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

This study guide is aligned to the IB Biology guide, first assessment 2025, and covers section D4.1 (Natural selection), statements D4.1.1 to D4.1.15. Statements D4.1.1 to D4.1.8 are for SL and HL. Statements D4.1.9 to D4.1.15 are additional higher level, so they are HL only. Hardy-Weinberg calculations are therefore HL work.

Use it with the revision notes and the practice set. Related pages: the IB Biology hub, the printable checklist, the evolution and speciation guide and the free diagnostics.

What this unit covers

Statement	What you must be able to do	Level
D4.1.1	Explain natural selection as the mechanism of evolution; define a paradigm shift	SL and HL
D4.1.2	Explain how mutation and sexual reproduction produce variation	SL and HL
D4.1.3	Link overproduction of offspring and competition to selection; give limiting resources	SL and HL
D4.1.4	Give abiotic factors as selection pressures, including density-independent ones	SL and HL
D4.1.5	Use differences in adaptation, survival and reproduction, and the idea of fitness	SL and HL
D4.1.6	Explain why only heritable traits change a population	SL and HL
D4.1.7	Explain sexual selection with an example such as birds of paradise	SL and HL
D4.1.8	Interpret data from Endler’s guppy experiments	SL and HL
D4.1.9	Define the gene pool	HL only
D4.1.10	Search databases for allele frequencies, with a human example	HL only
D4.1.11	Describe selection as a change in allele frequency (neo-Darwinism)	HL only
D4.1.12	Distinguish directional, disruptive and stabilizing selection	HL only
D4.1.13	Use p + q = 1 and p² + 2pq + q² = 1	HL only
D4.1.14	State the conditions for genetic equilibrium and use departures from it	HL only
D4.1.15	Describe artificial selection and separate it from natural selection	HL only

D4.1.1 Natural selection as the mechanism of change

Natural selection is the process by which individuals with heritable traits that suit their environment survive and reproduce more than others, so those traits become more common in later generations. It acts every generation and over billions of years, which is how today’s biodiversity arose.

Before Darwin, many people accepted that species change, but the mechanism was unclear. Lamarck proposed that traits acquired in life are passed on. Darwin’s mechanism explained the same observations and replaced that view. The guide calls this a paradigm shift: a fundamental change in the accepted framework of a field of science.

D4.1.2 Where variation comes from

• Mutation is a change in the base sequence of DNA. It produces new alleles. Only mutations in cells that form gametes can be passed to offspring.
• Sexual reproduction produces new combinations of existing alleles. Crossing over and independent assortment in meiosis, then random fertilisation, mean that no two offspring from the same parents (apart from identical twins) carry the same combination.

D4.1.3 and D4.1.4 Why selection happens

Many species produce far more offspring than can survive. A fish may release thousands of eggs, yet a stable population only needs two survivors per pair. The population cannot keep growing because the environment has a carrying capacity, the largest population it can sustain. Limiting resources include food, water, light, nesting sites and space. Individuals compete for them, and those with more suitable traits are more likely to get them.

Worked example. A fish lays 2000 eggs in her life and the population stays stable, so on average 2 of her offspring replace the parents. Percentage reaching adulthood = 2 ÷ 2000 × 100 = 0.1%. The other 99.9% die, and survival is not random: it depends partly on traits.

Abiotic factors are non-living selection pressures: temperature, rainfall, salinity, pH, fire and light. Extremes such as a severe frost or a drought are density-independent: they affect survival whatever the population size. Competition for food is usually density-dependent, because it gets worse as numbers rise.

D4.1.5 Fitness

Individuals of one species differ in how well they are adapted. Competition between members of the same species (intraspecific competition) is a major source of selection because they need the same resources.

Fitness here means how well a genotype does at surviving and reproducing: its survival value together with its reproductive potential. A long-lived animal that never breeds has a fitness of zero in evolutionary terms. What counts is the number of offspring that survive to reproduce.

D4.1.6 Heritability

A trait changes a population only if it is heritable, meaning determined by alleles passed through gametes. Traits gained during life, such as the muscle of a weightlifter, a scar, or a tan, come from the environment. They are not written into the base sequence of the genes, so they are not inherited. This is the key flaw in Lamarck’s idea.

Reasoning check. A cold snap kills more slow lizards than fast ones. Allele frequencies shift in the next generation only if speed has a genetic basis. If speed came only from diet, the survivors’ offspring would show no change.

D4.1.7 Sexual selection

In many animals one sex, usually the female, chooses mates, and males compete for access. Traits that help a male win a mate are favoured even if they cost survival. Physical or behavioural traits may signal overall fitness: bright colour, a long tail or an elaborate dance show that the male is healthy enough to afford them.

Birds of paradise. Males of many species have extravagant plumage and perform long courtship displays. Females choose among them. Males with the most striking features mate more, so alleles for those features spread over generations. Survival costs, such as being more visible to predators, limit how far it goes.

D4.1.8 Modelling selection: Endler’s guppies

John Endler studied guppies in streams in Trinidad and in experimental populations. Male guppies carry bright colour spots, and females prefer bright males. Predation pushes the other way, because bright males are easier to see. Sexual selection and natural selection therefore act in opposite directions, and the balance depends on the predators.

Endler controlled the selection pressure by keeping populations in pools with different levels of predators, or none, then compared the colour patterns after several generations. Where predators were absent or weak, males became more colourful. Where dangerous predators were present, males became duller.

