Genetic drift, gene flow and the Hardy-Weinberg principle: WACE Year 12 Human Biology Unit 4

Syllabus dot point

“Explain how genetic drift (including founder and bottleneck effects) and gene flow change allele frequencies, and use the Hardy-Weinberg principle to describe a non-evolving population”

8 min readUpdated 2 Jun 2026

Quick answer

Allele frequencies change not only by selection but by chance and by migration. Genetic drift is random change in allele frequencies, strongest in small populations, and includes the founder effect (a few individuals start a new population) and the bottleneck effect (a population is sharply reduced). Gene flow is the movement of alleles between populations. The Hardy-Weinberg principle describes the conditions under which allele frequencies would not change at all.

What this dot point is asking

WACE wants you to explain the non-selective mechanisms that change a gene pool and to understand the baseline of a non-evolving population. This complements the natural selection topic: selection is non-random and produces adaptation, while drift is random and does not.

Genetic drift

Genetic drift is a change in allele frequencies due to random chance rather than to differences in fitness. In every generation, which individuals happen to survive and reproduce, and which alleles happen to be passed on, involves an element of luck. In a large population these chance effects average out, but in a small population they can swing allele frequencies sharply from one generation to the next, even eliminating or fixing an allele regardless of whether it is beneficial.

Drift is random and matters most in small populations

Genetic drift changes allele frequencies by chance, not by fitness, so it does not produce adaptation. Its effect is large in small populations, where a single chance event can substantially change the gene pool, and small in large populations, where chance evens out. This is the opposite of natural selection, which is non-random and can act in populations of any size.

The founder effect

The founder effect occurs when a small group breaks away from a larger population to start a new one. The founders carry only a chance sample of the original population's alleles, so the new gene pool may differ markedly from the original, and some alleles may be over-represented while others are absent. This is why some isolated human populations have unusually high frequencies of particular alleles or genetic conditions.

The bottleneck effect

The bottleneck effect occurs when a population is drastically reduced in size by a disaster, disease or hunting, so that only a few individuals survive. The survivors carry only a chance sample of the original alleles, so genetic variation is reduced and the surviving allele frequencies may differ from the original. Even if the population later recovers in number, its genetic diversity stays low, which can make it more vulnerable.

Gene flow

Gene flow (migration) is the movement of alleles between populations when individuals or their gametes move and breed in a new population. Immigration adds alleles to a gene pool and emigration removes them. Gene flow tends to make separate populations more genetically similar and can introduce new alleles to a population. When gene flow is blocked (by isolation), populations are free to diverge, which links to speciation.

Gene flow

Gene flow is the transfer of alleles from one population to another through the movement and breeding of individuals or the dispersal of gametes. It tends to reduce genetic differences between populations and can introduce new alleles, so a lack of gene flow allows populations to diverge genetically over time.

The Hardy-Weinberg principle

The Hardy-Weinberg principle describes a theoretical population in which allele frequencies do not change from generation to generation, meaning no evolution is occurring. This genetic equilibrium holds only if five conditions are met: a very large population (no drift), no mutation, no gene flow (no migration), random mating, and no natural selection. Because real populations rarely meet all five, the principle is used as a baseline: if observed allele frequencies differ from the Hardy-Weinberg expectation, one of the conditions is broken and the population is evolving.

Using Hardy-Weinberg as a baseline

Why a frequency change signals evolution

The Hardy-Weinberg principle predicts that, with no drift, mutation, gene flow, selection or non-random mating, the allele frequencies in a gene pool stay constant generation after generation. So if you measure an allele at one frequency in a population and find a different frequency a few generations later, at least one Hardy-Weinberg condition has been violated and the population is evolving. To answer such a question, name which condition is most likely broken given the scenario: a small population suggests genetic drift, immigration suggests gene flow, and a survival advantage suggests natural selection. The principle itself describes a population that is not evolving; departures from it reveal that evolution is happening.

How this maps to the exam

Expect questions that ask you to distinguish genetic drift from natural selection, identify the founder or bottleneck effect from a scenario, explain how gene flow changes a gene pool, or state the conditions of the Hardy-Weinberg principle and use it to argue whether a population is evolving. You are unlikely to need heavy calculation, but you should understand the principle conceptually.

Treating genetic drift as a form of selection or confusing founder and bottleneck

Two errors recur. First, describing drift as favouring beneficial alleles; drift is random and does not depend on fitness, so it does not produce adaptation. Second, confusing the founder effect (a few individuals leave to start a new population) with the bottleneck effect (an existing population is sharply reduced). Both reduce variation by leaving a chance sample of alleles, but the cause differs, so identify the scenario carefully.

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-style5 marks

In a population in Hardy-Weinberg equilibrium, a recessive condition affects 4 percent (0.04) of individuals. Calculate the frequency of the recessive allele, the frequency of the dominant allele, and the percentage of the population that are heterozygous carriers. Show your working.

Show worked answer →

A 5 mark calculation needs the equations, substitution and correct values.

Equations: $p + q = 1$ and $p^2 + 2pq + q^2 = 1$ , where $q^2$ is the frequency of the homozygous recessive (affected) genotype.
Recessive allele frequency: $q^2 = 0.04$ , so $q = \sqrt{0.04} = 0.2$ .
Dominant allele frequency: $p = 1 - q = 1 - 0.2 = 0.8$ .
Heterozygous carriers: $2pq = 2 \times 0.8 \times 0.2 = 0.32$ , which is 32 percent of the population.

Markers reward taking the square root of $q^2$ to find $q$ , using $p = 1 - q$ , and the $2pq$ carrier calculation with working shown.

WACE 2024-style4 marks

Distinguish between the founder effect and the bottleneck effect as forms of genetic drift, and explain why both reduce genetic variation in the resulting population.

Show worked answer →

A 4 mark answer needs the contrast plus the shared reason variation falls.

Founder effect: A small number of individuals leave a population to start a new one, so the new population's allele frequencies are determined by chance from that small sample.
Bottleneck effect: A large population is suddenly reduced (for example by a disaster), and the survivors are a chance sample of the original.
Why variation falls: In both cases the small surviving or founding group carries only a fraction of the alleles present in the original population, and some alleles are lost by chance, so the resulting population has reduced genetic variation.

Markers reward the new-population versus crash distinction and the small-sample loss of alleles explanation.

Practise this

Sources & how we know this

Human Biology ATAR course: syllabus and support materials — SCSA
Past ATAR course examinations — SCSA