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

This guide covers 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: the factors that affect evolution, the Hardy-Weinberg principle, isolating mechanisms and the evolution of new species, and artificial selection with its ethics. Use it with the revision notes and the practice questions. Earlier ideas are in variation, adaptation and natural selection. The OCR A Level Biology hub, the checklist and the free diagnostics show the rest of the course.

What this unit covers

Section	What you must be able to do
6.1.2(e)	Explain factors that affect evolution: stabilising and directional selection, genetic drift, the genetic bottleneck and the founder effect
6.1.2(f)	Use the Hardy-Weinberg principle to calculate allele frequencies in populations
6.1.2(g)	Explain how isolating mechanisms lead to new species: geographical (allopatric speciation) and reproductive (sympatric speciation)
6.1.2(h)	Explain the principles and uses of artificial selection, with plant and animal examples; why genetic material (including wild types) must be kept; the ethics, including extreme cases such as dog breeds

The specification says the Hardy-Weinberg equations will be provided where needed and need not be recalled. You must still know what each symbol means.

Factors that affect evolution (6.1.2(e))

Evolution is a change in the allele frequencies of a population over generations. A population’s gene pool is all the alleles of all its genes. Mutation and sexual reproduction supply variation, and the processes below change which alleles are common.

Stabilising selection

The environment is stable and the extremes of a feature are at a disadvantage. Individuals near the mean survive and reproduce best, so alleles for the extremes become rarer. The mean stays the same and the spread narrows.

Worked example. Egg mass in a wading bird and the percentage of eggs that hatched:

Egg mass (g)	18	20	22	24	26
Hatched (%)	34	71	88	72	30

Hatching is highest at 22 g and falls on both sides. This is stabilising selection. Hatching at 18 g is 88 − 34 = 54 percentage points lower than at 22 g. As a percentage of the 22 g value, the fall is 54 ÷ 88 × 100 = 61.4%.

Directional selection

The environment changes, or a new selection pressure appears, and one extreme is favoured. Those individuals survive and reproduce more, so their alleles become more frequent and the mean shifts towards that extreme.

Worked example. A fishing net lets small fish escape. Over several generations the mean length of mature fish falls from 38.0 cm to 34.0 cm. That is a fall of 4.0 ÷ 38.0 × 100 = 10.5%. Large fish are caught before they breed, so alleles for fast growth become rarer.

Genetic drift

Genetic drift is a change in allele frequency by chance. It is not caused by any advantage. It matters most in small populations because a chance event affects a large share of the gene pool. Suppose a colony of 15 diploid lizards holds 30 copies of a gene and 3 copies of one allele fail to pass on by chance. The frequency of that allele changes by 3 ÷ 30 = 0.10. Among 6000 lizards (12 000 copies) the same loss changes it by 3 ÷ 12 000 = 0.00025. An allele can be lost altogether, or fixed at frequency 1.

Bottleneck and founder effect

A genetic bottleneck is a sharp fall in population size, for example from disease or hunting. The survivors carry only a sample of the original alleles. Even if numbers recover, the gene pool has fewer alleles and different frequencies, because chance decided who survived.

Worked example. A seal population of 5000 falls to 40 survivors. The survivors hold 80 gene copies, of which 12 are allele A. The frequency of A is 12 ÷ 80 = 0.15. Before the fall it was 0.30.

The founder effect is the same sampling problem in a new population. A few individuals colonise an island or a new habitat, and they carry only part of the parent gene pool.

Worked example. Twenty-five birds found an island population. Five are heterozygous for a rare recessive allele and the others have none. The island frequency is 5 ÷ 50 = 0.10. On the mainland it is 0.02. In an island generation of 400 birds in equilibrium, aa would be 0.10² × 400 = 4, against 0.02² × 400 = 0.16 at the mainland frequency. A rare allele can become common because of who founded the population.

The Hardy-Weinberg principle (6.1.2(f))

For a gene with two alleles, p is the frequency of one allele and q is the frequency of the other. Frequencies are between 0 and 1.

Equation	Meaning
p + q = 1	The two alleles together make up the whole gene pool
p² + 2pq + q² = 1	The three genotypes together make up the whole population
p², 2pq, q²	Homozygous dominant, heterozygous, homozygous recessive

The principle predicts that allele frequencies stay constant from generation to generation if the population is large, mating is random, and there is no mutation, migration or selection. Data that differ from the predictions mean one of those conditions is not met.

Method

1. If the recessive phenotype frequency is known, it is q².
2. Take the square root to find q. Then p = 1 − q.
3. Calculate the frequency wanted: p², 2pq or q². The dominant phenotype is p² + 2pq, or 1 − q².
4. To find a number of individuals, multiply by the population size.

Worked example 1 (from the recessive phenotype). A recessive metabolic condition affects 1 in 6400 people in a town of 32 000.

• q² = 1 ÷ 6400 = 0.00015625, so q = 0.0125.
• p = 1 − 0.0125 = 0.9875.
• Carriers: 2pq = 2 × 0.9875 × 0.0125 = 0.0246875, which is about 1 in 40.5.
• Number of carriers: 0.0246875 × 32 000 = 790.

Only 5 people in the town have the condition (0.00015625 × 32 000), yet 790 carry the allele. Most copies of a rare recessive allele sit in heterozygotes.

