AQA A-level Biology: Populations, Hardy-Weinberg, Selection and Speciation (7402) – Revision Notes

Condensed recall for AQA A-level Biology (7402) sections 3.7.2 and 3.7.3 (A-level only), from the AQA specification for first teaching 2015. For full explanations use the study guide, then test yourself with the practice questions. Course pages: the AQA A-level Biology hub, the printable checklist and the free diagnostics. Related: genetic diversity and natural selection and mutation and meiosis.

Definitions to know word for word

Term	Definition
Population	A group of organisms of the same species, occupying a particular space at a particular time, that can potentially interbreed
Gene pool	The complete set of alleles, for every gene, in a population at a given time
Allele frequency	How often an allele occurs in the gene pool, as a proportion or percentage
Evolution	A change in the allele frequencies in a population
Natural selection	Differential survival and reproduction because of predation, disease and competition; advantageous alleles are passed on more
Genetic drift	Chance change in allele frequency, important in small populations
Species	Organisms able to interbreed and produce fertile offspring

Hardy-Weinberg at a glance

Symbol	Meaning
p	Frequency of one allele (usually the dominant)
q	Frequency of the other allele (usually the recessive)
p + q	1
p²	Homozygous dominant genotype
2pq	Heterozygous genotype
q²	Homozygous recessive genotype (the recessive phenotype)
p² + 2pq + q²	1
p² + 2pq	Dominant phenotype

The principle predicts that allele frequencies stay the same from generation to generation. Conditions: a large population, random mating, and an absence of mutation, migration and selection.

Method in steps

1. Find q²: it is the recessive phenotype frequency (or homozygous recessive genotype frequency).
2. q = √(q²).
3. p = 1 − q.
4. Calculate the required frequency from p², 2pq or q².
5. For a number of individuals, multiply by the population size.
6. Check that p² + 2pq + q² adds up to 1.

Worked reminder 1 (from allele frequency). In a population of 2000, q = 0.15. Then p = 0.85. p² = 0.7225, 2pq = 0.255, q² = 0.0225. Numbers: 1445 homozygous dominant, 510 heterozygous, 45 homozygous recessive. Total 2000.

Worked reminder 2 (from the recessive phenotype). A recessive condition affects 81 in 10 000. q² = 0.0081, so q = 0.09 and p = 0.91. Carriers: 2 × 0.91 × 0.09 = 0.1638, which is 1638 per 10 000.

Worked reminder 3 (from genotype counts). Count alleles. If 60 of 200 alleles are A, p = 0.30, q = 0.70. Do not count individuals.

Common wrong turns

• Using the dominant phenotype frequency as p² (it is p² + 2pq).
• Forgetting to square-root.
• Reporting q when asked for carriers.
• Applying the equation to a population where a condition clearly fails, without comment.

Sources of variation

Cause	Detail
Mutation	Primary source of new alleles
Meiosis	Crossing over; independent segregation of homologous chromosomes
Random fertilisation	Which gamete fuses with which is a matter of chance
Environment	Diet, light, temperature and disease affect expression; phenotype = genotype + environment

For 23 pairs, independent segregation gives 2²³ = 8 388 608 gametes.

Natural selection: the explanation in steps

1. The population contains variation, arising from mutation, meiosis and random fertilisation.
2. A selection pressure (predation, disease, competition) acts.
3. Individuals with the advantageous phenotype leave more offspring.
4. Those offspring inherit the advantageous allele.
5. The allele frequency in the gene pool rises across generations.

Worked reminder 4. A population of 800 has q = 0.20 for a recessive allele (p = 0.80). Genotypes: AA 512, Aa 256, aa 32. If all aa die before breeding: survivors = 768; a alleles = 256 of 1536; q becomes 0.167. The allele falls, but slowly, because it is hidden in Aa.

Selection types

Feature	Stabilising	Directional	Disruptive
Favoured	The mean	One extreme	Both extremes
Acts against	Both extremes	The other extreme	The mean
Mean	Unchanged	Moves	May stay, but the curve has two peaks
Range	Narrows	Shifts	Widens
Environment	Stable	Changing	Varied

Genetic drift versus selection

Feature	Natural selection	Genetic drift
Cause	Differential survival and reproduction linked to phenotype	Chance in which alleles are passed on
Direction	Towards advantageous alleles	None
Population size	Any	Matters in small populations
Result	Adaptation	Loss or fixation of alleles with no advantage

Why small only. A population of 10 diploids has 20 copies of a gene. Losing 2 copies by chance changes the frequency by 2 ÷ 20 = 0.10. In a population of 10 000 (20 000 copies) the same loss is 0.0001.

