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How and why do allele frequencies change within a population over time?

Use the Hardy-Weinberg principle and describe the factors that change allele frequencies.

Gene pools, allele and genotype frequencies, the Hardy-Weinberg principle, and the forces that drive microevolution, for TCE Biology Unit 4.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-02

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Gene pools and allele frequencies

A population is a group of interbreeding organisms of the same species in an area. The gene pool is the total of all alleles for all genes in that population. Evolution, in genetic terms, is a change in allele frequencies in a gene pool over generations (often called microevolution).

Allele frequency is the proportion of a particular allele among all copies of that gene in the population. If 70 per cent of the alleles for a gene are the dominant form and 30 per cent are recessive, the allele frequencies are 0.7 and 0.3.

The Hardy-Weinberg principle

The Hardy-Weinberg principle describes the allele and genotype frequencies expected in a population that is not evolving. It acts as a null model: if real frequencies match the prediction, no evolutionary force is acting on that gene; if they do not, something is changing the population.

For a gene with two alleles, let p be the frequency of the dominant allele and q the frequency of the recessive allele. Then:

p + q = 1 (the allele frequencies add to 1).
p squared + 2pq + q squared = 1 (the genotype frequencies add to 1).

Here p squared is the frequency of homozygous dominant individuals, 2pq is the frequency of heterozygotes, and q squared is the frequency of homozygous recessive individuals.

The principle only holds under five conditions: no mutation, no natural selection, a very large population (no genetic drift), no gene flow (no migration), and random mating. Because these are rarely all true, the value of the principle is in showing when a population is changing.

Forces that change allele frequencies

Five processes can shift allele frequencies and so cause evolution:

Mutation: introduces new alleles into the gene pool. It is the ultimate source of variation but changes frequencies slowly on its own.
Natural selection: favours alleles that increase fitness, raising their frequency over generations.
Genetic drift: random changes in allele frequency due to chance, strongest in small populations where chance events can remove or fix alleles. The founder effect (a new population started by a few individuals) and the bottleneck effect (a sharp population crash) are examples that reduce genetic diversity.
Gene flow (migration): movement of alleles between populations as individuals or gametes move, which tends to make populations more similar.
Non-random mating: when mate choice depends on traits, certain genotype combinations become more common, changing genotype frequencies.

Genetic diversity and survival

Populations with high genetic diversity have more variation for selection to act on, so they are more likely to include individuals that can survive environmental change. Small populations lose diversity through drift and inbreeding, which raises extinction risk. This is why conservation programs try to maintain large, genetically varied populations and gene flow between them.

Exam-style practice questions

Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

TCE 20247 marksIn a population in Hardy-Weinberg equilibrium, a recessive condition affects

1

400

individuals. Using

p^2 + 2pq + q^2 = 1

and

p + q = 1

, calculate the frequency of the recessive allele

q

, the frequency of the dominant allele

p

, and the frequency of heterozygous carriers. State two conditions that must hold for the population to be in equilibrium.

Show worked answer →

A 7 mark answer works through all three frequencies and states two conditions.

Recessive allele $q$: Affected (homozygous recessive) frequency $q^2 = \dfrac{1}{400} = 0.0025$ . So $q = \sqrt{0.0025} = 0.05$ .
Dominant allele $p$: $p = 1 - q = 1 - 0.05 = 0.95$ .
Carriers ( $2pq$ ): $2pq = 2 \times 0.95 \times 0.05 = 0.095$ , that is about $9.5\%$ of the population are heterozygous carriers.
Two conditions (any two): No mutation; no migration (no gene flow); random mating; very large population (no genetic drift); no natural selection.

Markers reward $q = 0.05$ , $p = 0.95$ , carrier frequency $0.095$ , and two valid equilibrium conditions.

TCE 20225 marksExplain how genetic drift and gene flow each change allele frequencies in a population, and explain why genetic drift has a greater effect on a small population than a large one.

Show worked answer →

A 5 mark answer explains both mechanisms and the small-population effect.

Genetic drift: A random change in allele frequencies from chance events (such as which individuals happen to survive or reproduce). It can cause alleles to be lost or fixed by chance, not by fitness.
Gene flow: The movement of alleles between populations through migration and interbreeding. It introduces or removes alleles, tending to make populations more genetically similar.
Why drift hits small populations harder: In a small population, a chance event (for example a few deaths) removes a large proportion of the alleles, so frequencies swing sharply. In a large population the same chance event changes only a tiny fraction, so the effect averages out.

Markers reward correct mechanisms for drift (random) and gene flow (migration) and the sampling-error reasoning for small populations.