IB DP Biology: Natural selection and allele frequencies: Hardy-Weinberg and artificial selection (DP Biology) – Revision Notes

These revision notes follow the IB Biology guide, first assessment 2025, section D4.1 (Natural selection), statements D4.1.1 to D4.1.15. Statements D4.1.9 to D4.1.15 are HL only; the rest are SL and HL. For full explanations and worked examples, read the study guide, then test yourself with the practice set.

Also useful: the IB Biology hub, the printable checklist and the free diagnostics.

Key terms

Term	Meaning
Natural selection	Differential survival and reproduction of individuals with heritable traits, changing a population over generations
Paradigm shift	A fundamental change in the accepted framework of a science (Lamarckism to Darwin)
Mutation	A change in DNA base sequence; makes new alleles
Carrying capacity	The largest population an environment can sustain
Density-independent factor	Affects survival regardless of population size, such as a frost
Intraspecific competition	Competition between members of the same species
Fitness	Survival value plus reproductive potential of a genotype
Heritable	Passed on through alleles in gametes
Sexual selection	Selection by mate choice or competition for mates
Gene pool (HL only)	All genes and all their alleles in a population
Allele frequency (HL only)	Proportion of gene copies that are a given allele
Artificial selection (HL only)	Humans choose which individuals breed

The logic of natural selection

1. Populations show variation, from mutation and sexual reproduction.
2. More offspring are produced than the environment can support, so there is a struggle for resources.
3. Individuals differ in adaptation, so some survive and breed more.
4. If the traits are heritable, their alleles are more common in the next generation.
5. Repeated over many generations, the population changes.

Test each link before you write an answer. If any step is missing, the explanation is incomplete.

Sources of variation (D4.1.2)

• Mutation makes new alleles. Only changes in gamete-forming cells matter for inheritance.
• Meiosis: crossing over and independent assortment give new combinations.
• Random fertilisation: any sperm can fuse with any egg.
• Sexual reproduction does not create new alleles, only new combinations.

Selection pressures (D4.1.3 to D4.1.5)

• Biotic: food, predators, disease, mates, nesting sites.
• Abiotic: temperature, drought, salinity, pH, fire. Extremes such as frost are density-independent.
• Competition for food or space is density-dependent.
• Individuals do not choose to adapt. Variation exists first and the environment filters it.
• Fitness is judged by the number of offspring that survive to breed.

Heritable or not? (D4.1.6)

Trait	Heritable?	Reason
Bacterial resistance gene	Yes	In the DNA, passed on
Strong arm muscles from lifting	No	Environmental effect, not in gene base sequence
Lost tail from an accident	No	Injury does not alter the gametes
Eye colour	Yes	Alleles

Sexual selection and Endler’s guppies (D4.1.7 and D4.1.8)

• Traits that signal health, such as bright colour or elaborate displays, improve mating success.
• Costs include predation. The final trait balances mating benefit against survival cost.
• Birds of paradise: females choose, males with the most striking plumage and displays mate more.
• Guppy pattern: fewer predators means brighter males; more predators means duller males.
• Reading a table: find the starting value, compute percentage change = (final − start) ÷ start × 100, then link the direction to the pressure that was varied.

HL only: gene pools and allele frequency (D4.1.9 to D4.1.11)

Method for allele frequency from counts

1. Total alleles = 2 × number of individuals.
2. Count copies of one allele (homozygote counts twice, heterozygote once).
3. Divide by total alleles.
4. Frequency of the other allele = 1 − that value.

Worked reminder. 18 AA, 16 Aa, 6 aa: 40 individuals, 80 alleles. A = (2 × 18 + 16) ÷ 80 = 52 ÷ 80 = 0.65, so a = 0.35.

Isolated populations differ because there is no gene flow, conditions differ and mutations arise separately. When using a database, note population, sample size and allele, and include a human example.

Neo-Darwinism: natural selection plus genetics. Evolution is a change in allele frequency in the gene pool.

HL only: types of selection (D4.1.12)

Type	Favours	Result
Directional	One extreme	Mean shifts
Stabilizing	Mean	Spread narrows
Disruptive	Both extremes	Two peaks

All three alter allele frequencies.

HL only: Hardy-Weinberg (D4.1.13 and D4.1.14)

Quantity	Symbol	Value
Frequency of dominant allele	p	1 − q
Frequency of recessive allele	q	1 − p
Homozygous dominant	p²
Heterozygous	2pq
Homozygous recessive	q²

Equations: p + q = 1 and p² + 2pq + q² = 1.

