OCR A Level Biology A: Populations, Hardy-Weinberg, speciation and artificial selection (H420) – Revision Notes

These revision notes cover section 6.1.2(e) to (h) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. For full explanations and longer worked examples, go to the study guide. Then test yourself with the practice questions. The OCR A Level Biology hub, the checklist and the free diagnostics cover the rest of the course.

Key terms

Term	Meaning
Gene pool	All the alleles of all the genes in a population
Allele frequency	How common one allele is in the gene pool, from 0 to 1
Evolution	A change in allele frequencies in a population over generations
Species	Organisms that can interbreed to produce fertile offspring
Gene flow	Movement of alleles between populations by interbreeding
Isolating mechanism	A barrier that stops gene flow between populations
Wild type	The naturally occurring form of an organism or allele

6.1.2(e) Factors that affect evolution

Factor	What happens	Effect on the population
Stabilising selection	Extremes disadvantaged in a stable environment	Mean unchanged, spread narrows
Directional selection	One extreme favoured, often after an environmental change	Mean shifts towards that extreme
Genetic drift	Chance changes in allele frequency	Large in small populations; alleles can be lost or fixed
Genetic bottleneck	Population falls sharply, survivors are a chance sample	Fewer alleles and changed frequencies, even after recovery
Founder effect	A few individuals start a new population	New population has a sample of the parent gene pool

Selection steps (for any explanation). Variation exists, a selection pressure acts, individuals with the advantageous allele survive and reproduce more, they pass on the allele, and its frequency rises over generations.

Must-know distinction. Selection favours alleles that improve survival or reproduction. Drift does not. A bottleneck is a fall in numbers of an existing population. The founder effect is a new population started by a few migrants.

Worked reminders. Bottleneck: a population of 3000 falls to 25 survivors holding 50 gene copies, 9 of them allele B. Frequency of B = 9 ÷ 50 = 0.18. Directional selection: mean beak depth rises from 9.5 mm to 10.2 mm, a rise of 0.7 ÷ 9.5 × 100 = 7.4%.

Reading graphs. A single peak that narrows is stabilising selection. A peak that moves sideways is directional. A line for a small population that jumps up and down without a trend suggests drift.

6.1.2(f) Hardy-Weinberg

Symbol or equation	Meaning
p and q	Frequencies of the two alleles
p + q = 1	All alleles in the gene pool
p² + 2pq + q² = 1	All genotypes in the population
p², 2pq, q²	Homozygous dominant, heterozygous, homozygous recessive
p² + 2pq	Dominant phenotype

The equations are provided in assessments where needed. Conditions for constant frequencies: a large population, random mating, no mutation, no migration and no selection.

Method in steps

1. Identify what is given: a recessive phenotype frequency (this is q²), an allele frequency, or genotype counts.
2. From q², take the square root to get q. Then p = 1 − q.
3. From genotype counts, count alleles: each homozygote has two copies, each heterozygote has one of each.
4. Calculate the genotype or phenotype asked for.
5. Multiply by the population size if a number of individuals is wanted.
6. Check that the three genotype frequencies add up to 1.

Worked reminder. In a population of 1200, q = 0.25. Then p = 0.75. AA = 0.5625 × 1200 = 675. Aa = 0.375 × 1200 = 450. aa = 0.0625 × 1200 = 75. These add up to 1200.

Worked reminder (from a rare condition). 1 in 400 has a recessive disorder. q² = 0.0025, so q = 0.05 and p = 0.95. Carriers = 2 × 0.95 × 0.05 = 0.095. In 4000 people that is 380 carriers, against 10 affected.

Selection against a recessive phenotype. If aa individuals do not breed, the new q = pq ÷ (1 − q²). For q = 0.6 this is 0.24 ÷ 0.64 = 0.375. The value of q falls, but the fall slows as the allele becomes rare.

Interpreting results. If observed genotype numbers differ from those expected, a Hardy-Weinberg condition is probably not met. Fewer heterozygotes than expected suggests non-random mating. A change in allele frequency between generations shows evolution.

6.1.2(g) Isolating mechanisms and speciation

	Allopatric	Sympatric
Barrier	Geographical: river, mountains, sea	Reproductive: no physical barrier
Location	Populations in different places	Populations in the same area
Key isolating mechanisms	Distance or barrier prevents interbreeding	Temporal, behavioural, mechanical, gametic, hybrid sterility, polyploidy

Answer frame for any speciation question. Barrier or isolating mechanism, then no gene flow, then independent mutations, then different selection pressures and drift, then diverging allele frequencies, then inability to interbreed and produce fertile offspring, so they are different species.

