VCE Biology — Unit 4 AOS 1
Natural Selection — Flashcards & Quiz
Natural selection is the central mechanism of evolutionary change in VCE Biology Unit 4. You need to compare directional, stabilising and disruptive selection on a phenotype distribution, list the four conditions for selection (variation, inheritance, selection pressure, differential fitness), and explain how it acts on phenotypes rather than directly on genotypes. VCAA frequently tests sexual selection and balancing mechanisms such as heterozygote advantage — the sickle cell example is a must-know context.
Key Points
- Four conditions (VISA): Variation, Inheritance, Selection pressure, differential Advantage — all four required for natural selection to act.
- Selection acts on phenotypes (the observable traits), but changes genotype frequencies in the next generation.
- Modes: directional (favours one extreme, shifts mean), stabilising (favours mean, narrows range), disruptive (favours both extremes, can drive speciation).
- Heterozygote advantage (balancing selection): HbᴬHbˢ heterozygotes have resistance to malaria + mild sickle-cell symptoms — classic VCAA example.
- Sexual selection is a subtype: traits that reduce survival can still increase mating success (peacock tails, moose antlers).
- Common misconception: evolution has no goal — natural selection cannot produce "perfect" organisms, only ones better adapted to current pressures.
Common Mistakes to Avoid
- Claiming natural selection "makes" organisms fitter — it SELECTS from existing variation; it doesn't create new traits.
- Forgetting that natural selection acts on PHENOTYPES but changes GENOTYPE frequencies.
- Mixing up directional, stabilising and disruptive selection modes.
- Using "fitness" as strength or survival — fitness in biology = reproductive success (offspring left).
- Applying natural selection to individuals — it operates on POPULATIONS across generations.
Exam Strategy
VCAA Unit 4 AOS 1 natural selection questions ask you to apply the process to a named scenario. Structure: (1) identify variation in the starting population, (2) name the selection pressure, (3) explain differential survival/reproduction, (4) describe the shift in allele frequencies across generations. The three modes (directional, stabilising, disruptive) each correspond to a characteristic distribution shape — be ready to sketch them.
Sample Flashcards
Q1: Compare directional, stabilising and disruptive selection.
Directional selection: favours one extreme phenotype, shifts the mean (e.g., increasing beak size during drought). Stabilising selection: favours intermediate phenotypes, reduces variation (e.g., average birth weight in humans). Disruptive selection: favours both extreme phenotypes over intermediate, can increase variation and may lead to speciation (e.g., beak sizes in seed-crackers: very large or very small, not medium).
Q2: What conditions are necessary for natural selection to occur?
Four conditions (VISA): 1) Variation — individuals differ in their traits. 2) Inheritance — traits must be heritable (genetic basis). 3) Selection pressure — environmental factor that differentially affects survival/reproduction. 4) Advantage — individuals with certain traits have higher fitness (more offspring). When these conditions are met, advantageous alleles increase in frequency over generations. Natural selection acts on phenotypes but changes genotype frequencies.
Q3: What is sexual selection and how does it differ from natural selection?
Sexual selection is a form of natural selection where traits are favoured because they increase mating success rather than survival. Two types: 1) Intersexual selection (mate choice) — one sex chooses mates with attractive traits (e.g., peahen choosing elaborate peacock tail). 2) Intrasexual selection (competition) — same-sex competition for mates (e.g., male elephant seals fighting for dominance). Sexual selection can produce traits that reduce survival but increase reproductive success.
Q4: What is the heterozygote advantage and how does it maintain genetic variation?
Heterozygote advantage (overdominance) occurs when the heterozygous genotype (Aa) has higher fitness than either homozygote (AA or aa). This maintains both alleles in the population because selection favours heterozygotes, preventing either allele from being eliminated. This is a form of balancing selection that preserves genetic diversity.
Q5: What is coevolution?
Coevolution is the reciprocal evolutionary change between two interacting species, where each species acts as a selective pressure on the other. Types: 1) Mutualistic — both species benefit (e.g., flowers and pollinators evolving matching structures). 2) Antagonistic — evolutionary arms race (e.g., predator develops speed, prey develops faster escape; parasite evolves immune evasion, host evolves stronger immunity). Changes in one species drive changes in the other, creating a feedback loop.
Sample Quiz Questions
Q1: Stabilising selection favours individuals with extreme phenotypes over those with intermediate phenotypes.
Answer: FALSE
STABILISING selection favours INTERMEDIATE phenotypes and reduces variation. DISRUPTIVE selection favours extreme phenotypes. Stabilising selection narrows the distribution around the mean.
Q2: Natural selection acts directly on an organism's genotype.
Answer: FALSE
Natural selection acts on PHENOTYPES (observable traits). It is the phenotype that determines survival and reproduction. However, because phenotypes are influenced by genotypes, selection indirectly changes allele frequencies.
Q3: Heterozygote advantage maintains both alleles in a population because the heterozygous genotype has the highest fitness.
Answer: TRUE
When heterozygotes (Aa) have higher fitness than either homozygote (AA or aa), natural selection maintains both alleles in the population. Neither allele can be eliminated because the fittest genotype requires both.
Revision Tip
Selection modes and their distribution shapes are visual — use Revizi flashcards that pair a graph shape with its mode (directional shifts, stabilising narrows, disruptive splits).
Related Concepts
Last updated: March 2026 · 5 flashcards · 6 quiz questions