How is section 3.7 weighted and where does it appear in the AQA exams?

Section 3.7 is an A2 (second-year) topic assessed across Papers 1, 2 and 3 of the AQA A-Level Biology (7402) exams. It is a reliably tested topic because it carries the maths content the specification emphasises: Hardy-Weinberg, chi-squared and mark-release-recapture calculations recur most years. Expect at least one multi-mark calculation and one extended question on natural selection, speciation or succession. Paper 3 also includes the synoptic essay, for which evolution and variation are common themes.

Which calculations must I be able to do?

Three. First, the chi-squared test on offspring ratios (state a null hypothesis, work out expected values, compute the sum of (O minus E) squared over E, compare to the critical value at p = 0.05). Second, Hardy-Weinberg, where you find q from the recessive phenotype (q-squared), then p = 1 minus q, then p-squared and 2pq. Third, mark-release-recapture, where population equals first sample times second sample divided by the number marked in the second sample. Practise these until the steps are automatic.

What is the difference between codominance and incomplete dominance, and which does AQA test?

Codominance means both alleles are expressed fully and separately in the heterozygote (roan cattle show red and white hairs; blood group AB shows both A and B antigens). Incomplete dominance blends the two alleles to an intermediate (red plus white gives pink). AQA 3.7 names codominance and multiple alleles, so use codominant examples such as ABO blood groups or roan coat colour in your answers.

How do I tell allopatric and sympatric speciation apart?

Both require reproductive isolation that stops gene flow between two gene pools so they evolve independently until they can no longer interbreed. Allopatric speciation is caused by geographic isolation (a physical barrier such as a river, sea or mountain range). Sympatric speciation happens in the same area without a geographic barrier, through reproductive isolating mechanisms such as seasonal breeding differences, behavioural isolation or polyploidy in plants.

Why does conservation often mean stopping succession?

Left alone, succession drives a community toward a single climax (usually woodland in the UK), where a few dominant competitors reduce diversity. Many valued habitats (chalk grassland, heathland) are earlier successional stages with high diversity. To conserve them, managers deliberately halt succession by grazing, mowing, controlled burning or coppicing, holding the community at a diverse plagioclimax below the natural climax.

EnglandBiology

AQA A-Level Biology 3.7 Genetics, populations, evolution and ecosystems: a complete deep dive

A deep-dive AQA A-Level Biology guide to section 3.7. Covers inheritance and the chi-squared test, the Hardy-Weinberg principle, natural selection, genetic drift and speciation, populations and carrying capacity, mark-release-recapture, and succession and conservation, with the calculations and answer structures AQA repeats.

Generated by Claude Opus 4.824 min readAQA-7402-3.7Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What section 3.7 actually demands
Inheritance and predicting offspring
The chi-squared test
Populations, gene pools and Hardy-Weinberg
Natural selection, drift and speciation
Populations in ecosystems
Succession and conservation
How section 3.7 is examined
Check your knowledge

What section 3.7 actually demands

Section 3.7 is the capstone genetics-and-ecology topic of AQA A-Level Biology. It pulls together the genetics you met earlier and applies it at the level of whole populations: how alleles are inherited and tested statistically, how their frequencies are described and change, how that change produces new species, and how populations behave inside ecosystems that themselves change over time.

The topic rewards two distinct skills. The first is quantitative: three calculations (chi-squared, Hardy-Weinberg and mark-release-recapture) appear almost every year and are pure marks if you drill the steps. The second is structured explanation: natural selection, speciation and succession are each a fixed logical sequence that markers reward when you lay it out in order. Train both deliberately.

Inheritance and predicting offspring

A genotype is the alleles an organism carries; a phenotype is the observable trait produced by the genotype interacting with the environment. Most phenotypes are affected by more than one gene.

For a monohybrid cross of two heterozygotes ( $Aa \times Aa$ ) the genotypic ratio is $1:2:1$ and the phenotypic ratio is $3:1$ . For an unlinked dihybrid cross ( $AaBb \times AaBb$ ) the phenotypic ratio is $9:3:3:1$ . Modified versions of these ratios are the exam's signal that something more is going on:

Codominance: both alleles expressed fully in the heterozygote (roan cattle, blood group AB).
Multiple alleles: more than two alleles in the population (ABO blood groups, with $I^A$ and $I^B$ codominant and $I^O$ recessive).
Sex-linkage: genes on the X chromosome; males (XY) are hemizygous so express X-linked recessives more often than females.
Autosomal linkage: two genes on the same autosome inherited together, reducing recombinants and distorting the $9:3:3:1$ ratio.
Epistasis: one gene masks another, giving modified ratios such as $9:3:4$ (recessive epistasis) or $12:3:1$ (dominant epistasis).

The chi-squared test

The chi-squared ( $X^2$ ) test decides whether the difference between observed and expected counts is significant or just chance. It works only on raw counts with a clear expected ratio.

X^2 = \sum \frac{(O - E)^2}{E}

The method is always the same: state a null hypothesis (no significant difference), calculate expected counts from the ratio and the total, compute $X^2$ , find degrees of freedom (categories minus 1), and compare to the critical value at $p = 0.05$ . If $X^2$ is below the critical value, accept the null hypothesis (the data fit the ratio); if it is at or above it, reject the null hypothesis (the difference is significant).

