WACE Biology · Unit 3
Changes in a Population's Genetic Makeup
WACE Biology ATAR Unit 3 Continuity of Life on Earth includes changes in a population's genetic makeup — the evolutionary mechanisms that reshape species over time. These 20 free flashcards and 20 true/false quiz questions cover evidence for evolution (fossil record, comparative anatomy, molecular biology, biogeography), natural selection and its four conditions, types of selection (directional, stabilising, disruptive), allopatric and sympatric speciation, genetic drift (bottleneck and founder effects), gene flow, Hardy-Weinberg equilibrium, adaptive radiation, convergent and divergent evolution, coevolution, reproductive isolation, vestigial structures, and hominin/human evolution. Every card aligns with the SCSA Biology ATAR Year 12 syllabus (Unit 3 Continuity of life on Earth).
Key Terms
- Natural Selection
- The non-random process by which heritable traits that increase survival and reproduction become more common in a population across generations. The SCSA WACE Biology ATAR course assesses application of the four conditions to specific scenarios.
- Genetic Drift
- Random change in allele frequencies due to chance sampling in small populations, producing the bottleneck and founder effects. SCSA expects students to contrast drift (random) with selection (non-random, adaptive).
- Gene Flow
- Movement of alleles between populations through migration and interbreeding. Gene flow homogenises populations and counteracts the divergence driven by drift and selection, a distinction frequently tested in WACE exams.
- Hardy-Weinberg Equilibrium
- A null model (p + q = 1; p² + 2pq + q² = 1) describing constant allele and genotype frequencies. Deviations from equilibrium signal that one or more evolutionary forces are acting on the population.
- Allopatric Speciation
- Speciation driven by geographic isolation of populations, followed by independent evolution until reproductive isolation develops. WACE responses are expected to trace the full sequence from isolation to species status.
- Adaptive Radiation
- Rapid diversification of a single ancestral species into multiple niches — the Australian marsupial radiation being a signature SCSA example used to illustrate continental biogeography and ecological opportunity.
Sample Flashcards
Q1: How does the fossil record provide evidence for evolution?
The fossil record documents the history of life by preserving remains or traces of organisms in sedimentary rock. It shows: 1) a progression from simple to complex organisms over geological time, 2) transitional fossils linking major groups, 3) the appearance and extinction of species. Radiometric dating provides absolute ages of fossils.
Q2: Explain how comparative anatomy provides evidence for evolution.
Comparative anatomy reveals: Homologous structures — similar bone structure but different functions (e.g. human arm, whale flipper, bat wing), indicating common ancestry. Analogous structures — different structure but similar function (e.g. bird wing, insect wing), indicating convergent evolution. Vestigial structures — reduced or functionless remnants of once-functional organs (e.g. human appendix, whale pelvic bones).
Q3: How does molecular biology provide evidence for evolution?
Molecular evidence compares DNA sequences, amino acid sequences and proteins across species. Closely related species share more similar sequences than distantly related species. Molecular clocks estimate divergence times based on the rate of mutation accumulation. The universal genetic code (shared by all life) supports common ancestry.
Q4: What is biogeography and how does it support evolution?
Biogeography is the study of the geographic distribution of species. It supports evolution because: 1) closely related species are often found in nearby regions, 2) island species resemble mainland species but show unique adaptations, 3) continental drift explains why similar fossils are found on now-separated continents (e.g. Gondwanan distributions).
Q5: State the four conditions required for natural selection to occur.
1) Variation — individuals in a population differ in their traits. 2) Heritability — the variation has a genetic basis that can be inherited. 3) Differential survival and reproduction — some variants are better suited to the environment and produce more offspring. 4) Overproduction — more offspring are produced than can survive, creating competition for resources.
Q6: Distinguish between directional, stabilising and disruptive selection.
Directional selection: one extreme phenotype is favoured, shifting the population mean in one direction. Stabilising selection: intermediate phenotypes are favoured, reducing variation. Disruptive selection: both extreme phenotypes are favoured over the intermediate, potentially leading to bimodal distribution and speciation.
Q7: Explain allopatric speciation and give an Australian example.
Allopatric speciation occurs when a population is geographically separated (e.g. by a river, mountain range or sea-level change). The isolated populations experience different selection pressures and genetic drift, accumulating genetic differences over time until they can no longer interbreed — they become separate species.
Q8: What is sympatric speciation and how does it differ from allopatric?
Sympatric speciation occurs within the same geographic area without physical barriers. It can occur through polyploidy (especially in plants), habitat differentiation or temporal isolation. Unlike allopatric speciation, no geographic barrier separates the populations — reproductive isolation develops while populations overlap.
Sample Quiz Questions
Q1: The fossil record provides a complete and unbroken history of all life on Earth.
Answer: FALSE
The fossil record is incomplete due to the rarity of fossilisation, destruction of fossils through geological processes, and bias toward organisms with hard body parts. It provides valuable but fragmentary evidence of evolutionary history.
Q2: Homologous structures have similar underlying anatomy but may serve different functions, indicating common ancestry.
