SACE Biology — Stage 2
Natural Selection — 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: Outline Darwin's theory of natural selection.
Natural selection requires: 1) Variation — individuals differ in heritable traits. 2) Overproduction — more offspring are produced than can survive. 3) Competition — resources are limited, creating a struggle for survival. 4) Differential survival and reproduction — individuals with advantageous traits are more likely to survive and reproduce. 5) Inheritance — advantageous traits are passed to offspring, increasing their frequency in the population over generations.
Q2: Compare stabilising, directional and disruptive selection.
Stabilising selection: favours the intermediate phenotype, reducing variation (e.g. human birth weight — very small or very large babies have lower survival). Directional selection: favours one extreme phenotype, shifting the population mean (e.g. antibiotic resistance in bacteria). Disruptive selection: favours both extreme phenotypes over the intermediate, potentially leading to speciation (e.g. beak sizes for different seed types).
Sample Quiz Questions
Q1: Natural selection acts on the phenotype of an individual, not directly on the genotype.
Answer: TRUE
Natural selection acts on observable traits (phenotype) that affect survival and reproduction. Genotype is inherited, but selection pressure is exerted through the phenotype expressed.
Q2: Organisms deliberately develop beneficial mutations in response to environmental changes.
Answer: FALSE
Mutations are random and not directed by environmental need. Natural selection then favours individuals who happen to have beneficial mutations — the mutations pre-exist, they are not caused by the environment.
Q3: Directional selection favours one extreme phenotype, shifting the population mean in one direction.
Answer: TRUE
Directional selection favours individuals at one end of the phenotypic range, causing the population mean to shift towards that extreme over generations (e.g. increasing antibiotic resistance).
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.
Last updated: March 2026 · 2 flashcards · 3 quiz questions