How does natural selection act on genetic diversity to produce adaptation, and how do the three types of selection differ?
Genetic diversity within a population, expressed as the number of different alleles in a gene pool, is acted on by natural selection. Random mutation produces new alleles, and selection results in changes in allele frequency. Directional and stabilising selection produce different effects, and selection leads to anatomical, physiological and behavioural adaptations that increase the chance of survival and reproduction.
An exam-focused answer to the AQA A-Level Biology 3.4.5 dot point on natural selection. Explains how selection changes allele frequencies, contrasts stabilising, directional and disruptive selection, and covers anatomical, physiological and behavioural adaptations.
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What this dot point is asking
AQA wants you to explain how natural selection changes allele frequencies in a gene pool, to distinguish stabilising, directional and disruptive selection (and recognise which a given example shows), and to classify adaptations as anatomical, physiological or behavioural.
Natural selection and allele frequency
A population shares a gene pool, the total of all alleles of all genes in that population. Genetic diversity is the number of different alleles in that gene pool.
Natural selection works in a fixed sequence:
- Random mutation produces new alleles, so individuals show variation in phenotype.
- A selection pressure (predation, disease, competition, climate) means some phenotypes survive better than others.
- Individuals with advantageous alleles are more likely to survive, reproduce and pass those alleles on (they have greater reproductive success).
- Over many generations the frequency of the advantageous allele increases in the gene pool.
This is how a population becomes better adapted to its environment.
Three types of selection
AQA specifies stabilising and directional selection by name and example. Disruptive selection is the third pattern that completes the set; expect it in data questions where the graph shows two peaks emerging.
Reading selection graphs
- Stabilising - the curve becomes taller and narrower around the same mean.
- Directional - the whole curve shifts left or right; the mean moves.
- Disruptive - the single peak becomes two peaks with a dip in the middle.
Adaptations
An adaptation is a feature that increases an organism's chance of survival and reproduction in its environment. AQA classifies adaptations into three types:
| Type | What it is | Example |
|---|---|---|
| Anatomical | A structural (physical) feature | Streamlined body of a fish; thick fur of an Arctic fox |
| Physiological | An internal, biochemical or functional feature | Production of venom; hibernation metabolism; antibiotic production by some fungi |
| Behavioural | A way of acting | Migration to avoid winter; courtship displays; playing dead to avoid predators |
Common mistakes
Try this
Q1. Explain why human birth mass is an example of stabilising selection. [3 marks]
- Cue. Both very high and very low birth mass have lower survival; intermediate mass is favoured; the range narrows and the mean stays the same.
Q2. Heavy metal tolerance evolved in a population of grass growing on contaminated mine waste. Name the type of selection and explain the change in allele frequency. [4 marks]
- Cue. Directional selection; tolerant plants survive the selection pressure (metal), reproduce, and the frequency of the tolerance allele increases over generations.
Q3. Classify each adaptation as anatomical, physiological or behavioural: (a) a camel storing fat in its hump, (b) a snake injecting venom, (c) a meerkat keeping watch for predators. [3 marks]
- Cue. (a) anatomical, (b) physiological, (c) behavioural.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA Paper 25 marksAntibiotic resistance in a population of bacteria is an example of directional selection. Explain how this type of natural selection leads to a population of resistant bacteria.Show worked answer →
A 5-mark directional selection answer needs the full mechanism with named cause.
- Random mutation produces a new allele giving some bacteria resistance to the antibiotic.
- The antibiotic is a selection pressure; non-resistant bacteria are killed.
- Resistant bacteria survive and reproduce, passing on the resistance allele.
- Over many generations the frequency of the resistance allele increases.
- Eventually most of the population is resistant.
Markers reward mutation as the source, the antibiotic as the selection pressure, differential survival and reproduction, and increased allele frequency over generations.
2020 AQA Paper 23 marksBirth mass in human babies is an example of stabilising selection. Explain what is meant by stabilising selection and why it applies to human birth mass.Show worked answer →
A 3-mark answer needs the definition plus the applied reasoning.
- Stabilising selection favours individuals with the modal (intermediate) phenotype and selects against both extremes.
- Very low birth mass babies have lower survival; very high birth mass babies risk birth complications.
- So intermediate birth mass is selected for, reducing the range of phenotypes and keeping the mean constant.
Markers reward selection against both extremes, the survival reasoning for each extreme, and the link to a maintained mean.
Related dot points
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An exam-focused answer to the AQA A-Level Biology 3.4.1 dot point on DNA, genes and chromosomes. Compares prokaryotic and eukaryotic DNA, defines gene, locus, allele, genome and proteome, and explains exons, introns and the triplet code.
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