IB Biology · Theme D: Continuity and change · SL and HL
D4.1 Natural selection
A one-page summary of D4.1 Natural selection, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What processes can cause changes in allele frequencies within a population?
What is the role of reproduction in the process of natural selection?
What D4.1 covers
Natural selection as a mechanism
- D4.1.1Natural selection is one continuous mechanism, not four separate facts
Variation and selection pressures
- D4.1.2Mutation and sexual reproduction generate variation
- D4.1.3Overproduction of offspring causes competition for resources
- D4.1.4Abiotic factors act as selection pressures too
- D4.1.5Individuals differ in fitness, and compete within their species
- D4.1.6Only heritable traits can evolve by natural selection
- D4.1.7Sexual selection is driven by mate choice, not survival
- D4.1.8Endler modelled selection by experimentally controlling predation
- D4.1.8Removing predation let colour increase over 15 generations
The gene pool and allele frequencies
- D4.1.9 · HLThe gene pool is all the alleles in a population
- D4.1.10 · HLGeographically isolated populations can have different allele frequencies
- D4.1.11 · HLNeo-Darwinism: genetics explains how selection changes allele frequency
Types of selection
- D4.1.12 · HLDirectional, disruptive and stabilizing selection shape a trait differently
The Hardy–Weinberg equation
- D4.1.13 · HLThe Hardy–Weinberg equation predicts genotype frequencies
- D4.1.13 · HLWorked example: calculating allele frequencies from genotype counts
- D4.1.14 · HLFive conditions must hold for Hardy–Weinberg equilibrium
Artificial selection
- D4.1.15 · HLArtificial selection requires deliberate human choice of which individuals breed
D4.1 Natural selection: summary
The mechanism
- Heritable variation (mutation + sexual reproduction) → a selection pressure acts → differential fitness (survival and reproduction) → favourable alleles more frequent. Repeats every generation. A real paradigm shift replacing Lamarckism.
Pressures and examples
- Overproduction → competition for resources (carrying capacity). Abiotic pressures (temperature, drought, pH, salinity) are density-independent. Only heritable traits evolve — acquired traits (Lamarckism) do not.
Sexual selection & modelling
- Sexual selection: mate choice, not survival — birds of paradise. Endler’s guppies: transplant experiment, low predation → colour rises, high predation → colour falls, both from 8 spots.
HL · Gene pool & allele frequency
- Gene pool = all alleles of all genes in a population. Allele frequency change = evolution. Isolated populations diverge (reduced gene flow). Neo-Darwinism = Darwin + Mendelian genetics.
HL · Selection types & Hardy–Weinberg
- Directional (mean shifts), disruptive (splits in two), stabilizing (narrows, same mean) — all change frequencies. p + q = 1; p² + 2pq + q² = 1 needs no mutation/selection/migration, random mating, large population.
HL · Artificial selection
- Artificial selection: humans deliberately choose which individuals breed (crop/animal breeding, e.g. Brassica oleracea). Antibiotic resistance is NATURAL selection — humans create the pressure but do not choose survivors.
Key terms
- Natural selection
- The mechanism by which heritable variation, acted on by a selection pressure, causes differential survival and reproduction, changing allele frequencies over generations.
- Mutation
- A change in the DNA base sequence; the only source of genuinely new alleles.
- Carrying capacity
- The maximum population size an environment’s resources can sustain.
- Selection pressure
- Any factor, biotic or abiotic, that causes differential survival or reproduction.
- Fitness
- The relative contribution of a genotype to the next generation: survival AND reproduction.
- Intraspecific competition
- Competition between individuals of the SAME species for the same limited resources.
- Heritable
- Controlled by alleles that can be passed to offspring through gametes.
- Sexual selection
- Selection arising from competition for mates and mate choice.
- Gene pool
- All the genes, and every different allele of each gene, present across a population.
- Allele frequency
- The proportion of one specific allele among all alleles for that gene in a population.
- Neo-Darwinism
- The integration of Darwin’s natural selection with Mendelian and population genetics.
- Directional selection
- Selection favouring one extreme phenotype, shifting the population mean toward it.
- Disruptive selection
- Selection favouring both extremes over the intermediate phenotype.
- Stabilizing selection
- Selection favouring the intermediate phenotype, narrowing variation around the same mean.
- Hardy–Weinberg equation
- p² + 2pq + q² = 1 and p + q = 1; predicts genotype frequencies under equilibrium.
- Hardy–Weinberg equilibrium
- A population with no mutation, no selection, no migration, random mating and a large population size.
- Artificial selection
- Selection where humans deliberately choose which individuals breed, based on a desired trait.
Sample exam questions
Three of the 51 multiple-choice questions for D4.1. Try each one before opening the answer.
Question 1. Which statement best describes the role of natural selection as an evolutionary mechanism?
- It causes an individual organism's DNA to change in response to environmental stress within its own lifetime
- It causes cumulative, gradual changes in allele frequencies over many generations, based on differential survival and reproduction
- It guarantees that every population will eventually evolve toward a single 'ideal' phenotype
- It only operates over short timescales of a few generations before stopping
Show the answer
Answer: B. Natural selection produces gradual, cumulative allele-frequency change across many generations, driven by differential survival and reproduction — it has operated continuously over billions of years.
Question 2. What is meant by the term 'paradigm shift' in the context of Darwin's theory of natural selection?
- A single experiment that proves a theory correct beyond any doubt
- A minor adjustment to an existing theory that leaves its core framework unchanged
- The process by which a theory is renamed without any change in content
- A fundamental change in the explanatory framework accepted by the scientific community, replacing an older model with a new one
Show the answer
Answer: D. A paradigm shift is a fundamental change in the scientific community's explanatory framework — Darwin's mechanism replaced the earlier Lamarckian model of inheritance of acquired characteristics.
Question 3. Elaborate courtship traits favoured by sexual selection can persist even if they slightly reduce an individual's survival chances because:
- Sexual selection favours traits that increase mating success, which can outweigh a modest survival cost
- Sexual selection has no effect on allele frequency
- Predators are always attracted to dull, camouflaged individuals instead
- Traits favoured by sexual selection are never heritable
Show the answer
Answer: A. As long as the reproductive benefit of a trait (more successful mating) outweighs its survival cost, the trait can still increase in frequency — fitness combines both survival and reproduction.
Practise D4.1
Study notes, every question and full markschemes for D4.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.