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

This guide covers AQA A-level Biology (7402) sections 3.7.2, Populations, and 3.7.3, Evolution may lead to speciation, as set out in the AQA AS and A-level Biology specification for first teaching 2015. Both sections are A-level only, not AS. They sit in topic 7, which the assessment overview places on A-level Paper 2 and Paper 3. Course pages: the AQA A-level Biology hub, the printable checklist and the free diagnostics. Next steps: the revision notes and the practice questions.

What the specification asks for

Section	What you must be able to do
3.7.2	Define a population; use the terms gene pool and allele frequency; state what Hardy-Weinberg predicts and the conditions for it; calculate allele, genotype and phenotype frequencies with p² + 2pq + q² = 1
3.7.3	Explain why phenotype varies; explain natural selection and its effect on allele frequency; describe stabilising, directional and disruptive selection; define evolution; explain allopatric and sympatric speciation; explain why genetic drift matters only in small populations; explain how long-term change gives a great diversity of species

The practical for this topic area, required practical 12, is listed under 3.7.4 and belongs with the next unit. Natural selection and antibiotic resistance are also met in section 3.4.4: see genetic diversity and natural selection. Mutation and meiosis are in mutation and meiosis.

3.7.2 Populations and gene pools

A population is a group of organisms of the same species, living in a particular space at a particular time, that can potentially interbreed. All three parts of that definition earn credit: same species, same place and time, able to interbreed.

The gene pool is the complete set of alleles, for every gene, held by a population at a given time. The allele frequency is how often one allele occurs in the gene pool, written as a proportion (0 to 1) or a percentage. Frequencies are about alleles, so a heterozygote contributes one copy of each allele, and a homozygote contributes two copies of one.

The Hardy-Weinberg principle

The principle is a mathematical model. It predicts that allele frequencies will not change from generation to generation. It applies only when:

• the population is large, so chance has little effect;
• individuals pair up at random;
• mutation does not occur;
• no individuals enter or leave (no gene flow);
• natural selection is absent, meaning all genotypes survive and reproduce equally well.

For a gene with two alleles, p is the frequency of one allele (usually the dominant) and q is the frequency of the other (usually the recessive). The alleles are the only two, so:

p + q = 1

The genotype frequencies are given by p² + 2pq + q² = 1, where p² is the frequency of the homozygous dominant genotype, 2pq is the heterozygote and q² is the homozygous recessive.

Method for recessive-phenotype questions

1. Recessive phenotype frequency = q². Take the square root to get q.
2. p = 1 − q.
3. Work out whichever frequency is asked: p², 2pq, or the dominant phenotype (p² + 2pq = 1 − q²).
4. If a number of individuals is wanted, multiply the frequency by the population size.

Worked example 1. A recessive allele causes a hereditary eye disorder in dogs. In a breed population of 40 000, 25 dogs have the disorder. How many carriers are expected?

• q² = 25 ÷ 40 000 = 0.000 625
• q = √0.000 625 = 0.025
• p = 1 − 0.025 = 0.975
• 2pq = 2 × 0.975 × 0.025 = 0.048 75
• Carriers = 0.048 75 × 40 000 = 1950

Note how rare the disorder is (1 in 1600) compared with carriers (about 1 in 20). Most copies of a rare recessive allele are hidden in heterozygotes, which is why selection removes such alleles slowly.

Method when genotype counts are given

Count alleles, not individuals. Worked example 2. A clover population of 250 plants has 120 RR, 90 Rr and 40 rr.

• Total alleles = 2 × 250 = 500.
• R alleles = (2 × 120) + 90 = 330, so p = 330 ÷ 500 = 0.66.
• q = 1 − 0.66 = 0.34.
• Expected numbers if Hardy-Weinberg applied: RR = 0.66² × 250 = 108.9; Rr = 2 × 0.66 × 0.34 × 250 = 112.2; rr = 0.34² × 250 = 28.9.
• Observed: 120, 90, 40. There are fewer heterozygotes and more of both homozygotes than expected, so this population is not in Hardy-Weinberg equilibrium.

The model then helps you reason. A deficit of heterozygotes fits non-random mating, such as plants mostly breeding with close neighbours, or selection against Rr. State the evidence (the numbers), then link it to a named condition.

3.7.3 Variation within a population

Individuals in a population show a wide range of phenotypes. Two causes act together:

• Genetic factors. Mutation is the primary source of new alleles. Meiosis shuffles alleles through crossing over between homologous chromosomes and independent segregation of chromosomes. Random fertilisation then combines two gametes by chance. With 23 chromosome pairs, independent segregation alone gives 2²³ = 8 388 608 different gametes, and a random pairing of two such gametes gives 2⁴⁶ ≈ 7.0 × 10¹³ combinations, before crossing over is counted.
• Environmental factors. Diet, light, temperature and disease alter how a genotype is expressed. Genetically identical plants grown in different light can differ in height.

