A population has a gene pool, the full set of alleles its members carry. Allele frequencies are the proportions in that pool. Hardy-Weinberg predicts that those proportions stay constant unless an evolutionary force disturbs them. Deviation from the prediction is evolution at the population level.
A population has a gene pool, and allele frequency is a property of the population.
A population is a group of organisms of the same species occupying a particular area at a particular time and capable of interbreeding to produce fertile offspring. The gene pool is the complete set of alleles carried by all individuals in that population. Every diploid individual contributes two alleles per locus, so a population of N diploid individuals contributes 2N alleles at each locus.
Write
alleles, notgenes. The gene pool is the population's collection of alleles.Genes in a populationis an explicit reject; the credited phrase isall the alleles in a population.
Three components are required for the AQA-credited population definition. Same species. Particular space at a particular time. Capacity to interbreed, or to produce fertile offspring. All three are required; omitting any one loses the mark. A community (multiple species sharing a habitat) is not a population, and defining a community on a population stem scores zero.
All three components, or you lose marks.
Same species+particular space at a particular time+capacity to interbreed(orproduce fertile offspring). 2024 P3 Q06.1 saw 64% score zero on the definition; 5% earned both marks.
Allele frequency is the proportion of all alleles at a locus that are of a particular type, expressed as a decimal or a percentage. For a locus with two alleles where one has frequency p and the other has frequency q, p + q = 1, because the two alleles are the only options and the proportions must sum to one.
The Hardy-Weinberg equations predict genotype frequencies from allele frequencies.
Hardy-Weinberg is the null hypothesis for evolution at the population level. In a population meeting specified conditions, allele frequencies and genotype frequencies stay constant from one generation to the next. A population in equilibrium is not evolving; a population that deviates is evolving, and the direction of the deviation points to the force responsible.
Allele frequencies: p + q = 1. The two alleles are the only options at the locus. Genotype frequencies: p² + 2pq + q² = 1. p² is the homozygous dominant frequency, 2pq is the heterozygous frequency, and q² is the homozygous recessive frequency. The three genotypes sum to 1 because every individual must have one of them.
Write
2pqin full for the heterozygote frequency.pqalone,2pq², and2pq divided by 2are all explicit rejects. A heterozygote can be drawn two ways (allele A from one parent paired with a from the other, or the reverse), and the factor of 2 accounts for both routes.
q² is the homozygous recessive phenotype frequency, because recessive phenotypes are produced only by aa genotypes. Heterozygotes are not phenotypically distinguishable from homozygous dominants, so the dominant phenotype frequency is p² + 2pq, not p² alone. When the dominant phenotype frequency is the given figure, subtract from 1 to get q² before any further algebra.
Anchor every Hardy-Weinberg working on
q² = the recessive phenotype frequency. The dominant phenotype frequency is p² + 2pq, not p², so subtract from 1 to get q² when the dominant figure is given. Misidentifying q² is the single most common Hardy-Weinberg failure (2017 P2 Q07.1; 2023 P2 Q04.4).
A chi-squared test compares observed genotype counts with the expected counts predicted by Hardy-Weinberg. Degrees of freedom = number of phenotype categories minus one. A calculated chi-squared above the critical value at the 5% significance level means the observed frequencies depart significantly from Hardy-Weinberg expectation, so the population is not in equilibrium.
Five conditions must hold for Hardy-Weinberg equilibrium; violations are evolution in progress.
The equations predict constant frequencies only when all five conditions hold simultaneously. Violation of any one shifts allele frequencies between generations, which is the operational definition of evolution at the population level.
The five Hardy-Weinberg conditions and what violating each one causes.
| Condition | What violating it causes |
|---|---|
| Large population | Genetic drift: allele frequencies change by chance, not by selection |
| Random mating | Genotype frequencies depart from prediction even when allele frequencies stay constant |
| No natural selection | Fitter genotypes increase in frequency at the others' expense |
| No mutation | New alleles enter the gene pool, or existing allele frequencies shift |
| No migration | Immigration brings alleles at new frequencies; emigration removes alleles |
The four credit-bearing answers for Suggest two assumptions of Hardy-Weinberg: no natural selection, no mutation, no migration, random mating. Pick any two. Phrase each as what the model assumes is absent, not what is happening. Large population is the prerequisite and is less consistently credited; the four-item list above is the reliable set.
