Lesson focus: natural selectionIndividuals with variations better suited to their environment are more likely to survive and reproduce, passing on their alleles. changes populations when heritable variants differ in survival or reproductive success under particular conditions.
Explain natural selection as a multi-generation change in allele frequencyHow common one version of a gene is in a population. and evaluate evidence for selection.
Build on earlier learning
InheritanceInheritance explains how alleles pass to offspring. Natural selection explains how their frequencies can change across generations.
You will be successful when you can…
- Identify heritable variation.
- State the selection pressure.
- Explain differential reproduction.
- Describe population change across generations.
Words that carry the science
- natural selection
- Differential survival and reproduction associated with heritable variation.
- fitness
- Relative reproductive success in a particular environment.
- allele frequency
- The proportion of gene copies represented by an allele in a population.
- selection pressure
- An environmental factor that changes relative reproductive success.
- resistance
- Inherited ability to survive exposure that harms susceptible organisms.
- evolution
- Change in inherited characteristics or allele frequencies of a population across generations.
Build the full selection chain from variation to population change
01Selection requires heritable variation
Mutation creates alleles and sexual reproduction reshuffles them. If a variant is heritable and affects reproductive success, its frequency can change. Environmental differences that do not pass to offspring are not directly inherited by selection.
Selection acts on phenotypes in a particular environment, while evolutionThe change in the inherited characteristics of a population over many generations, by natural selection. is measured as population-level inherited change.
02Differential reproduction drives change
A selection pressureSomething in the environment, such as a predator or disease, that makes some living things more likely to survive than others. such as predation, drought or an antibiotic affects variants differently. Individuals with a favourable phenotype may survive longer or reproduce more, passing associated alleles to more offspring.
Across generations, those alleles can become more common. Individuals are selected; populations evolve.
03Resistance evolves in populations
A bacterial population may already contain resistant variants before antibiotic exposure. Treatment kills susceptible bacteria more effectively, leaving resistant bacteria to reproduce and transfer resistanceWhen living things, such as bacteria or insects, are no longer killed by a chemical that used to kill them. alleles.
Stopping misuse reduces selection pressure, but resistance may persist, spread between bacteria or carry costs. Antibiotics do not cause bacteria to choose useful mutations.
Natural selection in peppered moths
Birds eat the moths they can see. On sooty bark the dark moths are camouflaged, survive more often and pass on their genes, so dark moths become more common.
Controls
Switch to clean bark part-way through to see what happened after the Clean Air Acts. Survival rates are illustrative.
Explain pesticide resistance
A pesticide becomes less effective after repeated use.
- The original pest population contains heritable variation in susceptibility.
- The pesticide kills susceptible pests more often.
- Resistant survivors leave proportionally more offspring.
- Resistance alleles become more frequent over generations, so the pesticide kills a smaller fraction.
How to make the reasoning visible: The pesticide selects among existing variants; it does not teach individual pests to resist it.
Follow allele frequency through selection
The values are illustrative frequencies of a resistance allele.
| Generation | Before treatment | After reproduction | Selection interpretation |
|---|---|---|---|
| 0 | 0.05 | 0.05 | Rare standing variation |
| 1 | 0.05 | 0.12 | Resistant survivors contribute more offspring |
| 2 | 0.12 | 0.28 | Frequency rises |
| 3 | 0.28 | 0.51 | Resistance becomes common |
| No-treatment comparison | 0.05 | 0.04 | No increase under these conditions |
Why is the no-treatment population useful?
Why does resistance not need to reach 100%?
Check your understanding
Answer from memory, then check against the suggested answer.
What must variation be for selection to cause evolution?
Do individual organisms evolve?
What is fitness in evolution?
Explain why an antibiotic does not cause a bacterium to mutate in the direction it needs.
Explain why natural selection and genetic drift can both change allele frequency.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A population of bacteria becomes resistant to an antibiotic.
Explain why some bacteria survive when the antibiotic is used.
(4)Explain how the whole population becomes resistant.
(2)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., finish with an explicit change in the population; survival of one bacterium is not the whole evolutionary account.