Populations, gene pools and Hardy-Weinberg

A population is interbreeding organisms of one species in one place at one time. Its gene pool is all the alleles of all the genes present, and an allele frequency is the proportion of a gene's copies that are a particular allele. Phenotypic variation within a population comes from genetic and environmental factors together.

The Hardy-Weinberg principle models a population in which allele frequencies do not change between generations:

p + q = 1 \qquad p^2 + 2pq + q^2 = 1

Here $p$ and $q$ are the dominant and recessive allele frequencies, and $p^2$ , $2pq$ , $q^2$ are the homozygous dominant, heterozygous and homozygous recessive genotype frequencies. Equilibrium holds only when all five conditions are met: no mutation, no migration, no natural selection, no genetic drift (a large population) and random mating. Because these are rarely all true, real allele frequencies usually change, and that change is the evidence of evolution.

The reliable calculation route starts from the recessive phenotype, the only phenotype that maps to a single genotype:

Natural selection, drift and speciation

Variation (from mutation) plus a selection pressure gives some individuals greater reproductive success; they pass on the advantageous allele, whose frequency rises over generations. This differential reproductive success is natural selection.

Directional selection shifts the mean toward one extreme when the environment changes (antibiotic resistance in bacteria).
Stabilising selection favours the intermediate and selects against extremes in a stable environment, reducing variation (human birth weight).

Genetic drift is random change in allele frequency, important in small populations where chance can fix or lose alleles. The founder effect is drift when a small, unrepresentative group founds a new population with reduced genetic diversity.

Speciation occurs when two populations become reproductively isolated so their gene pools can no longer mix and they diverge:

Allopatric speciation follows geographic isolation (a physical barrier separates the populations).
Sympatric speciation occurs in the same area through reproductive isolating mechanisms (seasonal, behavioural, or polyploidy in plants).

Populations in ecosystems

An ecosystem is all the living organisms plus the abiotic conditions in an area; a community is all its populations. The carrying capacity is the maximum stable population the ecosystem can support, set by:

Abiotic factors: temperature, light, water, pH, oxygen, mineral availability.
Biotic factors: intraspecific competition (within a species, the key density-dependent control that returns a population to carrying capacity), interspecific competition (between species), and predation.

Predator-prey populations cycle with the predator peak lagging the prey peak: abundant prey lets predators increase, more predators reduce prey, scarce prey starves predators, fewer predators let prey recover.

To estimate population size you sample and scale up: random quadrats for non-motile organisms, transects for distribution along an environmental gradient, and mark-release-recapture for mobile animals.

\text{population estimate} = \frac{n_1 \times n_2}{n_m}

where $n_1$ is the number first marked, $n_2$ the second sample size, and $n_m$ the marked individuals recaptured. The method assumes marked individuals mix randomly, the mark does not affect survival or get lost, and there is no major birth, death or migration between samples.

Succession and conservation

Succession is the change in a community over time. Primary succession starts on bare ground with no soil; secondary succession starts on disturbed land that retains soil and is faster. Pioneer species adapted to harsh conditions colonise first and change the abiotic environment (building soil, retaining water, reducing harshness), letting more competitive species replace them through seres until a stable climax community forms. Diversity rises through the intermediate stages and may fall at the climax, where dominant competitors prevail.

Conservation frequently manages succession. Because succession tends toward a low-diversity climax, conserving diverse early-stage habitats (chalk grassland, heathland) often means deliberately halting succession by grazing, mowing, controlled burning or coppicing, holding the community at a plagioclimax. AQA also expects you to evaluate conflicting evidence in conservation, weighing ecological, economic and social interests to reach a justified conclusion.

How section 3.7 is examined

A typical AQA 3.7 profile across the papers:

Multiple choice and short answer: identifying inheritance patterns and modified ratios, naming selection types, distinguishing allopatric and sympatric speciation, reading predator-prey and succession data.
Calculations: chi-squared on offspring data, Hardy-Weinberg allele and genotype frequencies, mark-release-recapture estimates. These are the most reliable marks in the topic.
Extended response: explaining natural selection (for example antibiotic resistance) as a sequence, describing succession from pioneer to climax, or evaluating conservation evidence.
Synoptic essay (Paper 3): variation, selection and evolution are recurring essay themes that draw on 3.7.

AQA 3.7 ties genetics to populations and ecosystems. Predict offspring ratios (3:1, 9:3:3:1, and modified ratios for codominance, sex-linkage, linkage and epistasis) and test them with chi-squared. Describe gene pools and allele frequencies and use Hardy-Weinberg ( $p + q = 1$ , $p^2 + 2pq + q^2 = 1$ ) by finding $q$ from the recessive phenotype. Explain evolution through differential reproductive success, directional and stabilising selection, genetic drift and the founder effect, and speciation by geographic (allopatric) or reproductive (sympatric) isolation. Analyse populations in ecosystems via carrying capacity, limiting factors and predator-prey cycles, and estimate size with quadrats, transects and mark-release-recapture. Finally, use succession (pioneer to climax) to explain conservation, which often deliberately halts succession to maintain diversity.

