SACE Biology — Stage 2
Natural Selection — SACE Biology — Stage 2 Flashcards & Quiz
Natural selection is the mechanism by which populations evolve over time through differential survival and reproduction of individuals with advantageous traits. SACE Biology Stage 2 expects you to apply the conditions for selection to named case studies, distinguish the three modes (directional, stabilising, disruptive), and evaluate evidence for evolution from fossils, biogeography and molecular phylogenetics.
Key Points
- Four conditions (VISA): Variation in traits, traits must be Inheritable, a Selection pressure exists, and differential reproductive Advantage results.
- Natural selection acts on phenotypes but changes genotype frequencies across generations — it does not create new alleles (mutation does).
- Directional selection: favours one extreme, shifts the mean (e.g. antibiotic resistance).
- Stabilising selection: favours intermediate phenotypes, reduces variation (e.g. human birth weight).
- Disruptive selection: favours both extremes, can lead to speciation if reproductive isolation develops.
- Evidence for evolution: fossil record, comparative anatomy (homologous structures), biogeography, molecular phylogenetics (DNA/protein similarity).
Common Mistakes to Avoid
- Saying organisms "want to" or "try to" evolve — natural selection has no goal or intention.
- Claiming natural selection creates new alleles — it selects from existing variation; mutations create new alleles.
- Using "fitness" as strength or size — biological fitness means reproductive success.
- Confusing the three selection modes — draw the bell curve to remember how each shifts it.
- Forgetting that selection pressure must be consistent over generations for directional change to accumulate.
Exam Strategy
SACE Stage 2 natural selection questions give you a named scenario (peppered moths, Darwin's finches, antibiotic resistance) and ask you to apply VISA and identify the selection mode. Structure: (1) describe the variation in the starting population, (2) identify the selection pressure, (3) explain which phenotype is favoured and why, (4) describe the change in allele frequency across generations, (5) name the mode. Always link to specific evidence.
Sample Flashcards
Q1: Explain how natural selection results in evolution.
Individuals in a population vary, and some variation is inherited. More offspring are produced than can survive, so there is competition. Individuals with alleles that suit the environment (selection pressure) are more likely to survive and reproduce, passing those alleles on. Over generations, the frequency of advantageous alleles in the population increases — this change in allele frequency is evolution. The environment selects individuals; the population does not choose to “adapt”.
Q2: How does industrial melanism in peppered moths (Biston betularia) illustrate natural selection?
Peppered moths occur in a pale form and a dark (melanic) form. During the Industrial Revolution in England, soot darkened tree trunks and killed pale lichens, so pale moths were more visible to bird predators while dark moths were camouflaged. The dark form increased in frequency in industrial areas. After clean-air laws reduced pollution, trunks became lighter again and the pale form increased.
Sample Quiz Questions
Q1: In natural selection, individual organisms change during their lifetime to adapt to their environment.
Answer: FALSE
Individuals do not adapt by choice; individuals with suitable inherited variations survive and reproduce more, changing allele frequencies in the population.
Q2: The dark form of the peppered moth increased in frequency in polluted industrial areas because it was better camouflaged.
Answer: TRUE
On soot-darkened trunks, dark moths were less visible to birds, so they survived and reproduced more.
Revision Tip
VISA is a reusable framework — drill a Revizi deck that gives you a scenario and asks you to apply VISA systematically, varying the modes across examples.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards · 2 sample quiz questions