Edexcel A Level (IAL) Biology -4.20 Hardy-Weinberg Equation- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -4.20 Hardy-Weinberg Equation- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -4.20 Hardy-Weinberg Equation- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 4.20 (i) understand how the Hardy-Weinberg equation can be used to see whether a change in allele frequency is occurring in a population over time
(ii) understand that changes in allele frequency can come about as a result of mutation and natural selection
(iii) understand that reproductive isolation can lead to accumulation of different genetic information in populations, potentially leading to the formation of new species
Hardy-Weinberg Principle, Allele Frequency & Speciation
🌱 Introduction
Populations evolve over time when allele frequencies change. To check whether evolution is happening, scientists use the Hardy-Weinberg equation a mathematical model describing gene frequencies in a stable population.
⚖️ Hardy-Weinberg Principle
Definition:
Predicts allele and genotype frequencies in a population remain constant from generation to generation if no evolutionary forces act on it.
🧩 Equation:
\( (p + q)^2 = 1 \)
Expanded form:
\( p^2 + 2pq + q^2 = 1 \)
| Symbol | Meaning |
|---|---|
| p | Frequency of dominant allele (A) |
| q | Frequency of recessive allele (a) |
| p² | Frequency of homozygous dominant genotype (AA) |
| 2pq | Frequency of heterozygous genotype (Aa) |
| q² | Frequency of homozygous recessive genotype (aa) |
Note:
Since there are only two alleles: \( p + q = 1 \)
Example:
If 16% of a population shows recessive phenotype (aa):
\( q^2 = 0.16 \Rightarrow q = 0.4 \)
Then, \( p = 1 – 0.4 = 0.6 \)
Genotype frequencies:
AA (p²) = 0.36, Aa (2pq) = 0.48, aa (q²) = 0.16
Compare observed vs expected frequencies → evolution if they differ.
🚫 Assumptions of Hardy-Weinberg
- No mutation
- No migration (gene flow)
- No natural selection
- Large population (avoid genetic drift)
- Random mating
Violation → allele frequencies change → evolution occurs.
🧬 Causes of Changes in Allele Frequency
| Factor | Effect on Population |
|---|---|
| Mutation | Creates new alleles → introduces variation |
| Natural Selection | Favourable alleles increase in frequency |
| Genetic Drift | Random changes, especially in small populations |
| Gene Flow | Migration introduces or removes alleles |
| Non-random mating | Certain alleles become more common due to mate selection |
Example: Mutation giving bacteria antibiotic resistance → allele frequency increases over generations.
🌍 Reproductive Isolation & Formation of New Species
Reproductive isolation prevents gene flow → allows independent evolution.
| Type | Explanation | Example |
|---|---|---|
| Geographical (Allopatric) | Physical barrier separates populations | River or mountain divides species |
| Behavioral | Different courtship/mating rituals | Bird songs differ between groups |
| Temporal | Breed at different times | Two frog species breed in different seasons |
| Mechanical | Incompatible reproductive organs | Different flower structures prevent cross-pollination |
| Gametic | Sperm and egg don’t fuse | Coral species releasing gametes simultaneously but not compatible |
Over time: Mutations accumulate → genetic differences → if interbreeding impossible → new species forms (speciation).
🧩 Summary Table
| Concept | Definition / Formula | Key Point |
|---|---|---|
| Hardy-Weinberg Equation | \((p + q)^2 = 1\) | Predicts stable allele frequencies |
| Allele Frequency Change | Due to mutation, selection, drift, migration | Drives evolution |
| Reproductive Isolation | Stops gene flow between populations | Leads to new species |
⚡ Quick Recap
H-W equation: \( p^2 + 2pq + q^2 = 1 \)
Frequencies constant → no evolution.
Mutation & natural selection change allele frequencies.
Isolation stops gene flow → populations evolve separately.
Over time → speciation occurs.