Write what the model assumes is
absent, not what is happening.Natural selection is occurringis a converse statement and scores zero; the credit-bearing phrasing isno natural selection.
Crossing over,independent assortment,codominance, andrandom fertilisationare sources of variation, not Hardy-Weinberg assumptions. None of them scores on aSuggest two assumptionsstem.
Pitfall — No births or deaths is not a Hardy-Weinberg condition
No births or deathsis not a Hardy-Weinberg condition.Births and deaths are an assumption of the mark-release-recapture method for estimating population size, covered at 3.7.4 (Populations in ecosystems). They are routinely confused with Hardy-Weinberg conditions. Births and deaths happen normally in a population at Hardy-Weinberg equilibrium; what does not happen is selection on which individuals survive to reproduce.
This confusion was the dominant error on 2023 P3 Q06.2, which scored 14% mastery, the lowest score on any year for this topic.
Genetic drift is the small-population consequence in detail. The alleles passed to the next generation are a small sample of the parental gene pool, and small samples vary from expectation by chance. An allele can become more or less frequent, or be lost entirely, by chance alone, without any selection pressure. Inbreeding is the related consequence when mating partners are closely related: the heterozygote frequency falls below Hardy-Weinberg expectation, producing a small gene pool and low genetic diversity.
The full credited drift answer pairs the mechanism (genetic drift; chance) with the outcome (allele frequency changes by chance, not by natural selection; or low genetic diversity; or small gene pool). Either element alone caps the answer at one mark.
Write
the allele, notthe mutation, in drift answers. The heritable variant in the gene pool whose frequency is changing by chance isthe allele.The mutationimplies a new event and is rejected at the drift step.
Variation comes from random fertilisation, meiosis, and mutation; environment modifies the phenotype.
Variation within a population is the raw material on which evolution acts. Three genetic sources contribute. Environmental factors also modify the phenotype, but environmental modifications are not heritable and are not subject to natural selection.
Sperm and egg fuse at random rather than in a pre-determined pairing. Each gamete produced by meiosis carries a unique combination of alleles, so random pairing produces an unpredictable combination of parental alleles in every zygote. Across a population, random fertilisation generates wide genotypic diversity.
Independent assortment in meiosis I distributes homologous chromosome pairs to daughter cells randomly: for each pair, either chromosome can go to either daughter cell. This generates millions of possible gamete chromosome combinations per individual. Crossing over in prophase I exchanges segments of chromatid between homologous chromosomes, producing recombinant chromosomes that carry new allele combinations.
Mutation alters the DNA base sequence and introduces new alleles into the gene pool. It is the only source of genuinely new genetic information; the other two sources reshuffle existing alleles. Germ-line mutations are heritable and can spread through the population by selection or by drift.
Name
independent assortmentandcrossing overas the credit-anchor mechanisms for meiosis-generated variation.Meiosisalone is a fallback accepted only when neither specific mechanism is named.
Continuous variation produces a normal distribution; discontinuous variation produces discrete classes.
Phenotypic variation falls into two broad patterns depending on how many genes contribute to the trait and how much the environment matters.
Continuous and discontinuous variation compared.
| Type | Pattern | Genetic basis | Example |
|---|---|---|---|
| Discontinuous | Discrete classes; no intermediates | One gene (or a few) with little environmental contribution | ABO blood group |
| Continuous | Continuous range; many intermediates; normal distribution | Many genes acting additively (polygenic) plus environmental contribution | Human height |
Continuous variation produces a normal distribution because many independent small additive contributions, by the standard sampling property, produce a symmetrical bell-shaped curve when a sufficiently large sample is plotted. The continuous/discontinuous distinction also connects to 3.7.3, where directional, stabilising, and disruptive selection are described in terms of their effects on the shape of a continuous phenotype distribution.
Key terms
- population
- gene pool
- allele
- allele frequency
- Hardy-Weinberg
- 2pq
- genetic drift
- independent assortment
- crossing over
- random fertilisation
- mutation
- natural selection