Answer: TRUE
Homologous structures (e.g. human arm, whale flipper, bat wing) share a similar bone arrangement inherited from a common ancestor but have been modified for different functions through divergent evolution.
Q3: Analogous structures indicate that two species share a recent common ancestor.
Answer: FALSE
Analogous structures (e.g. bird wing and insect wing) have similar functions but different underlying anatomy. They result from convergent evolution — similar environmental pressures, not common ancestry.
Q4: Molecular comparisons of DNA and protein sequences can be used to construct phylogenetic trees.
Answer: TRUE
Species with more similar DNA/protein sequences are more closely related. The degree of molecular similarity reflects evolutionary distance, allowing construction of phylogenetic trees showing evolutionary relationships.
Q5: Natural selection acts on the genotype of an organism, not its phenotype.
Answer: FALSE
Natural selection acts on the PHENOTYPE (observable traits) because this is what interacts with the environment. Organisms are selected based on their physical characteristics, behaviour and physiology — which are expressions of their genotype.
Why It Matters
Changes in a population's genetic makeup is the evolutionary half of WACE Biology Unit 3 Continuity of Life on Earth. WACE exams test your ability to explain how mutation, natural selection, genetic drift and gene flow reshape allele frequencies, to apply Hardy-Weinberg equations as a null model, and to evaluate the multiple lines of evidence (fossil, anatomical, molecular, biogeographical) that support evolution. Mastering speciation mechanisms (allopatric, sympatric, polyploidy in plants), adaptive radiation, convergent vs divergent evolution and coevolution lets you interpret Australian examples — marsupial radiation, quokka drift, Banksia–honey possum coevolution — that WACE examiners routinely set as applied contexts. This unit also connects to Unit 3 heredity (DNA, meiosis, alleles) and sets up the conservation-biology framing of Unit 4.
Key Concepts
Evidence for Evolution
The fossil record, homologous vs analogous structures, molecular comparisons and biogeography each provide independent evidence for common ancestry and change over time. WACE responses that integrate multiple evidence lines score above those citing a single source.
Natural Selection
The four conditions (variation, heritability, differential survival and reproduction, overproduction) explain how populations adapt. Distinguish directional, stabilising and disruptive selection with before–after distribution diagrams.
Speciation and Reproductive Isolation
Allopatric speciation (geographic isolation) and sympatric speciation (especially polyploidy in plants) both require reproductive isolation to persist. Pre- and post-zygotic mechanisms prevent gene flow between emerging species.
Genetic Drift, Gene Flow and Hardy-Weinberg
Random allele-frequency change (drift) dominates in small populations; gene flow homogenises populations; the Hardy-Weinberg equations (p + q = 1; p² + 2pq + q² = 1) serve as a null hypothesis for detecting evolutionary forces.
Common Mistakes to Avoid
- Treating natural selection as goal-directed — selection acts on existing variation and does not “design” adaptations. SCSA marking guides penalise teleological phrasing (“the species evolved to…”) in favour of mechanism-led explanations.
- Confusing homologous with analogous structures — homologous indicates common ancestry (divergent evolution), analogous indicates convergent evolution from similar selection pressures. WACE exams test this distinction repeatedly.
- Applying Hardy-Weinberg without stating its five assumptions (no mutation, no selection, random mating, large population, no gene flow) — SCSA requires the assumptions to be named when the model is invoked.
- Describing human evolution as a linear ladder from apes to humans — hominin evolution is a branching bush with multiple coexisting species (Neanderthals, Denisovans, Homo sapiens). WACE examiners reward the nuanced branching model.
Study Tips
- Draw branching phylogenetic trees from memory using Australian examples (marsupial radiation, hominin evolution) and check against your notes.
- Practise Hardy-Weinberg problems — start from q², solve for q and p, then compute all genotype frequencies, showing working for full marks.
- Build a table comparing directional, stabilising and disruptive selection with distribution diagrams before and after selection.
- Pair each evolutionary mechanism (selection, drift, gene flow, mutation, non-random mating) with a specific Australian example for applied questions.
- Distinguish homologous (divergent) from analogous (convergent) structures with one worked example for each in your notes.
- Review past WACE exam questions on speciation and evidence for evolution and practise structured extended responses.
- Work through the practice questions in this set at least twice using spaced repetition — testing yourself repeatedly is the most effective revision strategy for long-term retention.
Related Topics
Frequently Asked Questions
What does this WACE Biology Unit 3 topic cover?
Changes in a population's genetic makeup covers evidence for evolution (fossil, anatomical, molecular, biogeographical), natural selection, speciation (allopatric and sympatric), genetic drift, gene flow, Hardy-Weinberg equilibrium, adaptive radiation and hominin evolution.
What are the four conditions required for natural selection?
Natural selection requires variation, heritability, differential survival and reproduction, and overproduction of offspring. These four conditions are a frequent short-answer question in the WACE exam.
Are these flashcards aligned to the SCSA WACE syllabus?
Yes — every flashcard and quiz question is mapped to the SCSA Biology ATAR Year 12 syllabus for Unit 3 Continuity of life on Earth (changes in a population's genetic makeup).
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the SCSA Curriculum