Natural Selection
BiologyΒ· 18.2 Natural selectionΒ· 15 min read
1. Mechanism of Natural Selectionβ β ββββ± 5 min
Natural selection
The process by which individuals with inherited characteristics well suited to their environment produce more offspring than less well suited individuals, leading to changes in allele frequency over generations.
Example:
Peppered moths with dark colouration survive better in polluted environments, so pass on the dark allele more frequently.
Natural selection relies on five key observations and inferences first outlined by Darwin and Wallace:
Populations produce more offspring than can survive to reproduce, leading to competition for resources.
There is genetic variation between individuals in a population for most traits.
Certain variations (adaptations) make individuals more likely to survive and reproduce in a given environment.
Adaptive traits are heritable, so are passed on to offspring.
Over generations, advantageous alleles increase in frequency, while harmful alleles decrease in frequency.
Explain how natural selection led to an increase in the frequency of the allele for dark body colour in peppered moths (Biston betularia) during the Industrial Revolution in the UK.
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- Original population: Most moths were light-coloured, with a rare recessive allele for dark colour. Light moths were camouflaged on lichen-covered tree bark, so avoided predation by birds.
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- During the Industrial Revolution, soot from factories killed lichen and blackened tree bark, creating a new selection pressure: increased predation on light-coloured moths.
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- Dark-coloured moths were now better camouflaged, so had higher survival and produced more offspring than light moths.
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- The dark allele was passed to offspring generation after generation, so its frequency increased from less than 1% to over 90% in 100 years.
Exam tip:
Always link selection pressure to survival, reproduction and change in allele frequency in 3+ mark questions; CIE awards separate marks for each of these linked steps.
2. Three Main Types of Natural Selectionβ β β βββ± 6 min
Natural selection acts on polygenic traits (controlled by multiple genes) in three distinct ways, depending on which phenotype is favoured by the environment. Each type produces a different change to the distribution of phenotypes:
Selection types
Categories of natural selection grouped by which part of the phenotype distribution is favoured by environmental conditions.
Example:
Human birth weight is under stabilizing selection.
Type of selection | Favoured phenotype | Effect on variation | Example |
|---|---|---|---|
Stabilizing | Intermediate | Reduces variation, preserves mean trait | Human birth weight |
Directional | One extreme | Shifts distribution toward the extreme | Antibiotic resistance |
Disruptive | Both extremes | Increases variation, may lead to speciation | Finch beak size |
A population of squirrels has body mass ranging from 300g to 800g, with a mean of 550g. After many generations, the mean remains 550g, but the range narrows to 450g to 650g. What type of selection is this, and why?
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Step 1: Identify the change: The mean phenotype has not changed, but the range of variation has decreased, with extreme phenotypes eliminated from the population.
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Step 2: Match to selection type: This matches stabilizing selection, which favours intermediate phenotypes and selects against both small and large extremes.
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Step 3: Explanation: Intermediate body mass is adaptive: very small squirrels cannot retain enough heat, and very large squirrels cannot escape predators easily, so intermediate individuals have the highest survival and reproduction.
Check your understanding:
Which type of selection increases genetic variation in a population?
Stabilizing
Directional
Disruptive
None
Reveal answer
Disruptive βCorrect! Disruptive selection favours both extreme phenotypes, so both alleles for the trait remain common in the population, increasing overall variation.
3. Allele Frequency Change from Selectionβ β β βββ± 5 min
Natural selection is measured by the change in allele frequency in a population over generations. A selection pressure that favours one allele will consistently increase its frequency from generation to generation.
In a population of bacteria, 80% of alleles for a ribosomal protein are streptomycin-sensitive (S), and 20% are resistant (s). Streptomycin is added, and all SS bacteria die. Ss bacteria have 50% the reproductive success of ss bacteria. Calculate the new frequency of s after one generation of selection.
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Step 1: Use Hardy-Weinberg to get starting genotype frequencies, with and :
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Step 2: Apply fitness (relative reproductive success): SS fitness = 0, Ss fitness = 0.5, ss fitness = 1. Calculate weighted frequencies after selection:
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Step 3: Calculate total population frequency after selection, then find new :
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Step 4: Conclusion: The frequency of the resistant allele s increased from 0.2 to 0.6 in one generation, demonstrating the effect of selection.
