Cambridge International AS & A Level Biology 9700: Selection and evolution – Revision Notes

These notes condense topic 17, Selection and evolution, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sections 17.1 Variation, 17.2 Natural and artificial selection and 17.3 Evolution. It is A Level content, examined on Paper 4; the t-test can also appear on Paper 5. For full explanations and worked examples, use the selection and evolution study guide.

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17.1 Variation

Causes

Cause	Example
Genetic only	ABO blood group
Environmental only	A scar; the language you speak
Genetic and environmental	Human height and body mass; plant height (alleles plus light, water, mineral ions)

Must-know distinction: discontinuous vs continuous

	Discontinuous	Continuous
Classes	Few, distinct	A range, no clear classes
Genes	One or a few; alleles with large effects	Many (polygenes), each small and additive
Environment	Little effect	Large effect
Graph	Bar chart	Histogram, often normal

Genetic basis in one line each.

• Discontinuous: different alleles of a single gene give clearly different phenotypes; with dominance or codominance you get a few fixed classes.
• Continuous: each additive allele of many unlinked genes adds a small amount; the number of combinations is large, and the environment smooths the steps into a continuous range.

Worked reminder. Three additive genes, AaBbCc × AaBbCc → 0 to 6 additive alleles in proportions 1 : 6 : 15 : 20 : 15 : 6 : 1 (out of 64): seven classes already approach a bell curve.

Method: t-test

1. Check the conditions: continuous data, normally distributed populations, similar standard deviations.
2. Null hypothesis: no significant difference between the two means.
3. Calculate x̄ and s for each sample (s formula provided).
4. t = (x̄₁ − x̄₂) / √(s₁²/n₁ + s₂²/n₂) (formula provided).
5. Degrees of freedom = n₁ + n₂ − 2 (not provided).
6. Compare t with the critical value at p = 0.05. t greater → significant difference, reject the null hypothesis. t smaller → not significant, accept it.

Worked reminder. Cloned plants in high and low light: x̄ = 35.5 cm and 30.0 cm, s = 2.878 and 2.000, n = 8 each → t = 4.44, 14 df, critical value 2.14 → significant; the cause must be environmental.

17.2 Natural and artificial selection

Natural selection in five steps

1. Over-production of offspring.
2. Variation, partly genetic.
3. Competition for limited resources: the struggle for existence.
4. The best adapted are more likely to survive and reproduce.
5. Their alleles are passed on, so advantageous allele frequencies rise.

Forces of selection

Force	Selected against	Result	Environment
Stabilising	Both extremes	Mean the same, narrower range	Stable
Directional	One extreme	Mean shifts	Changing
Disruptive	The middle	Two peaks; can lead to speciation	Two different niches favoured

Must-know distinctions: selection, drift, founder, bottleneck

Process	Cause of allele-frequency change	Key word
Natural selection	Some alleles give a survival or reproductive advantage	Non-random
Genetic drift	Chance: which individuals happen to reproduce	Random; strongest in small populations
Founder effect	A few individuals start a new population with a non-representative sample of alleles	Colonisation
Bottleneck effect	A large population is suddenly reduced; the survivors’ alleles are a random sample	Catastrophe; low diversity afterwards

Antibiotic resistance in steps

1. Random mutation → resistance allele (not caused by the antibiotic).
2. Antibiotic kills susceptible bacteria; resistant ones survive.
3. Survivors reproduce by binary fission; allele also spread on plasmids.
4. Resistance allele frequency increases: directional selection.

Hardy–Weinberg (equations provided)

Symbol	Meaning
p	frequency of dominant allele
q	frequency of recessive allele
p²	frequency of homozygous dominant
2pq	frequency of heterozygotes
q²	frequency of homozygous recessive

Method in steps.

1. Find q² = number with recessive phenotype ÷ total.
2. q = √q²; p = 1 − q.
3. Work out 2pq or p², and multiply by the population size if a number is asked for.
4. With codominance, count alleles directly: p = (2 × AA + Aa) ÷ (2 × total).

Conditions: large population; random mating; no mutation; no migration; no selection.

Worked reminder (codominance). 500 individuals: 180 AA, 240 AB, 80 BB. p = (2 × 180 + 240) ÷ 1000 = 0.6, so q = 0.4. Expected numbers: p² × 500 = 180, 2pq × 500 = 240, q² × 500 = 80. Observed and expected match, so the population is consistent with Hardy–Weinberg equilibrium for this gene.