Interpreting data. The figures below are invented to show the pattern, not Endler’s results. The mean number of spots per male was 6.0 at the start in each tank.

Tank	Mean spots after several generations
No predators	8.1
Strong predation	4.2

No predators: (8.1 − 6.0) ÷ 6.0 × 100 = +35%. Strong predation: (4.2 − 6.0) ÷ 6.0 × 100 = −30%. With predators removed, female choice pushes colour up. With strong predators, natural selection against conspicuous males dominates. Only predator level changed, so predation is the likely cause.

D4.1.9 and D4.1.10 Gene pools and isolated populations (HL only)

A gene pool is all the genes, with all their different alleles, in a population. The allele frequency is the proportion of copies of a gene that are one particular allele.

Worked example. A diploid population of 50 plants has 30 BB, 15 Bb and 5 bb. Total alleles = 50 × 2 = 100. Copies of B = 30 × 2 + 15 = 75, so frequency of B = 75 ÷ 100 = 0.75. Copies of b = 15 + 5 × 2 = 25, so frequency of b = 0.25. Check: 0.75 + 0.25 = 1.

Geographically isolated populations often differ in allele frequency. There is little or no gene flow between them, each meets different conditions, and mutations arise independently.

Using databases (skills). Public databases such as the ALFRED allele frequency database or the 1000 Genomes browser let you search a gene and compare populations. Record the population, sample size and allele. Use at least one human example: the allele for lactase persistence is common in some populations with a history of dairy farming and rare in many others. Treat small samples with caution.

D4.1.11 Selection changes allele frequency (HL only)

Neo-Darwinism combines Darwin’s theory of natural selection with genetics: evolution is a change in allele frequency in the gene pool, driven by differences in survival and reproduction between individuals with different heritable traits.

D4.1.12 Three types of selection (HL only)

Type	Favoured	Effect on the distribution	Example
Directional	One extreme	Mean shifts toward it	Insecticide resistance in mosquitoes
Stabilizing	Intermediate	Extremes lost, spread narrows	Human birth mass
Disruptive	Both extremes	Splits towards two peaks	Bill size in seedcracker finches

All three change allele frequencies. Even stabilizing selection removes alleles that produce extremes.

D4.1.13 Hardy-Weinberg calculations (HL only)

For a gene with two alleles, let p be the frequency of the dominant allele and q the frequency of the recessive allele.

• p + q = 1
• p² + 2pq + q² = 1, where p² is the frequency of homozygous dominant, 2pq of heterozygous and q² of homozygous recessive.

Method. Start from whichever value you can find. The recessive phenotype is usually the only one that reveals a genotype, so start with q².

Worked example 1. In a beetle population, 16% are green (recessive, aa). q² = 0.16, so q = √0.16 = 0.4 and p = 1 − 0.4 = 0.6. Homozygous dominant p² = 0.36, heterozygous 2pq = 2 × 0.6 × 0.4 = 0.48, homozygous recessive q² = 0.16. Check: 0.36 + 0.48 + 0.16 = 1. In 500 beetles: 180 AA, 240 Aa, 80 aa.

Worked example 2. A recessive condition affects 1.44% of a population. q² = 0.0144, so q = 0.12 and p = 0.88. Carriers = 2pq = 2 × 0.88 × 0.12 = 0.2112, about 21%. Most recessive alleles sit in carriers.

Worked example 3 (from genotype counts). In 400 snails there are 150 AA, 180 Aa, 70 aa. Alleles = 800. A = (2 × 150 + 180) ÷ 800 = 0.60, so q = 0.40. Expected under Hardy-Weinberg: AA 144, Aa 192, aa 64.

D4.1.14 Hardy-Weinberg conditions (HL only)

Allele and genotype frequencies stay constant from one generation to the next only if:

1. the population is large;
2. mating is random;
3. there is no mutation;
4. there is no migration (gene flow) in or out;
5. there is no selection: all genotypes survive and reproduce equally.

Real populations rarely meet all five, so the equation is a baseline. If genotype counts do not fit it, at least one condition fails. In example 3, heterozygotes are fewer than expected (180 against 192), which suggests non-random mating or lower survival of heterozygotes. A small difference could also be chance.

Worked example 4: one generation of selection. Use the beetles: AA 0.36, Aa 0.48, aa 0.16. Suppose only half of the aa beetles survive to breed. Relative numbers are 0.36, 0.48 and 0.08, total 0.92. New q = (0.48 ÷ 2 + 0.08) ÷ 0.92 = 0.32 ÷ 0.92 = 0.348 (3 s.f.), down from 0.4. Selection against the recessive homozygote has lowered the frequency of its allele.

D4.1.15 Artificial selection (HL only)

In artificial selection humans choose which individuals breed, picking those with desirable traits in crops and domesticated animals. Repeating this over generations shifts the population, as in higher-yielding maize and dairy cattle that give more milk. The cost can be reduced variation.

Unintended results are different. When antibiotics kill most of a bacterial population, resistant bacteria survive and multiply. Nobody chose them for breeding, so this is natural selection, with the antibiotic acting as the selection pressure.

Common errors

• Writing that individuals evolve. Populations evolve; individuals do not change their alleles.
• Treating acquired characteristics as heritable.
• Using p² for the recessive phenotype. The recessive homozygote is q².
• Forgetting to square-root q² before finding p.
• Calling antibiotic resistance artificial selection.

Official syllabus

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