Evolution by natural selection
CIE A-Level BiologyΒ· 45 min read
1. Core Principles of the Darwin-Wallace Theoryβ β ββββ± 15 min
Natural Selection
The process by which heritable traits that increase an organism's chance of survival and reproductive success are more likely to be passed on to subsequent generations, leading to changes in allele frequency over time.
Example:
Peppered moths with dark colouration survive better in polluted woodland.
The theory of evolution by natural selection was independently proposed by Charles Darwin and Alfred Russel Wallace, and is built on four core observations:
All populations produce more offspring than can survive to reproduce, leading to competition for limited resources
Genetic variation exists between individuals within all populations
Variation is heritable, meaning it is passed from parent to offspring
Individuals with traits better suited to their environment (adaptations) are more likely to survive, reproduce and pass on their advantageous alleles
Outline how natural selection led to an increase in the frequency of dark-coloured peppered moths (Biston betularia) in 19th century industrial Britain.
- 1
- Existing genetic variation: There was already variation for wing colour in the peppered moth population, with pale speckled and rare dark melanic forms.
- 2
- Change in selection pressure: Industrial soot coated tree bark and killed light-coloured lichens. Pale moths were now easily visible to bird predators, while dark moths were camouflaged.
- 3
- Differential survival and reproduction: More dark moths survived predation, reproduced successfully, and passed the allele for dark colouration to their offspring.
- 4
- Change in allele frequency: Over generations, the frequency of the dark colour allele increased from less than 1% to over 90% in polluted areas.
Exam tip:
In CIE exams, you must explicitly link natural selection to a change in allele frequency, not just trait frequency, to gain full marks.
2. Selection Pressures and Adaptationsβ β β βββ± 20 min
Selection Pressure
An environmental factor that affects the survival and reproduction of individuals with different phenotypes, increasing the reproductive success of individuals with certain favourable traits.
Example:
Antibiotics act as a strong selection pressure on bacterial populations, favouring resistant individuals.
Selection pressures can be biotic (living) or abiotic (non-living). Common examples include predation, competition for resources, disease, temperature, and water availability. Adaptations are heritable traits that increase an organism's fitness (ability to survive and reproduce) in its environment. They can be anatomical, physiological, or behavioural.
Explain how natural selection leads to the evolution of antibiotic resistance in bacterial populations.
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- Random mutation: A rare spontaneous mutation creates an allele for antibiotic resistance in a small number of individual bacteria.
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- Selection pressure acts: When antibiotics are introduced, all non-resistant bacteria are killed, leaving only the rare resistant individuals.
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- Differential reproduction: Resistant bacteria have no competition, so they reproduce rapidly and pass the resistance allele to all their offspring (via asexual reproduction).
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- Outcome: Over a short period of time, the entire population becomes resistant to the antibiotic.
3. Natural vs Artificial Selectionβ β ββββ± 10 min
Natural selection is often contrasted with artificial selection, the process by which humans intentionally select for desirable traits in domesticated plants and animals. The core difference is the source of selection pressure and the outcome.
Key differences between the two processes are summarised below:
Natural Selection
Selection pressure comes from the natural environment. Traits are selected because they increase survival and reproductive success for the organism. Process is slow, occurring over many generations. Results in adaptations that benefit the organism.
+ Pros: Produces adaptations well-suited to the natural environment
β Cons: Slow process, not directed towards a specific human goal
Artificial Selection
Selection pressure comes from intentional human selection. Traits are selected because they are useful or desirable to humans, regardless of benefit to the organism. Process is much faster, occurring over tens to hundreds of generations. Results in domesticated varieties that meet human needs.
+ Pros: Fast, produces varieties with desired human traits
β Cons: May produce traits that reduce survival of the organism in the wild
Use the example of domestic wheat to contrast natural and artificial selection.
- 1
- Selected trait: In natural selection, wheat is selected for seed dispersal to spread offspring. In artificial selection, farmers select for retained seeds that stay on the plant for easy harvest.
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- Source of selection: In natural selection, selection pressure comes from the natural environment (competition, predation). In artificial selection, selection pressure comes from intentional human selection of parent plants.
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- Timescale: Natural selection for wild wheat occurs over thousands of generations. Modern high-yield domestic wheat was developed over less than 100 years of artificial selection.
4. Common Pitfalls
Wrong move:
Claiming that natural selection causes mutations in response to a selection pressure
Why:
Mutations are random and spontaneous, and occur before selection acts. Natural selection only acts on existing variation in the population.
Correct move:
State that random mutation creates the advantageous allele first, then natural selection increases the frequency of this allele in the population.
Wrong move:
Confusing evolution with natural selection and using the terms interchangeably
Why:
Evolution is the outcome (change in a population over time), while natural selection is the mechanism that causes evolution. Exam markers will penalise this confusion.
Correct move:
Define evolution as the change in allele frequency in a population over time, and natural selection as the process that causes this change when advantageous traits increase survival and reproduction.
Wrong move:
Forgetting to mention that variation is heritable in explanations of natural selection
Why:
Only heritable genetic variation can be passed to offspring and lead to long-term evolutionary change. Non-heritable acquired variation cannot be selected for.
Correct move:
Always explicitly state that the advantageous variation is genetically determined and heritable when describing the process of natural selection.
Wrong move:
Claiming that natural selection produces perfect adaptations
Why:
Natural selection can only act on existing variation, and environmental conditions change over time. No adaptation is ever perfect.
Correct move:
Describe adaptations as traits that increase fitness in the current environment, not as perfect or optimal traits.
5. Quick Reference Cheatsheet
Concept | Key Exam Point |
|---|---|
Core Observations | Overproduction β competition; heritable genetic variation exists in populations |
Process of Natural Selection | Advantageous heritable allele β higher survival/reproduction β passed to offspring β increased allele frequency over generations |
Selection Pressure | Environmental factor that favours certain phenotypes; can be biotic (predation) or abiotic (temperature) |
Adaptation | Heritable trait that increases an organism's fitness (survival + reproduction) |
Natural vs Artificial Selection | Natural: environment selects, benefits organism; Artificial: human selects, benefits human |
6. Frequently Asked
Is natural selection the same as evolution?
No. Evolution is the change in heritable characteristics of a population over generations. Natural selection is the mechanism that causes evolutionary change.
Can natural selection act on non-heritable traits?
No. Only genetically encoded traits that are passed to offspring can be selected for by natural selection. Acquired traits cannot be inherited, so they cannot drive long-term evolutionary change.
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
Explanation of antibiotic resistance by natural selection
- 2021 Β· 1
Core principles of natural selection MCQ
- 2023 Β· 4
Compare natural and artificial selection
Going deeper
What's Next
Evolution by natural selection is the foundation of all modern biology, and underpins every other topic in selection and evolution. Understanding this core process allows you to explain everything from the global crisis of antibiotic resistance to the origin of the diversity of life on Earth. Next, you will build on these principles to explore how natural selection leads to the formation of new species, and how other factors like genetic drift also change allele frequencies in populations. Mastery of this sub-topic is critical for answering almost all extended response questions in the selection and evolution unit of CIE A-Level Biology exams.