OxfordAQA International AS and A-level Biology: Natural selection and speciation (9610) – Revision Notes

These notes condense sections 3.3.8.1 and 3.3.8.2 of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. Unit 3 is listed as International A-level only. For full explanations and worked examples, read the study guide first, then test yourself with the practice questions. Course links: the OxfordAQA A-level Biology hub, the checklist and the free diagnostics.

Key definitions

Term	Meaning
Evolution	A change in the allele frequencies in a population
Gene pool	The complete set of alleles of all the genes in one population
Allele frequency	The fraction of the gene copies in the gene pool that are a particular allele
Phenotypic variation	The range of phenotypes in a population, from genes and the environment
Differential survival and reproduction	Individuals differ in how long they live and how many offspring they leave, because of predation, disease and competition
Selective advantage	A phenotype that makes an individual more likely to survive and leave more offspring
Species (in the sense used by this unit)	Members that can interbreed and produce fertile offspring
Reproductive separation	Populations of one species cannot, or do not, exchange alleles

The sequence of natural selection

1. Variation exists in the population (heritable differences).
2. Predation, disease or competition removes some individuals more than others.
3. Individuals with advantageous phenotypes survive and reproduce more.
4. They pass on the favourable alleles.
5. The allele frequency rises in the next generation.

Write all five links when asked to explain. Missing the inheritance step is the commonest slip.

Calculating allele frequency

Quantity	Working
Total gene copies	2 × number of individuals
Frequency of allele R	(2 × RR + Rr) ÷ (2 × n)
Frequency of allele r	1 − frequency of R
Percentage change	(new − old) ÷ old × 100

Reminder example. A beetle population of 100 had 18 RR, 44 Rr and 38 rr. After a pesticide, 15 RR, 33 Rr and 12 rr survived.

• Before: R = (36 + 44) ÷ 200 = 0.40
• After: R = (30 + 33) ÷ 120 = 0.525 (0.53)
• Change: +0.125, which is +31% relative to the starting frequency (31.25%)
• Survival: RR 15 ÷ 18 = 83%, Rr 33 ÷ 44 = 75%, rr 12 ÷ 38 = 32%. Resistance is conferred by R; survivors bred, so R becomes more frequent.

State whether a change is absolute (0.125) or relative (31%).

Three types of selection

	Stabilising	Directional	Disruptive
Favoured	Mean phenotype	One extreme	Both extremes
Mean	Unchanged	Shifts	May stay similar
Spread	Narrows	Similar, moved	Widens, two peaks
Typical conditions	Stable environment	Changing environment or new pressure	Two distinct niches
Allele frequencies	Intermediate alleles retained	Allele for extreme rises	Alleles for both extremes rise

Method for naming the type from a graph or table

1. Find the mean or peak before and after.
2. Peak moved to one side: directional.
3. Peak stayed, curve taller and narrower: stabilising.
4. Single peak became two peaks: disruptive.
5. Name the type, and state what was removed or favoured, quoting data.

Reproductive separation and speciation

• Reproductive separation: populations stop exchanging alleles, so gene pools can differ.
• Why gene pools diverge: independent mutations, different selection pressures, chance changes in allele frequency.
• New species: genetic differences lead to inability to interbreed and produce fertile offspring.
• Test for two species: crosses between the populations give no offspring, or offspring that are sterile.

Allopatric versus sympatric

	Allopatric	Sympatric
Meaning	In different places	In the same place
Separation by	Geographical barrier	Reproductive barrier without a geographical one
Gene flow stopped by	Distance or barrier	Breeding time, behaviour, niche, chromosome number
Example	Populations on separate islands	Polyploid plants; two forms using different food

Polyploid reminder

Diploid 2n = 18, tetraploid 4n = 36. Tetraploid × diploid gives 27 chromosomes (triploid), which cannot pair into sets at meiosis, so the hybrid is sterile. The tetraploids form a new reproductively separate group in one step.

Model explanation to learn

Question type: “Explain how natural selection led to a higher frequency of an allele.”

• There was phenotypic variation, caused partly by different alleles of the gene.
• A selection pressure (predation, disease or competition) acted on the population.
• Individuals with the advantageous phenotype were more likely to survive.
• They reproduced more and produced more offspring.
• They passed on the favourable allele, so its frequency in the gene pool rose in the next generation.

Add a figure from the data wherever the question gives you one, such as “survival of the resistant genotype was 83% compared with 32%”.