Worked example 2 (from genotype counts). In 250 field voles, 135 are CC, 90 are Cc and 25 are cc.

• Alleles: 2 × 250 = 500. C = (2 × 135) + 90 = 360, so p = 360 ÷ 500 = 0.72 and q = 0.28.
• Expected numbers if in equilibrium: CC = 0.72² × 250 = 129.6, Cc = 2 × 0.72 × 0.28 × 250 = 100.8, cc = 0.28² × 250 = 19.6.
• Observed Cc (90) is lower and cc (25) higher than expected. Fewer heterozygotes than expected can mean mating is not random, for example between relatives, or that selection is acting.

Worked example 3 (effect of selection). In a population q = 0.50. A pesticide stops every aa individual from breeding, but AA and Aa breed.

• Before breeding: AA 0.25, Aa 0.50, aa 0.25. The breeders are 0.75 of the population.
• Allele a in the breeders: Aa individuals carry one copy each, so a makes up 0.25 of all the alleles in the breeders. Breeders are 0.75 of the population, so q = 0.25 ÷ 0.75 = 0.333.
• In the next generation, aa = 0.333² = 0.111, down from 0.25.

Selection against a recessive phenotype lowers q, but slowly once the allele is rare, because heterozygotes hide it.

Isolating mechanisms and speciation (6.1.2(g))

A species is a group of organisms that can interbreed to produce fertile offspring. New species form when populations stop exchanging genes (gene flow) and their gene pools diverge. An isolating mechanism stops gene flow.

Geographical mechanisms: allopatric speciation

Allo- means other, and -patric refers to place. A physical barrier such as a river, mountain range or sea splits a population.

1. The barrier stops gene flow between the two populations.
2. Mutations arise independently in each.
3. The environments differ, so different alleles are favoured by natural selection. Genetic drift also acts, especially in a small population.
4. Allele frequencies diverge over many generations.
5. If the barrier is removed, the groups cannot interbreed to produce fertile offspring, so they are separate species.

Reproductive mechanisms: sympatric speciation

Sym- means same. The populations share an area but stop interbreeding because of a reproductive barrier. Examples:

Mechanism	How it stops gene flow
Temporal	Breeding or flowering at different times of day or year
Behavioural	Courtship signals, such as song or display, are not recognised by the other group
Mechanical	Reproductive structures do not fit together
Gametic	Gametes do not fuse
Hybrid sterility	Hybrids survive but are infertile, so alleles do not pass between the groups
Polyploidy	A change in chromosome number makes plants unable to produce fertile offspring with the parent type

Polyploidy is common in plants. Bread wheat has six sets of chromosomes, a result of hybridisation between different grass species. A plant with a new chromosome number can be isolated in one generation.

Use of a different host or niche in the same place can also split a population. The sequence is always no gene flow, independent mutation and selection, divergence in allele frequencies, then inability to produce fertile offspring.

Artificial selection (6.1.2(h)(i))

In artificial selection (selective breeding) humans choose which individuals breed, so human choice is the selection pressure. The steps are:

1. Decide on the characteristic wanted.
2. Select the individuals that show it most strongly.
3. Breed them together.
4. Select the best offspring and repeat for many generations.

Allele frequencies for the chosen feature rise each generation.

Plants. Crossing a high-yielding wheat with a plant carrying disease resistance, then choosing resistant offspring over generations, gives a high-yielding resistant variety. Tomatoes can be selected for fruit size or firmness.

Animals. Dairy cattle are selected for milk yield. Bulls give no milk, so a bull may be chosen on his daughters’ yields. Hens can be selected for egg numbers.

Worked example. Mean milk yield in a herd rises from 6200 to 6950 litres per cow per year over five generations. The rise is 750 litres, which is 750 ÷ 6200 × 100 = 12.1%. The mean rise is 750 ÷ 5 = 150 litres per generation.

Keeping a genetic resource

Intensive breeding uses few lines, so alleles are lost. Seed banks, rare breeds and wild populations act as a store. Wild types and wild relatives hold alleles for disease resistance, drought tolerance or other features that modern varieties have lost. If a new disease or a changed climate appears, breeders can cross these alleles in. A lost allele cannot be recovered.

Ethics of artificial selection (6.1.2(h)(ii))

Ethics means weighing benefits against harms. Say who benefits and who is harmed.

Benefits include more food from less land, crop loss cut by disease resistance, and animals bred for temperament and work. Concerns are that animals cannot consent, that breeding for appearance can cause pain or disability, and that inbreeding and a narrow gene pool raise the frequency of harmful recessive alleles.

The specification asks you to consider extreme cases such as dog breeds. Some breeds have been selected for very short noses, which can cause breathing problems, and others for features that raise the risk of joint disease. For: companionship, tradition and owner choice. Against: avoidable suffering and small gene pools. A balanced answer might suggest health-first breed standards or outcrossing, and gives a supported view.

Common errors

• Writing that individuals evolve. Populations evolve, by changes in allele frequency.
• Taking the recessive phenotype frequency as q rather than q².
• Treating the dominant phenotype as p² only, instead of p² + 2pq.
• Saying that drift happens because an allele is better. It is chance.
• Writing “the population adapted” for speciation. Name the barrier, the lack of gene flow and the different selection.
• Writing that artificial selection adds new alleles. It changes the frequencies of existing ones.

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).