fix or lose alleles. This is why small, isolated populations lose genetic diversity quickly.
Gene flow
Gene flow is the movement of alleles between populations when individuals migrate and breed, or when pollen or seeds are carried between plant populations. Gene flow tends to make populations more genetically similar and introduces new alleles into a population. A barrier that stops gene flow, such as a mountain range or ocean, allows populations to diverge.
The founder effect
When a small number of individuals leave a population and establish a new one, they carry only a sample of the original gene pool. The new population's allele frequencies may differ markedly from the parent population purely by chance, and some alleles may be over-represented while others are absent. This is the founder effect, a special case of drift.
Track the allele through the founders
A few birds blown to a remote island start a new population. By chance, two of the founders happen to carry a rare allele that was uncommon in the mainland population. Because the founding group is tiny, that allele now makes up a large fraction of the island gene pool. The island population therefore differs genetically from the mainland from the very start, not because of selection but because of which individuals happened to found it.
The bottleneck effect
A bottleneck occurs when a population crashes sharply, for example through disease, drought or hunting, leaving only a small number of survivors. The survivors carry only a fraction of the original genetic diversity, often not a representative sample. Even if the population recovers in numbers, its genetic diversity stays low. Reduced diversity leaves a population less able to adapt to future change.
Putting the forces together
Natural selection, drift, gene flow, founder effects and bottlenecks all change allele frequencies, and they often act at the same time. Selection is directional and non-random; drift, founder effects and bottlenecks are random; and gene flow connects populations. In conservation biology these forces explain why small, isolated Australian populations of threatened species need careful genetic management to avoid losing diversity.
Gene pool: all the alleles in a population. Genetic drift: random change in allele frequencies. Gene flow: movement of alleles between populations. Bottleneck: a sharp reduction in population size that lowers genetic diversity.
Why this matters for continuity
These processes show that evolution is not only about selection. A small Australian population recovering from a bottleneck, or a founder population on an island, can diverge from its parent population by chance alone. Understanding them is essential for conservation, because a species with a depleted gene pool may survive in the short term yet lack the variation needed to adapt to future threats.
- Saying drift affects large and small populations equally
- Drift has a much stronger effect in small populations.
- Confusing the founder effect and a bottleneck
- A founder effect starts a new small population; a bottleneck is a crash in an existing one. Both reduce diversity by chance.
- Treating gene flow as always harmful
- Gene flow adds alleles and can restore diversity to a small population.
- Calling drift adaptive
- Drift is random; it does not improve a population's fit to its environment.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksDistinguish between genetic drift and gene flow, and explain why genetic drift has a greater effect on small populations than on large ones.Show worked answer →
A 6 mark answer needs a clear distinction plus the small-population reasoning.
- Genetic drift
- Random change in allele frequencies from one generation to the next due to chance alone (which individuals happen to survive and reproduce). It is not driven by fitness and can fix or lose alleles.
- Gene flow
- The movement of alleles between populations when individuals (or pollen or seeds) migrate and breed. It introduces new alleles and tends to make populations more similar.
- Distinction
- Drift is a random change within a population; gene flow is a transfer of alleles between populations.
- Why drift is stronger in small populations
- In a large population, chance fluctuations in each direction tend to cancel out, so frequencies stay stable. In a small population a chance event (such as a few individuals failing to reproduce) changes a large fraction of the gene pool, so allele frequencies can swing sharply and alleles can be lost or fixed quickly.
Markers reward correct definitions, the within- versus between-population distinction and the averaging-out reasoning for population size.
WACE 2024-style5 marksA koala population is reduced to a few individuals by disease, then recovers in number over several decades. Explain how this bottleneck affects the genetic diversity of the population and its ability to adapt to future change.Show worked answer →
A 5 mark answer needs the bottleneck mechanism plus the consequence for adaptation.
- The bottleneck
- A sharp crash leaves only a small number of survivors. These survivors carry only a fraction of the original gene pool, and which alleles survive is largely down to chance rather than fitness, so the surviving sample is often not representative.
- Effect on diversity
- Many alleles are lost in the crash. Even when numbers recover, the population is rebuilt from the survivors' limited alleles, so genetic diversity stays low.
- Effect on adaptation
- Low diversity means less variation for natural selection to act on. If the environment changes (new disease, climate shift), the population is less likely to contain alleles that confer an advantage, so it is more vulnerable to extinction.
Markers reward loss of alleles by chance, persistently low diversity after recovery and reduced capacity to adapt.