Data-interpretation checklist

Step	What to do
Read the axes and units	Say what is measured and over what time
Find the pattern	Peak shifts, narrows or splits; allele frequency rises or falls
Quote numbers	Give start and end values, and the change
Calculate	Mean from a frequency table: sum of (midpoint × frequency) ÷ total
Link to selection	Which phenotype survived better, and why
Finish with inheritance	Advantageous allele passed on, so frequency changes

Mean from a table, reminder. Seed mass classes 2, 4, 6 mg with 10, 30, 10 seeds: mean = (20 + 120 + 60) ÷ 50 = 4.0 mg. If a later generation has 5, 25, 20 seeds, mean = (10 + 100 + 120) ÷ 50 = 4.6 mg: a shift towards heavier seeds, so directional selection if the whole distribution moved.

Speciation reasoning in short

When asked whether two populations are separate species, look at the cross data. Compare the percentage of pairs producing fertile offspring within each population with the percentage between populations. Offspring that are rare or sterile in between-population crosses support separation. Say that the species criterion requires the inability to produce fertile offspring, so a low but non-zero success rate means the populations are diverging and may not yet be two species.

Selection pressures in context

Pressure	How it produces differential survival	Typical data you might see
Predation	Conspicuous or slow individuals are caught more often	Survival rate by colour or size class
Disease	Individuals with resistance alleles survive infection	Allele frequency in survivors compared with the starting population
Competition for resources	Individuals better at obtaining food, light or space leave more offspring	Growth or offspring number by phenotype

Each pressure acts on phenotypes. The effect on the gene pool appears only after survivors breed, so allele frequency should be compared between generations or between a starting group and the breeding survivors.

Must-know distinctions

• Evolution versus natural selection: evolution is the change in allele frequency; selection is one cause of it.
• Allele versus phenotype frequency: you must say which is changing. A recessive phenotype can be rare while the allele is common.
• Barrier versus speciation: a barrier stops gene flow; speciation needs divergence plus loss of fertile interbreeding.
• Variation versus mutation: mutation creates new alleles; sexual reproduction reshuffles them.
• Heritable versus environmental variation: only heritable variation responds to selection.

Quick self-test

1. Define evolution in terms of alleles.
2. A population has 30 FF, 50 Ff and 20 ff. Calculate the frequency of F.
3. Name three selection pressures in the specification.
4. After a hard winter, the mean wing length of a bird population is unchanged but the range is narrower. Name the selection.
5. A fish population divides into small-bodied and large-bodied groups with few intermediates. Name the selection.
6. Give the percentage change in an allele frequency rising from 0.20 to 0.35.
7. State why a sterile hybrid does not prove the parents are one species.
8. What does allopatric mean, and what stops gene flow?
9. A plant with 2n = 22 gives a tetraploid. How many chromosomes does it have, and how many would a tetraploid × diploid hybrid have?
10. Why must an allele frequency be measured across generations?

Answers

1. A change in the allele frequencies in a population.
2. F = (2 × 30 + 50) ÷ (2 × 100) = 110 ÷ 200 = 0.55.
3. Predation, disease and competition for resources.
4. Stabilising selection.
5. Disruptive selection.
6. (0.35 − 0.20) ÷ 0.20 × 100 = 75% (a rise of 0.15 in absolute terms).
7. Species are defined by producing fertile offspring. A sterile hybrid shows the parents cannot pass genes on across the group boundary, so the parents are different species.
8. In other places. A geographical barrier stops gene flow.
9. Tetraploid 4n = 44. The hybrid has 22 + 11 = 33.
10. Evolution is a property of the population’s gene pool, which only changes when alleles pass to offspring.

Where marks are usually lost

• Saying organisms “evolve because they need to” instead of describing selection of existing variation.
• Leaving out that survivors reproduce and pass on the allele.
• Quoting a phenotype frequency when the question asks for allele frequency.
• Confusing absolute change with percentage change, or forgetting to give units or labels.
• Naming the selection type without quoting figures from the data.
• Writing “cannot interbreed” without “produce fertile offspring”.
• Calling a geographical barrier the cause of the new species, rather than the reason gene flow stopped.
• Treating a variation caused only by environment as a basis for evolution.
• Describing disruptive selection as “selection against all individuals”; it removes the intermediates only.
• Forgetting that polyploid hybrids with odd chromosome sets are sterile because chromosomes cannot pair.

Official syllabus

OxfordAQA, International AS and A-level Biology (9610), specification Version 5.1, sections 3.3.8.1 and 3.3.8.2 (Unit 3, Populations and genes).