Explain evolution as change in gene pools
Describe evolution as change in inherited allele frequencies and formation of species from pre-existing populations.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What changes during evolution in a population?
What you will learn
- Outline evolution as change in gene pools between generations
- Relate accumulated genetic change to formation of new species
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Generation 1 | 20% stated phenotype |
| Generation 2 | 35% |
| Generation 3 | 52% |
| Interpretation | Trend only; inheritance and reproductive evidence needed |
Build the evolutionary explanation
A population’s gene pool comprises its alleles. Evolution is a change in allele frequencies over generations, caused by processes such as selection, mutation, migration and genetic drift. Accumulated differences between populations can contribute to speciation when gene flow becomes restricted and reproductive isolation develops. New species arise from pre-existing populations rather than appearing fully formed.
Interpret data with care
A change in one individual’s phenotype is not evolution. A population can evolve without every individual changing, because allele frequencies shift across generations.
Pause and explain
Two populations become isolated and accumulate inherited differences. Explain how this can lead to new species.
Worked example
A resistance allele increases from 2% to 35% over many generations. What population-level change is described?
Show the worked solution
- Allele frequency changed.
- The change occurred across generations.
Answer Evolution: the allele frequency in the population gene pool changed over generations.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how natural selection can contribute to evolution.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Selection causes unequal reproductive success.
- Allele frequencies change across generations.
If phenotypes are heritable, individuals with higher reproductive success pass associated alleles to more offspring, changing allele frequencies in the population over generations.
Look for these in your work
- Population process identified
- Cause and consequence linked
A population’s average body size changes during one organism’s lifetime. Is this necessarily evolution?
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- Evolution is population-level inherited change.
- An individual’s growth is not a gene-pool change.
No. An individual changing size as it grows is not evolution; evolution requires a change in inherited allele frequencies across generations.
Look for these in your work
- Evidence interpreted
- Assumptions stated
Two populations become isolated and accumulate inherited differences. Explain how this can lead to new species.
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Restricted gene flow allows divergence.
- Reproductive isolation can prevent successful interbreeding.
With restricted gene flow, mutation, selection and drift can cause populations to diverge. If reproductive isolation develops, they may form distinct species.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Describing an individual as evolving during its lifetime
- Treating evolution as a deliberate process with a goal
What can you explain now?
What changes during evolution in a population?
Compare with the explanation
Allele frequencies in its gene pool
The change occurs across generations.
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Make it stick
What is the population-level definition of evolution?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.