Most phenotypes reflect both. In an explanation, name a genetic source (mutation, meiosis, random fertilisation) and an environmental one, and say that the phenotype is the product of genotype and environment.

Natural selection and allele frequency

Predation, disease and competition for the means of survival (food, water, mates, space) cause differential survival and reproduction. Organisms whose phenotype gives a selective advantage are more likely to survive, breed and pass on the favourable allele. The allele frequency in the gene pool rises; unfavourable alleles become less frequent.

Evolution is a change in the allele frequencies in a population. Selection is one cause of it, and the Hardy-Weinberg conditions give a baseline for spotting it.

Worked example 3. In a population of 1000 beetles, q (pale allele, a) = 0.30, so p = 0.70. Predators eat every pale (aa) beetle before it breeds.

• Before selection: AA = 0.70² × 1000 = 490; Aa = 2 × 0.70 × 0.30 × 1000 = 420; aa = 0.30² × 1000 = 90.
• Survivors: 490 + 420 = 910 beetles, holding 1820 alleles.
• A alleles = (2 × 490) + 420 = 1400; a alleles = 420.
• New q = 420 ÷ 1820 = 0.231, down from 0.30.
• The next generation would then contain about 0.231² = 5.3% pale beetles, compared with 9% before.

Because the recessive allele hides in Aa survivors, it falls only gradually.

Three types of selection

Type	Environment	Effect on the phenotype distribution	Example
Stabilising	Stable	Extremes selected against; the mean stays and the range narrows	Wing length in a bird on an exposed coast: very long and very short wings both reduce survival in strong winds
Directional	Changing	One extreme favoured; the mean shifts that way	Insecticide resistance in an insect pest after repeated spraying
Disruptive	Varied, with more than one niche	Both extremes favoured over the mean; the distribution becomes bimodal	Black-bellied seedcracker finches have either large or small bills, each suited to different seeds

In each case, selection acts on phenotypes and changes the allele frequencies in the gene pool.

Genetic drift

Genetic drift is a change in allele frequency produced by chance, not by advantage. Which alleles reach the next generation depends partly on which individuals happen to breed and which gametes happen to fuse.

It matters in small populations because a few chance events represent a large share of the gene pool. Take a population of 10 diploid individuals, with 20 copies of a gene. If three copies fail to pass on by chance, the frequency changes by 3 ÷ 20 = 0.15. In a population of 10 000 (20 000 copies), the same three copies change it by 3 ÷ 20 000 = 0.000 15, which is negligible. In a small population an allele held by one or two individuals may be lost altogether, with no selective cause. In a large population, chance effects average out.

Compare the two causes:

• Selection is directional in effect and depends on the phenotype and the environment.
• Drift has no direction and does not depend on advantage.

Speciation

A species here is a group of organisms that can interbreed and produce fertile offspring. New species arise from existing species when two populations undergo reproductive separation and their gene pools accumulate differences, until members of the two groups are unable to breed together and leave fertile offspring.

Allopatric speciation

Allopatric speciation follows geographical separation, for example a river changing course or a rise in sea level splitting a population.

1. A barrier splits one population into two, with no gene flow between them.
2. Mutations arise independently in each, and the two environments may differ.
3. Natural selection favours different alleles in each population, and genetic drift acts in any small one. Allele frequencies diverge.
4. The gene pools accumulate differences (for example in courtship behaviour, anatomy or breeding season).
5. If the barrier is removed, the two populations cannot interbreed to produce fertile offspring: they are separate species.

Sympatric speciation

Sympatric speciation happens without geographical separation: the populations share a range. Reproductive isolation comes from other barriers, such as:

• different breeding seasons or flowering times (seasonal isolation);
• different courtship behaviour (behavioural isolation);
• changes in chromosome number, such as polyploidy in plants. Common cord-grass in Britain, Spartina anglica, arose from a hybrid whose chromosome set doubled, restoring fertility, while it could not breed fertilely with its parent species.

The same logic applies as in the allopatric case: separation, then divergence of gene pools by mutation and selection, then loss of fertile interbreeding.

Diversity over long periods

Repeated cycles of variation, selection and isolation, over a very long time, split lineages again and again. Each split adds a species, which is how one ancestral population can give rise to the great diversity of species.

Common errors

• Writing that individuals “adapt” or “evolve”. Individuals do not evolve; populations do, by allele frequency change.
• Treating p² as the frequency of the dominant phenotype. The dominant phenotype is p² + 2pq.
• Starting a calculation from the dominant phenotype. Only the recessive phenotype gives q² directly.
• Giving counts of individuals instead of counting alleles.
• Saying drift “favours” an allele. It is chance.
• Omitting “fertile” from the definition of a species.
• Giving geographical separation as the cause of sympatric speciation.

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.