WACE Human Biology Unit 4 deep dive: human variation and evolution for the 2026 exam

WACEHuman BiologyStudy guide17 min readUpdated 23 Sep 2026

Revision deep dive for WACE Human Biology Unit 4: DNA and the genetic code, sources of variation, inheritance patterns including multiple alleles and sex linkage, population genetics and Hardy-Weinberg, natural selection, drift and gene flow, speciation, primates, hominin evolution, evidence and dating, and the spread of modern humans, with worked examples and links to every Unit 4 dot point.

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How Unit 4 fits the exam
1. DNA, genes and the genetic code
2. Sources of variation
3. Patterns of inheritance
4. Population genetics
5. Natural selection and speciation
6. Primates and hominins
7. Evidence, dating and the spread of modern humans
Common mistakes
Check your knowledge

How Unit 4 fits the exam

Unit 4 (human variation and evolution) is the other half of the WACE Human Biology exam content. It mixes calculation-style questions (genetic crosses, allele frequencies, half-lives) with evidence-based explanations (hominin trends, the spread of modern humans). This deep dive links every Unit 4 dot point on the site. For exam format and timing, see the exam strategy guide.

1. DNA, genes and the genetic code

Dot point: DNA, genes and the genetic code.

A gene is a section of DNA that codes for a polypeptide; alleles are alternative forms of a gene. The genotype is the allele combination; the phenotype is the observable trait, shaped by genotype and environment. The genetic code is read in triplets (codons); it is universal and degenerate (several codons can code for the same amino acid).

2. Sources of variation

Dot point: sources of human variation.

Mutation (gene or chromosomal) is the only source of new alleles.
Crossing over in prophase I and independent assortment in metaphase I create new allele combinations.
Random fertilisation combines any of millions of genetically different gametes.

3. Patterns of inheritance

Dot point: patterns of inheritance and variation.

Multiple alleles: ABO blood groups

$I^A$ and $I^B$ are codominant; $i$ (or $I^O$ ) is recessive. A parent with genotype $I^Ai$ (blood group A) and a parent with $I^Bi$ (group B) can produce:

	$I^B$	$i$
$I^A$	$I^AI^B$ (AB)	$I^Ai$ (A)
$i$	$I^Bi$ (B)	$ii$ (O)

Each of the four blood groups has a probability of $\frac{1}{4}$ .

Sex linkage

Red-green colour blindness is X-linked recessive. A carrier mother ( $X^BX^b$ ) and a father with normal vision ( $X^BY$ ):

Daughters: $X^BX^B$ or $X^BX^b$ , so none are colour blind, but half are carriers. Sons: $X^BY$ or $X^bY$ , so each son has a $\frac{1}{2}$ chance of being colour blind.

Males are affected more often because they have only one X chromosome: a single recessive allele is expressed.

Polygenic traits (height, skin colour) are controlled by many genes and show continuous variation, often further shaped by the environment. Single-gene traits usually show discontinuous variation.

4. Population genetics

Dot points: population genetics and change, genetic drift, gene flow and the Hardy-Weinberg principle.

The gene pool is all the alleles in a population. Evolution is a change in allele frequencies over generations, driven by natural selection, genetic drift, gene flow and mutation.

Hardy-Weinberg principle

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

p^2

: homozygous dominant,

2pq

: heterozygous,

q^2

: homozygous recessive.

Carrier frequency of a recessive condition

A recessive condition affects 1 in 2500 people. So $q^2 = \dfrac{1}{2500} = 0.0004$ and $q = 0.02$ . Then $p = 0.98$ and the carrier frequency is $2pq = 2(0.98)(0.02) = 0.0392$ , about 1 in 26 people.

Most copies of the recessive allele are hidden in carriers, which is why selection against a rare recessive allele is slow.

Genetic drift is a random change in allele frequency, strongest in small populations. The founder effect and bottlenecks are special cases. Gene flow (migration between populations) makes gene pools more similar.

5. Natural selection and speciation

Dot points: natural selection and types of selection, speciation and isolating mechanisms.

Directional selection favours one extreme (the mean shifts).
Stabilising selection favours the intermediate (the spread narrows); human birth mass is the classic example.
Disruptive selection favours both extremes (the distribution can become bimodal).

Allopatric speciation: a geographical barrier separates a population; different selection pressures, drift and mutation act on each group; over time reproductive isolation develops, so the groups can no longer produce fertile offspring together. Isolating mechanisms may act before fertilisation (behavioural, temporal, mechanical) or after it (hybrid inviability or sterility).

6. Primates and hominins

Dot points: primate characteristics and classification, hominin evolution.

Primate features: forward-facing eyes with stereoscopic colour vision, grasping hands with opposable thumbs, nails instead of claws, a large brain relative to body size, and extended parental care.

Trends in hominin evolution:

Bipedalism: foramen magnum moves forward and under the skull, the spine gains an S-shaped curve, the pelvis becomes shorter and broader, the femur angles inward (carrying angle), and the big toe aligns with the other toes.
Skull and brain: cranial capacity increases, the face flattens, the brow ridge reduces, the jaw becomes smaller and parabolic, and a chin appears.
Culture: from simple stone tools to more complex technologies, use of fire, art and language.

7. Evidence, dating and the spread of modern humans

Dot points: evidence for human evolution, the spread of modern humans.

Evidence comes from fossils, comparative anatomy (homologous structures), comparative biochemistry (amino acid sequences) and DNA comparisons, including mitochondrial DNA (inherited through the mother) and the Y chromosome (through the father).

Radiometric dating

Carbon-14 has a half-life of about 5730 years. A bone sample retains 25 percent of its original carbon-14. Two half-lives have passed ( $100 \to 50 \to 25$ percent), so the sample is about $2 \times 5730 = 11\,460$ years old.

Carbon-14 is only useful for relatively recent organic remains (up to about 50 000 years). Older hominin fossils are dated using isotopes with longer half-lives, such as potassium-argon dating of volcanic layers above and below the fossil.

Out of Africa (recent African origin) proposes that modern humans evolved in Africa and spread across the world, largely replacing other hominins. The multiregional hypothesis proposes that modern humans evolved in several regions from earlier populations linked by gene flow. The greater genetic diversity of African populations and the mitochondrial DNA evidence support a recent African origin, while DNA evidence of limited interbreeding with Neanderthals and Denisovans adds nuance.

Common mistakes

Where Unit 4 marks go missing

Writing that a mutation happens "because the organism needs it". Mutations are random; selection acts on the variation that exists.
Confusing genotype frequency ( $q^2$ ) with allele frequency ( $q$ ) in Hardy-Weinberg questions.
Forgetting that fathers pass their X chromosome to daughters, not sons.
Describing speciation without reproductive isolation, which is what defines a new species.
Stating the Out of Africa or multiregional hypothesis without evidence. Answers must cite specific evidence, such as fossil ages or mitochondrial DNA diversity.

Check your knowledge

In a population, 16 percent of people show a recessive trait. Find the frequency of the dominant allele. (Answer: $q = 0.4$ , so $p = 0.6$ .)
Which type of selection narrows the range of a trait? (Answer: stabilising selection.)
Why is mitochondrial DNA useful for tracing maternal ancestry? (Answer: it is inherited from the mother only and does not recombine.)

Then try the Unit 4 practice quiz.

Sources & how we know this

Human Biology ATAR course Year 12 syllabus — School Curriculum and Standards Authority (SCSA)
Human Biology past ATAR course examinations — School Curriculum and Standards Authority (SCSA)
Human Biology ATAR course: syllabus and support materials — SCSA
Past ATAR course examinations — SCSA

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