Compare continuous and discontinuous variation
Distinguish a continuous range from discrete categories and relate each to measurement.
Section 17.1 · Variation
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 is the defining pattern of discontinuous variation?
What you will learn
- Define continuous and discontinuous variation
- Classify examples by whether phenotypes form a range or distinct categories
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Feature | Pod body length in mm |
| Sample extremes | 42 mm and 76 mm |
| Sample range | 76 − 42 = 34 mm |
| Missing value | No record is not proof of impossibility |
Build the evolutionary explanation
Continuous variation shows a range of phenotypes with many intermediate values, such as measured height or mass. Discontinuous variation places individuals into distinct categories with no intermediates for the measured characteristic, such as ABO blood group. Measurement precision and the way a trait is defined affect how data are recorded, but do not by themselves explain its genetic basis.
Interpret data with care
Do not classify a trait only by whether it is numeric. A count may be discrete, while a measured trait can be continuous. Use the distribution and phenotype categories given.
Pause and explain
A survey records number of offspring per nest. Why might the values be discrete even though they are numerical?
Worked example
A sample of leaf lengths includes many intermediate values. Classify the variation.
Show the worked solution
- The characteristic forms a range.
- Intermediate phenotypes are present.
Answer Continuous variation.
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.
Classify the range of human blood groups A, B, AB and O.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- The listed outcomes are separate phenotype categories.
- There is no intermediate blood-group category.
Discontinuous variation.
Look for these in your work
- Population process identified
- Cause and consequence linked
A measured seed mass ranges from 0.2 g to 1.1 g with many intermediate values. Classify it.
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- The values form a continuous range.
- Intermediate measurements occur.
Continuous variation.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A survey records number of offspring per nest. Why might the values be discrete even though they are numerical?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Offspring are counted in whole numbers.
- There are separate count values, not every intermediate decimal.
The count is discrete because offspring number takes whole-number categories, not a continuous range of intermediate values.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Calling continuous variation “many alleles” without evidence
- Assuming every numerical measurement is continuous
What can you explain now?
What is the defining pattern of discontinuous variation?
Compare with the explanation
Distinct categories with no intermediates
Individuals fall into separate phenotype groups for the characteristic.
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Make it stick
How does a range of intermediates differ from separate categories?
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.