Recognise variation within one species without confusing it with species differences
Variation compares individuals of the same species; similarity is not genetic identity.
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
- Recall genes, alleles, genotype and phenotype from the inheritance pathway. Use the stated organism, units and sample; the records in these lessons are invented.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must be shared for the definition of variation here?
What you will learn
- Identify differences within a species
- Separate observation from genetic inference
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Definition | Differences among individuals of one species |
| Observed length | 48 mm versus 63 mm: 15 mm difference |
| Genetic cause | Not established by length alone |
| Species comparison | Pea versus oak does not by itself meet this definition |
Identify differences within a species
Variation means differences between individuals of the same species. Compare two pea plants of the same identified species: their body length, body mass or seed features may differ. These are examples of variation within that species. Comparing a pea plant with an oak tree is a comparison between species, so it does not by itself illustrate the definition required here. Establish which individuals belong to the comparison before classifying a difference.
An observed difference is a phenotype observation, not automatically evidence of a different allele. Body length can depend on inherited information and growing conditions. Two plants with different lengths might have different genotypes, different environments or both. Conversely, two plants with similar lengths need not have identical genotypes. Keep what was seen separate from the explanation that additional evidence would need to support.
Separate observation from genetic inference
State the feature precisely enough that another observer could compare it. For a plant, body length measured from a defined reference point in millimetres is more useful than calling one specimen bigger. Body mass in grams is a different feature from length. Use comparable developmental stages and methods; a younger plant compared with an older one introduces a possible age explanation rather than isolating inherited differences.
A sample describes the individuals actually examined. It need not contain every phenotype possible in the species. A tray in which all sampled peas have round seeds does not prove that the species lacks other seed shapes. The written investigations in this pathway use original invented records and safe observable specimens; interpreting them is not a substitute for supervised practical competence or a teacher’s evaluation of the whole qualification.
Sources for this lesson
Pause and explain
Two same-species plants differ in length. What is established?
Worked example
Two identified pea plants differ in length, while a pea and an oak differ in leaf form. Which comparison meets the definition of variation?
Show the worked solution
- Check the species membership of the individuals in each comparison.
- The two pea plants belong to one species and differ in a measured feature; the pea–oak comparison crosses species.
Answer The two pea plants show variation within a species. The pea–oak contrast is a species comparison and does not by itself meet this definition.
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.
Two confirmed plants of the same pea species are 48 mm and 63 mm long. Identify the variation and calculate the difference.
Give me a hint
Name the shared species and the measured feature.
Compare my reasoning
- The individuals share species membership; length is the compared phenotype.
- Subtract 48 from 63 using millimetres.
They show body-length variation within the species; the difference is 15 mm.
Look for these in your work
- Within-species comparison
- Length difference with mm units
A worksheet compares the mass of a pea plant and a bean plant. Why is that comparison insufficient for the definition of variation within a species?
Give me a hint
Check whether the individuals belong to one species.
Compare my reasoning
- The worksheet supplies two different species.
- A mass difference between them does not establish differences among individuals within either species.
The comparison is between species. The required definition needs individuals of the same species; within-species observations would be needed.
Look for these in your work
- Correct species boundary
- No claim that mass cannot vary within a species
Ten same-species plants have the same recorded height. A student concludes that their genotypes are identical and the species has no variation. Evaluate both claims.
Give me a hint
A single phenotype and one sample leave information missing.
Compare my reasoning
- Shared height does not identify every allele or equalise every growing condition.
- The sample need not include all individuals or all features in the species.
Neither conclusion follows. The record shows equal sampled heights, not identical genotypes or absence of all variation in the species.
Look for these in your work
- Phenotype–genotype distinction
- Sample limitation
Common mistakes
- Comparing different species as the definition of variation
- Inferring a new allele from any visible difference
- Treating one sample as every possible phenotype
What can you explain now?
Two same-species plants differ in length. What is established?
Compare with the explanation
A phenotype difference in the supplied sample
Length is an observed feature; its cause needs more evidence.
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
Without your notes, identify differences within a species and separate observation from genetic inference. Keep units, the stated sample and the limits of the evidence clear.
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.