Allopatric or sympatric? Ask whether a physical barrier separates the populations. If the answer is yes, it is allopatric. If the populations share an area and a reproductive barrier separates them, it is sympatric.

Polyploidy. A change in chromosome number can isolate a plant from its parent species in one generation. It is a route to sympatric speciation in plants.

6.1.2(h) Artificial selection

Principles. Humans choose the individuals that breed. Select the best, breed them, select the best offspring and repeat. The frequency of alleles for the wanted feature rises.

Example	Feature selected
Wheat	Yield and disease resistance
Tomato	Fruit size and firmness
Dairy cattle	Milk yield (bulls judged on daughters)
Hens	Egg numbers

Genetic resource. Keep wild types, wild relatives, rare breeds and seed banks. They hold alleles for disease resistance and other features that have been lost from intensively bred lines. Lost alleles cannot be recovered.

Speed of change. Many generations of selection are needed, so species with short life cycles can change in fewer years than cattle.

Ethics. For: more food, disease resistance, working animals, livelihoods. Against: animal welfare, consent, breeding for appearance, inbreeding and a narrow gene pool. For extreme cases such as dog breeds with breathing problems, weigh the harm to the animal against owner preference, and suggest a remedy such as health-first breed standards.

Command words

Command word	What to do
Calculate	Show the working and give units or a frequency to a sensible number of figures
Explain	Give reasons, linking cause to effect
Describe	State what the data or process shows, quoting values
Discuss	Give arguments on both sides, then a supported view
Suggest	Apply your knowledge to an unfamiliar case

Quick self-test

1. State three conditions needed for allele frequencies to stay constant.
2. In a population q = 0.30. Calculate the frequency of heterozygotes.
3. A recessive phenotype has frequency 0.0064. Calculate the frequency of carriers.
4. A recessive disorder affects 1 in 900 people. How many carriers are expected in 9000 people?
5. A sample of 100 has 30 AA, 50 Aa and 20 aa. Calculate the frequency of A.
6. Describe the difference between a bottleneck and the founder effect.
7. A population of 20 diploid animals loses 3 gene copies by chance. Calculate the change in frequency.
8. Distinguish stabilising from directional selection.
9. Name two reproductive isolating mechanisms and say how each stops gene flow.
10. Give one reason for keeping wild relatives of a crop.
11. State one ethical objection to breeding dogs for extreme appearance.

Answers

1. Any three from: large population, random mating, no mutation, no migration, no selection.
2. p = 0.70. 2pq = 2 × 0.70 × 0.30 = 0.42.
3. q² = 0.0064, so q = 0.08 and p = 0.92. 2pq = 0.1472.
4. q = 1/30 = 0.0333 and p = 0.9667. 2pq = 0.0644. 0.0644 × 9000 = 580.
5. A copies = (2 × 30) + 50 = 110 of 200. Frequency = 0.55.
6. A bottleneck is a fall in the numbers of an existing population, leaving a chance sample of alleles. The founder effect is a new population started by a few individuals carrying a sample of the parent gene pool.
7. 40 gene copies. 3 ÷ 40 = 0.075.
8. Stabilising selection favours the mean and narrows the spread, with the mean unchanged. Directional selection favours one extreme and moves the mean.
9. Temporal: breeding at different times. Behavioural: courtship signals are not recognised. (Other correct pairs are credited.)
10. They hold alleles, such as for disease resistance, that the crop has lost.
11. Animals cannot consent, and extreme features can cause pain or breathing difficulty.

Where marks are usually lost

• Using the recessive phenotype frequency as q rather than q².
• Taking the dominant phenotype as p² instead of p² + 2pq.
• Counting individuals instead of alleles when finding allele frequency.
• Explaining drift as an advantage. Drift is chance.
• Writing that individuals evolve or adapt, instead of changes in allele frequency.
• Leaving out the loss of gene flow in a speciation answer.
• Stopping an explanation of selection before the allele frequency rises.
• Writing about ethics without weighing benefit against harm.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025: section 6.1.2(e) to (h), Patterns of inheritance (stabilising and directional selection, genetic drift, the genetic bottleneck and founder effect, Hardy-Weinberg, isolating mechanisms and artificial selection).