Explain natural selection in populations
Link inherited variation, competition, differential reproductive success and allele transmission over generations.
Section 17.2 · Natural and artificial selection
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
At what level do allele frequencies change through natural selection?
What you will learn
- Explain why overproduction and competition create a struggle for existence
- Explain how better-adapted individuals leave more offspring and pass on alleles
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| 1 · Variation | Inherited short and long beaks |
| 2 · Struggle | Many chicks; limited food |
| 3 · Reproduction | A greater chance in the stated food context |
| 4 · Next generation | Associated alleles can be passed to offspring |
Build the evolutionary explanation
Populations can produce more offspring than available resources can support. Individuals vary, and some variation is heritable. Environmental conditions create competition; individuals with phenotypes better suited to those conditions are more likely to survive and reproduce. Their alleles are passed to more offspring, so allele frequencies can change over generations. Natural selection acts on phenotypes, while evolution is the resulting change in inherited variation in populations.
Interpret data with care
Individuals do not evolve because they need to adapt. Selection changes reproductive success among existing variants; mutations arise independently of an organism’s need, and selection can alter their frequency.
Pause and explain
A population shows an allele-frequency change after several generations. What additional evidence supports natural selection rather than drift?
Worked example
A drought favours plants with deeper roots. Explain how the trait may become more common.
Show the worked solution
- Root variation already exists and is heritable.
- Better-adapted plants produce more offspring carrying relevant alleles.
Answer Plants with deeper roots are more likely to survive and reproduce during drought. If root depth has an inherited component, alleles contributing to it can become more frequent over generations.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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 an inherited resistance trait can increase in a population exposed to a pathogen.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Individuals vary in resistance.
- More resistant individuals leave more offspring carrying associated alleles.
If resistance is heritable, individuals with greater resistance are more likely to survive infection and reproduce. Their offspring inherit resistance-associated alleles, increasing their frequency over generations.
Look for these in your work
- Population process identified
- Cause and consequence linked
A student writes that giraffes stretched their necks and passed the longer necks to offspring. Correct this natural-selection explanation.
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- Variation exists among individuals.
- Those with heritable advantageous variation reproduce more.
Individuals vary in neck length. If longer necks improve reproductive success in the environment and the trait is heritable, associated alleles can become more frequent over generations; individuals do not lengthen their inherited necks by stretching.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A population shows an allele-frequency change after several generations. What additional evidence supports natural selection rather than drift?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Show that the trait affects survival or reproduction in that environment.
- Show the variation is heritable.
Evidence that the allele-associated phenotype is heritable and changes survival or reproductive success in the environment supports selection. A frequency change alone can also result from drift or migration.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Saying individuals evolve during their lifetime
- Saying the environment creates a needed allele
What can you explain now?
At what level do allele frequencies change through natural selection?
Compare with the explanation
In populations over generations
Differential reproduction changes the inherited allele frequencies of a population.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does differential reproductive success change allele frequencies?
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.