Review Supplement selection and resistance
Integrate adaptation, phenotype records, bacterial resistance and the two selection processes.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Use the supplied biological context. Resistance examples are educational and not medical advice or lab protocols.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement correctly describes population adaptation?
What you will learn
- Explain adaptation as a population process over generations
- Evaluate a resistance record and compare selection mechanisms
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Adaptation | Population; many generations |
| Resistance | Bacteria; specific drug context |
| Natural selection | Environmental effects on reproductive chance |
| Artificial selection | Human choice, crossing and offspring selection |
Explain adaptation as a population process over generations
Adaptation by natural selection describes populations becoming more suited to their environment over many generations. Link inherited variation with resource competition and differential reproductive outcomes. Individuals do not acquire needed alleles because of a short-term challenge; descendants inherit alleles through reproduction.
Resistance is a bacterial example. A population may contain variants with different responses to a particular antibiotic. The drug context can select for bacteria more likely to survive; resistant bacteria that reproduce can pass resistance information to descendants. Resistance belongs to germs, not to a person, and the model does not prescribe treatment.
Evaluate a resistance record and compare selection mechanisms
For numerical records, calculate each phenotype fraction using the total at that stage. The resistant proportion can rise even when resistant counts fall. A phenotype fraction is not automatically an allele frequency. Keep the origin of variation, the selection event, subsequent reproduction and any horizontal transfer distinct.
Natural and artificial selection both rely on variation and inheritance. In natural selection, environmental conditions affect reproductive chances. In artificial selection, people choose desirable parents, cross them and select offspring. State which mechanism a question supports and leave later syllabus areas, practical competence and cumulative assessment within their explicit limits.
Sources for this lesson
Pause and explain
Which pair correctly identifies the two selection mechanisms?
Worked example
A bacterial dataset gives 10 resistant and 90 susceptible before exposure; 8 and 9 survive. A separate crop programme deliberately chooses parents for seed size. Calculate the resistant-share change and distinguish the processes.
Show the worked solution
- Calculate 10/100 before and 8/17 after.
- The bacterial drug context is natural selection; deliberate crop parent choice is artificial selection.
Answer The resistant fraction rises from 10% to about 47.1% while its count falls from 10 to 8. The bacterial case illustrates selection by a drug context; deliberate crop parent choice is artificial selection. The figures are stipulated phenotype counts, not allele frequencies.
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.
For 10 resistant, 90 susceptible before and 8, 9 survivors, calculate the resistant share at both stages and state the absolute count change.
Give me a hint
Use totals 100 and 17.
Compare my reasoning
- Before 10%; after 8/17≈47.1%.
- Resistant count changes 10→8, a fall of 2.
The share rises from 10% to about 47.1%, while the resistant count falls by 2; the data are stipulated phenotype groups.
Look for these in your work
- Correct stage percentages
- Absolute count change
A wild plant population becomes more drought-suited over many generations; farmers separately choose drought-tolerant crop parents. Compare.
Give me a hint
Separate environmental effects from deliberate breeding decisions.
Compare my reasoning
- The wild-population case is natural selection if inheritance and differential reproduction are supported.
- The crop programme is artificial selection because people choose parents.
Both involve inheritance and reproduction; the primary selection pressure differs between environment and human parent choice.
Look for these in your work
- Correct classification
- Shared basis and distinction
A report shows more resistant bacteria after an antibiotic, but gives no pre-exposure sample, genotype count or comparison group. State one observation and two limits.
Give me a hint
Keep observed detection separate from origin, selection and allele-frequency claims.
Compare my reasoning
- Resistant bacteria were detected at the later stage.
- No baseline means the change cannot be quantified.
The observation is that resistance was detected later; the record cannot quantify a before–after shift or establish when resistance arose, nor calculate allele frequency without genotype data.
Look for these in your work
- Observation stated
- Two limits
Common mistakes
- Using the same denominator at both stages
- Calling the antibiotic artificial selection by itself
- Treating phenotype counts as allele counts
What can you explain now?
Which pair correctly identifies the two selection mechanisms?
Compare with the explanation
Environment affects reproduction; people choose breeding parents
The selector and process differ.
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Make it stick
Without your notes, explain adaptation as a population process over generations and evaluate a resistance record and compare selection mechanisms. Explain the mechanism, use the given population or breeding records and state what the evidence does not establish.
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.