Must-know distinction: natural vs artificial selection

	Natural selection	Artificial selection
Selection pressure	The environment (predators, disease, climate, competition)	Humans choose which individuals breed
Features favoured	Those that increase survival and reproduction	Those useful to humans, e.g. yield or uniformity
Speed and diversity	Usually slow; diversity often kept	Can be fast; genetic diversity often reduced

Selective breeding

Principle: humans select parents with the desired trait, breed them, select the best offspring and repeat over many generations.

Example	What is done	Why it works
Wheat and rice	Cross a high-yield variety with a disease-resistant variety or wild relative; repeatedly cross resistant offspring back to the high-yield variety	Combines the resistance allele with high-yield alleles
Maize	Inbreed to make homozygous inbred lines; cross two lines to make F1 hybrids	F1 heterozygous at many loci → hybrid vigour; all the same genotype → uniform crop
Dairy cattle	Choose bulls by their daughters’ milk yield (progeny testing); artificial insemination of high-yield cows	Bulls carry yield alleles but cannot show them

17.3 Evolution

Evolution: formation of new species from pre-existing species over time, through changes to gene pools from generation to generation. A gene pool is all the alleles of all the genes in a population; selection, drift, founder effects and mutation all change it.

DNA sequence data: compare the base sequence of the same gene in different species. Fewer differences → more recent common ancestor → more closely related. If mutations accumulate at a roughly steady rate, the number of differences can estimate the time since divergence.

Must-know distinction: allopatric vs sympatric

	Allopatric	Sympatric
Isolation	Geographical barrier	Ecological or behavioural, in the same area
Example of barrier	Mountain range, sea, river	Different food, habitat or breeding time; different courtship
Common steps	No gene flow → different mutations, selection pressures and drift → gene pools diverge → can no longer interbreed to give fertile offspring	Same

Quick self-test

1. Is ABO blood group an example of continuous or discontinuous variation?
2. Two unlinked additive genes: AaBb × AaBb. How many phenotype classes are expected, and what fraction of offspring have exactly two additive alleles?
3. Two samples have n = 12 and n = 10. How many degrees of freedom for a t-test?
4. A t-test gives t = 1.62 with 20 degrees of freedom. The critical value at p = 0.05 is 2.09. What do you conclude?
5. In a population, 9% of individuals show a recessive phenotype. Calculate the frequency of heterozygotes.
6. 1% of a population shows a recessive phenotype. What fraction are carriers?
7. Which force of selection keeps the mean the same but narrows the range?
8. A few lizards reach a new island and start a population. Name the effect on allele frequencies.
9. Why are F1 hybrid maize plants uniform?
10. The same 600-base section of a gene differs at 9 positions between two species. Calculate the percentage difference.
11. A river changes course and splits a population of snails. Name the type of speciation that may follow.
12. State four conditions needed for the Hardy–Weinberg principle to apply.

Answers

1. Discontinuous.
2. Five classes (0–4 additive alleles, 1 : 4 : 6 : 4 : 1); exactly two = 6/16 = 3/8.
3. 12 + 10 − 2 = 20.
4. 1.62 < 2.09, so the difference is not significant; accept the null hypothesis.
5. q² = 0.09, q = 0.3, p = 0.7, 2pq = 0.42.
6. q = 0.1, p = 0.9, 2pq = 0.18.
7. Stabilising selection.
8. The founder effect.
9. Both parents are homozygous inbred lines, so every F1 plant receives the same alleles and has the same genotype.
10. 9/600 × 100 = 1.5%.
11. Allopatric speciation.
12. Any four of: large population, random mating, no mutation, no migration, no selection.

Where marks are usually lost

• Writing “the organism adapts to survive” instead of “individuals with the advantageous allele are more likely to survive and reproduce”.
• Leaving out “passes on alleles to offspring”: survival alone is not natural selection.
• Using “genes” where “alleles” is needed: selection changes allele frequencies, not gene frequencies.
• Saying the antibiotic “causes” the resistance mutation, or that bacteria become “immune”.
• Taking the square root of the dominant phenotype frequency to find p.
• Forgetting degrees of freedom, or stating that the t-test “proves” a difference.
• Confusing genetic drift (random) with natural selection (non-random).
• Explaining hybrid vigour as homozygosity, or forgetting why farmers buy F1 seed every year.
• Describing allopatric speciation without the step “no gene flow between the populations”.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027 (Version 1), Cambridge University Press & Assessment – topic 17, Selection and evolution.