Method in steps

1. Identify what is given: usually the recessive phenotype frequency, which equals q².
2. Square-root to get q.
3. p = 1 − q.
4. Compute p², 2pq, q².
5. Check that the three genotype frequencies add to 1.
6. For numbers of individuals, multiply by the population size.

Worked reminder. A recessive trait occurs in 0.49% of a population. q² = 0.0049, q = 0.07, p = 0.93. Carriers = 2 × 0.93 × 0.07 = 0.1302, about 13.0%.

Worked reminder: effect of selection. Start with q = 0.3, so p = 0.7 and genotypes AA 0.49, Aa 0.42, aa 0.09. If aa individuals have a relative survival of 0.4, aa becomes 0.09 × 0.4 = 0.036. Total = 0.49 + 0.42 + 0.036 = 0.946. New q = (0.42 ÷ 2 + 0.036) ÷ 0.946 = 0.246 ÷ 0.946 = 0.260 (3 s.f.). The recessive allele has fallen from 0.3 to 0.26 in one generation, and it falls slowly because it is hidden in heterozygotes.

If you are given the number of heterozygotes or homozygous dominants only, use the matching term: p² gives p directly.

Conditions for genetic equilibrium: large population; random mating; no mutation; no migration; no selection. If observed genotypes differ from expected, at least one is unmet. Example: too few heterozygotes suggests non-random mating or selection against heterozygotes.

HL only: artificial selection (D4.1.15)

• Humans pick parents with desired traits in crops and domesticated animals, and repeat each generation.
• Examples: higher-yielding crops, dogs bred for behaviour, cattle for milk.
• Antibiotic resistance in bacteria is natural selection: the antibiotic is a selection pressure, but nobody chooses which bacteria breed.

Frame for an explain question

For any “explain how a population changed” question, write four short linked points: there was heritable variation; more offspring were produced than survive; individuals with the advantageous trait survived and reproduced more; so the allele became more frequent. Add the named selection pressure from the stem. For HL, finish by stating the change in allele frequency.

Must-know distinctions

• Natural selection (environment filters) against artificial selection (humans choose).
• Mutation (new alleles) against sexual reproduction (new combinations).
• Acquired (not heritable) against inherited characteristics.
• Density-dependent against density-independent factors.
• Natural selection (survival) against sexual selection (mating success).
• Allele frequency against genotype frequency.

Quick self-test

1. What does the term paradigm shift mean? Give the example from this unit.
2. Name the two sources of variation and what each produces.
3. Give one density-independent abiotic selection pressure.
4. Define fitness.
5. Why is a weightlifter’s muscle size not inherited?
6. In an experiment, predators are removed from a guppy pool. Predict the change in male colour.
7. (HL only) Define gene pool.
8. (HL only) Which type of selection favours both extremes?
9. (HL only) A recessive trait has frequency 0.0049. What proportion are carriers?
10. (HL only) In 600 individuals with p = 0.8, how many are heterozygous?
11. (HL only) State the five Hardy-Weinberg conditions.
12. (HL only) Bacteria become resistant after antibiotic use. Natural or artificial selection?

Answers

1. A fundamental change in the accepted framework of a field of science. Darwin’s theory replaced Lamarckism.
2. Mutation makes new alleles. Sexual reproduction makes new combinations of alleles.
3. A severe frost (also drought or fire).
4. How well a genotype does at surviving and reproducing: its survival value and reproductive potential.
5. It was gained through use, an environmental effect, not coded in the base sequence of genes, so not passed on in gametes.
6. Males become brighter or more colourful over generations because female preference is no longer opposed by predation.
7. The complete set of genes in a population, including every allele of each.
8. Disruptive selection.
9. q = 0.07, p = 0.93, 2pq = 0.1302, so about 13.0%.
10. q = 0.2, 2pq = 0.32, 0.32 × 600 = 192.
11. Large population, random mating, no mutation, no migration, no selection.
12. Natural selection.

Where marks are usually lost

• Saying individuals evolve or adapt during their lives, rather than populations changing.
• Writing “survival of the fittest” without linking to reproduction and heritability.
• Omitting the step that variation is heritable.
• Calling mutation the source of new combinations; it makes new alleles.
• Using p² or q instead of q² for the recessive phenotype.
• Forgetting to take the square root of q².
• Not checking that genotype frequencies sum to 1.
• Describing a graph without naming the selection type and the direction of change.
• Describing antibiotic resistance as artificial selection.
• Giving percentage change without the sign or without dividing by the starting value.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section D4.1 Natural selection.