Check your knowledge

A mix of definitional, calculation and exam-style questions covering the whole of section 3.7. Attempt all under timed conditions, then check against the solutions block.

Define genotype, phenotype and gene pool, and explain why the dominant phenotype cannot be mapped to a single genotype. (4 marks)
A dihybrid self-cross expected to give a 9:3:3:1 ratio produces offspring counts of 90, 28, 32 and 10 (total 160). Carry out a chi-squared test and state your conclusion. Critical value at p = 0.05 and 3 degrees of freedom = 7.815. (5 marks)
A recessive allele causes a metabolic condition affecting 1 in 400 people. Use Hardy-Weinberg to calculate (a) the frequency of the recessive allele, (b) the frequency of carriers, and (c) the percentage of the population that are homozygous dominant. (4 marks)
(a, 3) Explain how directional selection has led to a population of insects becoming resistant to an insecticide. (b, 2) State how stabilising selection differs, with a named example. (5 marks)
(a, 3) Describe the role of geographic isolation in allopatric speciation. (b, 2) Explain how sympatric speciation can occur without a geographic barrier. (5 marks)
A researcher captures and marks 64 grasshoppers, releases them, and later captures 48, of which 16 are marked. (a, 2) Estimate the population size. (b, 3) State and explain two assumptions of the method. (5 marks)
Using the concept of succession, explain why a heathland nature reserve carries out controlled burning and grazing. (6 marks)

Solutions

Q1: Genotype is the alleles an organism carries; phenotype is the observable characteristic produced by the genotype interacting with the environment; a gene pool is all the alleles of all the genes in a population. The dominant phenotype cannot be mapped to one genotype because it is produced by both the homozygous dominant ( $p^2$ ) and heterozygous ( $2pq$ ) genotypes, which look identical, so only the recessive phenotype ( $q^2$ ) reveals its genotype directly.
Q2: Null hypothesis: no significant difference between observed and expected ratios. Expected values (total 160 in 9:3:3:1, units of 160/16 = 10): 90, 30, 30, 10. $X^2 = \dfrac{(90-90)^2}{90} + \dfrac{(28-30)^2}{30} + \dfrac{(32-30)^2}{30} + \dfrac{(10-10)^2}{10} = 0 + 0.133 + 0.133 + 0 = 0.27$ . Degrees of freedom = 3; critical value 7.815. Since 0.27 < 7.815, accept the null hypothesis: the data fit the 9:3:3:1 ratio (difference not significant).
Q3: (a) $q^2 = 1/400 = 0.0025$ , so $q = \sqrt{0.0025} = 0.05$ . (b) $p = 1 - 0.05 = 0.95$ ; carriers $2pq = 2 \times 0.95 \times 0.05 = 0.095 = 9.5$ percent. (c) Homozygous dominant $p^2 = 0.95^2 = 0.9025 = 90.25$ percent.
Q4: (a) Random mutation produces a few insects with an allele giving insecticide resistance (variation). When the insecticide is applied it is a selection pressure that kills non-resistant insects; resistant insects survive and reproduce (differential reproductive success), passing the resistance allele to offspring, so its frequency increases over generations and resistance becomes common. This is directional selection: the mean shifts toward the resistant extreme. (b) Stabilising selection favours the intermediate phenotype and selects against both extremes in a stable environment, reducing variation, for example human birth weight, where very small and very large babies historically had lower survival.
Q5: (a) A geographic barrier (such as a river, sea or mountain range) physically separates a population into two, preventing gene flow between them. The two gene pools then experience different selection pressures and undergo independent mutation and genetic drift, so their allele frequencies and phenotypes diverge over many generations until they can no longer interbreed to produce fertile offspring, forming two species. (b) Sympatric speciation occurs in the same area through reproductive isolating mechanisms: for example seasonal isolation (breeding at different times), behavioural isolation (different courtship), or polyploidy in plants, which immediately produces individuals with an incompatible chromosome number that cannot interbreed with the parent population.
Q6: (a) Population estimate $= \dfrac{64 \times 48}{16} = \dfrac{3072}{16} = 192$ grasshoppers. (b) Any two, explained: (i) marked individuals mix randomly back into the population before the second sample, otherwise the recapture proportion would not represent the whole population; (ii) the mark does not affect survival or behaviour and is not lost, otherwise the number recaptured would be biased; (iii) there is no significant birth, death, immigration or emigration between samples, otherwise the population would have changed and the estimate would be wrong.
Q7: Left unmanaged, the heathland would undergo succession: pioneer and intermediate species change the abiotic environment (building soil and reducing harshness), allowing more competitive species such as scrub and tree seedlings to colonise and outcompete the heathland plants, eventually reaching a woodland climax community with lower diversity of the characteristic heathland species. Controlled burning removes accumulated growth and resets succession to an earlier stage, and grazing removes tree and shrub seedlings, both preventing the community from progressing to woodland. This holds the heathland at a diverse plagioclimax, conserving its characteristic plant and animal species. Markers reward using the succession sequence, explaining how the management halts it, and linking this to conserving diversity.