Repeat crop selection over many generations
Crop improvement uses repeated choice, crossing and offspring selection with records.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Use the stated case, sample and assumptions. Diagrams and numerical records in this pathway are original teaching models; written answers are self-checked.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes crop selection artificial selection?
What you will learn
- Apply artificial selection to crop plants
- Track the same desired feature through multiple generations
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Generation 1 mean | 2.0 g per seed |
| Generation 3 mean | 2.4 g per seed |
| Difference | +0.4 g in the stated records |
| Caution | Comparable environment and repeated selection needed |
Apply artificial selection to crop plants
Artificial selection in a crop plant is carried out over many generations. People choose parent plants with a desirable feature, cross them to produce seeds and select offspring that show that feature. The chosen offspring can become parents for the next generation. Repeating the sequence can make the target feature more common.
Use a crop goal that can be measured consistently. For an invented bean line, the breeder selects for seed mass under a stated growing protocol. Soil, water, spacing and harvest stage should be kept comparable when comparing generations; otherwise an environmental difference could be mistaken for a breeding change.
Track the same desired feature through multiple generations
Records should include the number of plants assessed, the number selected, the measurement method and which generation is shown. A higher average seed mass in a later cohort can support an observed trend, but it does not by itself prove selective breeding caused it if growing conditions or sampling changed.
Crop improvement can involve trade-offs. Selecting larger seeds may not also increase seed number, germination or drought performance. State the feature targeted and keep other outcomes visible. The syllabus asks learners to outline repeated artificial selection and apply it to crop contexts, not to assume every selected trait improves every aspect of a plant.
Sources for this lesson
Pause and explain
Which comparison best tracks repeated crop selection?
Worked example
In a controlled example, the mean seed mass of selected-line beans is 2.0 g in generation 1 and 2.4 g in generation 3. State the change and one limit before attributing it to breeding.
Show the worked solution
- Subtract the earlier mean from the later mean: 2.4 − 2.0 = 0.4 g.
- Check whether measurement and growing conditions were comparable across generations.
Answer The mean increased by 0.4 g in the stated records. To attribute the change to repeated selection, the breeding and environmental records must support that explanation.
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.
A bean breeder assesses 80 plants per generation and selects 20 with the largest seeds for crossing. State the selected fraction and the repeated next step.
Give me a hint
Use the 80 assessed plants for the denominator.
Compare my reasoning
- 20/80 = 25%.
- Cross selected plants, grow the next cohort and measure/select offspring using the same protocol.
One quarter were selected; the process must be repeated through new crosses and offspring selection over generations.
Look for these in your work
- Fraction calculated
- Repeated cycle described
Mean seed mass is 1.8 g in an earlier cohort and 2.1 g in a later cohort grown in a different soil. What can be stated and what remains uncertain?
Give me a hint
Calculate the difference, then consider the changed condition.
Compare my reasoning
- 2.1 − 1.8 = 0.3 g.
- Different soil may affect seed mass, so breeding cannot be isolated from these records.
The later mean is 0.3 g greater, but the soil difference means the record alone cannot attribute the change to selection.
Look for these in your work
- Numeric difference correct
- Causal limit
A line selected for large seeds shows fewer seeds per plant in generation 4. Evaluate whether the crop has been improved.
Give me a hint
Use the stated goal and consider other relevant outcomes.
Compare my reasoning
- Large seed mass improved only if measured comparably.
- Lower seed number is a possible trade-off; total yield or the farmer’s goal is needed.
The target feature may have improved, but the overall judgement depends on the goal and trade-offs; report seed mass and seed number separately.
Look for these in your work
- Target assessed
- Trade-off stated
Common mistakes
- Calling one cross “many generations”
- Ignoring changed growing conditions
- Claiming improvement in all crop traits
What can you explain now?
Which comparison best tracks repeated crop selection?
Compare with the explanation
Comparable measurements and selection records across generations
The method, conditions and selection steps need to be recorded.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, apply artificial selection to crop plants and track the same desired feature through multiple generations. 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.