Speciation

Feature	Allopatric	Sympatric
Barrier	Geographical (river, mountain range, sea)	None geographical; same area
Isolating mechanism	Physical separation	Seasonal, behavioural, ecological or chromosome-number change (polyploidy)
Gene flow	Stopped by the barrier	Stopped by reproductive isolation
Result	Gene pools diverge until fertile interbreeding fails	Same

Answer frame: separation, independent mutation, different selection and drift, change in allele frequencies, inability to interbreed and produce fertile offspring. Long-term repetition of this gives the diversity of species.

Linking the ideas

Hardy-Weinberg is the baseline: if all five conditions hold, allele frequencies stay constant and there is no evolution. Each real process breaks one condition. Mutation adds new alleles, migration moves alleles in or out, selection removes some genotypes more than others, non-random mating changes genotype frequencies, and small size lets drift act. Evolution is what you see when the frequencies change, so the equation lets you detect it and then explain it.

Reading data and graphs

• Allele frequency over generations. A steady rise or fall points to selection. Irregular changes, especially in a small population, point to drift. Frequencies that stay flat fit Hardy-Weinberg conditions.
• Distribution curves. Compare the curve after selection with the one before. Narrower with the same mean: stabilising. Mean has moved: directional. Two peaks with a dip in the middle: disruptive.
• Observed against expected genotypes. Calculate p and q from the data, work out the expected numbers, then compare. An excess of homozygotes (a deficit of heterozygotes) fits non-random mating. A change in q between generations fits selection, migration, mutation or drift.
• Percentage change. (new value minus old value) divided by the old value, times 100. A fall of an allele from 0.50 to 0.35 is a decrease of 30%.

Command words in this topic

Command word	What the answer needs
State	A short fact: a condition, a definition
Calculate	Working shown, with the frequency or number clearly identified
Explain	A reason joined to a cause, using “because” or “so”
Describe	What happens, quoting figures from any data
Suggest	A sensible biological idea, applied to the context given

Quick self-test

1. Define a population.
2. What is a gene pool?
3. State the Hardy-Weinberg conditions.
4. In a population, q = 0.4. Calculate the frequency of heterozygotes.
5. A recessive trait appears in 4 of every 10 000 individuals. Find q.
6. Which type of selection narrows the range of a trait?
7. Why is drift unimportant in a large population?
8. Name two ways a sympatric population can become reproductively isolated.
9. Why must “fertile” be in the definition of a species?
10. A population of 500 has 80 aa individuals. What is the frequency of the recessive allele?
11. What does it mean to say evolution is a change in allele frequencies?
12. Name the two genetic sources of variation produced during sexual reproduction, other than mutation.

Answers

1. A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
2. The complete set of alleles, for every gene, in a population at a given time.
3. Large population, random mating, and no mutation, migration or selection.
4. p = 0.6, so 2pq = 2 × 0.6 × 0.4 = 0.48.
5. q² = 0.0004, so q = 0.02.
6. Stabilising selection.
7. Chance events involve a tiny fraction of the gene pool and average out, so frequencies barely change.
8. Any two of: breeding or flowering at different times; different courtship behaviour; chromosome-number change (polyploidy).
9. Some different species can mate, but their offspring are sterile, so the gene pools remain separate; fertility shows that gene flow is possible.
10. q² = 80 ÷ 500 = 0.16, so q = 0.4.
11. The proportion of copies of each allele in the population’s gene pool differs between generations.
12. Meiosis (crossing over and independent segregation) and random fertilisation.

Where marks are usually lost

• Describing evolution of individuals rather than a change in a population’s allele frequencies.
• Leaving out “fertile” or “interbreed” in species definitions.
• Using p² for the dominant phenotype, or q for the recessive phenotype frequency.
• Not stating which Hardy-Weinberg condition is broken when a result does not fit.
• Describing selection without saying that the allele is passed on.
• Treating drift as adaptive.
• Giving a geographical barrier as the cause of sympatric speciation.
• Reading a graph with two peaks as stabilising rather than disruptive selection.
• Giving a count of individuals when a frequency was asked for.

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, published by AQA: sections 3.7.2 and 3.7.3.