Exam tip:
Always remember: natural selection acts on individual organisms, but the evolutionary change occurs in populations, changing overall allele frequency. Individuals do not evolve.
4. Key Exam Examplesβ β ββββ± 4 min
CIE examiners expect you to reference named, specific examples of natural selection in written answers, usually for 3-5 mark extended questions. Three of the most common examples are:
Antibiotic resistance in bacteria: Random mutation creates a resistance allele in one bacterium. Antibiotics act as a selection pressure, killing all non-resistant bacteria. Resistant bacteria survive and reproduce, passing on the resistance allele, leading to a fully resistant population.
Peppered moth industrial melanism: As described earlier, soot pollution changed camouflage and predation pressure, leading to increased frequency of the dark allele; clean air laws have since reversed this change.
Beak size in Darwin's finches: Drought reduces the supply of small soft seeds, leaving only large hard seeds. Finches with larger stronger beaks survive better, so average beak size increases in the population over a few generations.
5. Common Pitfalls
Wrong move:
Claiming individuals change their alleles or adapt during their lifetime in response to selection pressure.
Why:
Evolution acts on populations over generations, not individual organisms. Individuals cannot change their inherited DNA.
Correct move:
State that advantageous alleles already exist in the population due to random mutation; selection increases the frequency of these alleles over generations.
Wrong move:
Confusing disruptive selection with directional selection.
Why:
Directional selection favours one extreme phenotype, while disruptive selection favours both extremes.
Correct move:
Remember: Disruptive disrupts the middle, so it eliminates intermediate phenotypes and keeps both extremes.
Wrong move:
Stating that natural selection is the only cause of evolution.
Why:
Other mechanisms including genetic drift, gene flow and mutation also cause changes in allele frequency.
Correct move:
State that natural selection is the only mechanism that causes adaptive evolution; other processes cause non-adaptive changes.
Wrong move:
Forgetting to name the selection pressure when explaining natural selection in an exam answer.
Why:
CIE awards a specific marking point for explicitly identifying the environmental factor driving selection.
Correct move:
Always explicitly name the selection pressure (e.g. predation by birds, antibiotic exposure) in your explanation.
Wrong move:
Claiming stabilizing selection changes the mean phenotype of a population.
Why:
Stabilizing selection selects against extreme phenotypes, so the population mean stays the same, while variation decreases.
Correct move:
Only directional selection shifts the mean phenotype of the population toward the favoured extreme.
6. Quick Reference Cheatsheet
Concept | Key Definition | Key Effect |
|---|---|---|
Natural selection | Differential survival/reproduction of heritable traits | Change in allele frequency over generations |
Stabilizing selection | Favours intermediate phenotypes | Mean stays same, variation decreases |
Directional selection | Favours one extreme phenotype | Mean shifts toward the favoured extreme |
Disruptive selection | Favours both extreme phenotypes | Variation increases, may lead to speciation |
Selection pressure | Environmental factor affecting survival/reproduction | Determines which traits are selected for |
7. Frequently Asked
Is evolution the same as natural selection?
No. Evolution is defined as the change in allele frequency in a population over generations. Natural selection is one mechanism that causes evolution, specifically adaptive evolution.
Do I need to memorize specific examples for the exam?
Yes. CIE examiners expect you to use named examples (e.g. antibiotic resistance, peppered moths) to illustrate natural selection in extended response answers, and marks are awarded for correct, relevant examples.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Explain the process of natural selection
- 2023 Β· 1
Identify type of selection from description
- 2024 Β· 4
Discuss antibiotic resistance as selection example
Going deeper
What's Next
Natural selection is the core mechanism of adaptive evolution, and this concept connects to all further topics in selection and evolution, including speciation and artificial selection. Understanding how natural selection changes allele frequency over generations is also foundational for analysing real-world evolution, such as the impacts of climate change on wild populations and the growing threat of antibiotic resistance in disease-causing bacteria. CIE exams regularly test links between natural selection and these applied topics, so mastering this sub-topic will help you access full marks on extended response questions.