VCE Biology Unit 4 evolution case studies: the 2026 guide

What this guide is for

VCE Biology Unit 4 AoS 2 covers evolution and the origin of species. The SAC and the exam expect knowledge of specific case studies. This guide covers the four most-tested case studies (peppered moth, antibiotic resistance, Darwin's finches, human evolution) and the writing moves that secure top marks.

Mechanisms of evolution

Five mechanisms in the VCAA syllabus:

Natural selection. Differential survival and reproduction based on heritable variation. Requires: variation, heritability, fitness differences, time.

Genetic drift. Random changes in allele frequencies. Most significant in small populations. Founder effect and bottleneck are special cases.

Gene flow. Migration of individuals (and their genes) between populations.

Mutation. Random changes in DNA. The source of new genetic variation.

Non-random mating. When individuals do not choose mates randomly, allele frequencies change.

Natural selection is the most-tested. Practice answering "how does natural selection explain X" questions.

Case study 1: Peppered moth

Species. Biston betularia, common in Britain.

Pre-Industrial Revolution. Light (typica) form dominant. Bark covered with light-coloured lichens; dark moths conspicuous to predators (birds).

Industrial Revolution. Pollution killed lichens; bark darkened with soot. Light moths now conspicuous; dark (carbonaria) form had a camouflage advantage.

Outcome. Dark form rose from < 1% to > 95% in industrial Manchester by 1900.

Post-Clean Air Acts. UK Clean Air Act 1956. Pollution declined. Lichens returned. Light form recovered.

Significance. Textbook example of directional natural selection in real time, driven by environmental change.

Case study 2: Antibiotic resistance

Mechanism. Bacteria evolve resistance through:

• Spontaneous mutation conferring resistance.
• Horizontal gene transfer (plasmids carrying resistance genes).
• Selection by antibiotic use (sensitive bacteria die, resistant bacteria reproduce).

MRSA. Methicillin-resistant Staphylococcus aureus. Major hospital infection. Evolved in 1960s; now widespread.

Multi-drug-resistant TB. Mycobacterium tuberculosis with resistance to multiple antibiotics. Treatment requires combinations of antibiotics over months.

Significance. Rapid evolution observable in real time (over months to years). Public health crisis. Demonstrates natural selection on accelerated timescales.

Case study 3: Darwin's finches

Setting. Galapagos Islands. Approximately 15 species of finches descended from a common ancestor about 2-3 mya.

Adaptive radiation. Each species specialised for particular food types through beak morphology.

Peter and Rosemary Grant's study. 40-year observational study on Daphne Major (1973 onwards). Documented:

• Beak size variation within species.
• Selection during drought years (large-beaked survived because only large seeds remained).
• Selection during wet years (smaller beaks favoured).
• New species formation through hybridisation.

Significance. Real-time natural selection observed; speciation in action.

Case study 4: Human evolution

Hominin timeline.

• Last common ancestor with chimpanzees: ~6-7 mya.
• Australopithecus afarensis (Lucy): c. 3.2 mya. Bipedal but small-brained.
• Homo habilis: c. 2.4-1.4 mya. Larger brain, stone tools.
• Homo erectus: c. 1.9 mya - 110 kya. First out of Africa.
• Neanderthals (Homo neanderthalensis): c. 430-40 kya. Europe and Western Asia.
• Homo sapiens: c. 300 kya - present.

Out of Africa. Modern humans dispersed from Africa starting c. 70-50 kya, replacing or interbreeding with other hominins.

Interbreeding. Non-African modern humans carry 1-4% Neanderthal DNA, indicating interbreeding c. 50-60 kya.

Significance. Demonstrates the long evolutionary process producing modern humans; the complexity of hominin diversification; the role of geography and climate.

Other case studies worth knowing

• HIV evolution. Rapid mutation; evasion of immune response; multi-drug resistance.
• Coral reef bleaching. Selection for heat-tolerant coral variants.
• Pesticide resistance in insects.
• Sickle cell trait. Heterozygote advantage in malaria-endemic regions.

Writing about evolution in SACs and exams

A strong response:

1. Names the case study explicitly. Don't write generically.
2. Identifies the mechanism. Natural selection? Drift? Gene flow?
3. Specifies the heritable trait. Specifically what allele or phenotype was selected for.
4. Explains the selection pressure. What environmental factor created the differential.
5. Identifies the outcome. What allele frequencies changed and how.

A response with all five steps secures top band.

In one sentence

VCE Biology Unit 4 evolution requires knowledge of mechanisms (natural selection, genetic drift, gene flow, mutation, non-random mating) and specific case studies (peppered moth as classic directional selection, antibiotic resistance as real-time evolution, Darwin's finches as adaptive radiation, human evolution as long-term hominin diversification); strong responses name the case study, identify the mechanism, specify the trait under selection, explain the selection pressure, and identify the outcome.