17.2.517-18 · Selection, Evolution, Classification and Conservation
Natural Selection- Hardy-Weinberg Principle — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 11 of 36 1 quiz
Lesson summary
17.2.5 Natural Selection: Hardy-Weinberg Principle — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: State the Hardy-Weinberg conditions and use the equations to find allele and genotype frequencies.The principle — predicts allele frequencies in a population; in a lar…
- Allele frequency
- Genotype frequency
- Antibiotic
- Resistance
- It acts as a null model for a population that is not evolving
- It applies accurately only when these conditions are met:No natural selection is taking place
Part 1 of 7The Hardy-Weinberg principle
The principle
- The principle predicts allele frequencies in a population;
- in a large randomly mating population frequencies stay constant from generation to generation.
It acts as a null model for a population that is not evolving.
It applies accurately only when these conditions are met:
- No natural selection is taking place.
- There is no migration into or out of the population.
- Mating is random.
- The population is large.
- No mutations are occurring.
If observed frequencies differ from predicted ones, a condition is not met and evolution is occurring.
Part 2 of 7Alleles and genotypes in the equations
Alleles and genotypes in the equations
The equations apply to a trait controlled by one gene with two alleles (B and b).
Three genotypes are possible:
- Homozygous dominant (BB).
- Heterozygous (Bb).
- Homozygous recessive (bb).
Frequencies are written as proportions of the population, numbers out of 1.
Allele frequency
- Allele frequency the proportion of all alleles in the population that are of one form.
- p = frequency of the dominant allele (B).
- q = frequency of the recessive allele (b).
Genotype frequency
- Genotype frequency the proportion of individuals with a particular genotype.
Part 3 of 7The two equations
| Equation | Meaning |
|---|---|
| p + q = 1 | Allele frequencies add to 1 because there are only two alleles at the locus |
| p2 + 2pq + q2 = 1 | Genotype frequencies add to 1 |
| p2 | Frequency of homozygous dominant individuals |
| 2pq | Frequency of heterozygous individuals |
| q2 | Frequency of homozygous recessive individuals |
Part 4 of 7Finding allele frequency from counts
1
In a population there are two alleles per individual, so 100 individuals have 200 alleles.
2
Allele frequency = number of copies of the allele ÷ total number of alleles at the locus.
3
Once p is known, q = 1 − p, and the reverse.
4
Number of individuals with a genotype = genotype frequency × population size.
Part 5 of 7Calculation strategy
1
Decide which part of the equation you are given and which part you must find.
2
The recessive phenotype is the only phenotype whose genotype is certain (homozygous recessive), so questions usually give q2.
3
Convert a percentage to a decimal; this is q2.
4
Take the square root to find q.
5
Calculate p = 1 − q.
6
Calculate p2 and 2pq; the carrier (heterozygote) frequency is 2pq.
7
Check that p2 + 2pq + q2 = 1.
Part 6 of 7Using and limiting the principle
1
Used to estimate the frequency of carriers of genetic diseases from the frequency of the recessive phenotype.
2
Allele frequencies measured in successive generations show evolution if they change.
- Deviation from the predicted values shows selection, drift, migration, mutation or non-random mating is acting.
3
Real populations never meet all conditions exactly, so results are approximations.
4
Chi-squared can test whether observed genotype numbers differ significantly from Hardy-Weinberg expectations.
5
Sex-linked genes need modified calculations.
Part 7 of 7More from the specification
More from the specification
Hardy-Weinberg equilibrium calculation
- Hardy-Weinberg equilibrium calculation expanding binomial allele frequencies p squared plus 2pq plus q squared provides the mathematical yield for genotype frequencies.
Natural Selection and Environmental Selection Pressures
- Environmental selection factors (such as predation, disease,
- or climatic stress) favor individuals with advantageous phenotypes, enabling them to survive, reproduce,
- and pass alleles to their offspring.
Evolution of Antibiotic Resistance in Bacteria
- Heavy antibiotic use exerts strong selective pressure,
- where rare mutant bacteria possessing resistance alleles survive and multiply, demonstrating rapid adaptation in microbial species.
Artificial Breeding vs Genetic Drift
- In selective breeding, humans deliberately cross plants or animals with desirable traits,
- whereas in small populations genetic drift causes allele frequencies to fluctuate purely by random chance.
Disruptive selection
- Selection favouring both extreme phenotypes.
Variation
- Differences between individuals of the same species.
Selective breeding
- Humans choosing which individuals breed.
Artificial selection
- Selective breeding of organisms with desirable traits by humans.
Selection pressure
- A factor, such as an antibiotic, that favours survival of some individuals.
Founder effect
- Loss of genetic variation when a few individuals found a new population.
Bottleneck effect
- Loss of genetic variation after a sudden fall in population size.
Directional selection
- Selection favouring one extreme phenotype.
Stabilising selection
- Selection favouring the average phenotype.
Antibiotic
- A drug that kills bacteria or stops their growth and does not affect viruses.
Resistance
- The ability of bacteria to survive an antibiotic.
Species
- A group of organisms that can reproduce to produce fertile offspring.
Growth
- A permanent increase in size and dry mass.
Key terms
| Term | Meaning |
|---|---|
| Hardy-Weinberg principle | Allele and genotype frequencies stay constant in a large, randomly mating population with no selection, migration or mutation. |
| p | Frequency of the dominant allele. |
| q | Frequency of the recessive allele. |
| 2pq | Frequency of heterozygous genotype. |
| Allele frequency | Proportion of all alleles at a locus that are of one form. |
| Genotype frequency | Proportion of individuals in a population with a particular genotype. |
Avoid these mistakes
1
Giving the recessive allele frequency as q2 — q2 is the recessive genotype and phenotype frequency; q is the allele frequency.
2
Applying the equations to populations that are small, selected or migrating — the conditions must hold.
3
Forgetting 2pq for heterozygotes — it is the carrier frequency.
4
Starting with the wrong term — identify first which term the question gives.
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