Compare three species concepts
Distinguish biological, morphological and ecological definitions of a species and their limits.
Section 18.1 · Classification
A Level; Cambridge International Biology 9700, 2028–2030. Written responses are self-checked; this does not certify supervised fieldwork or practical competence.
Before you begin
- Classification compares shared characteristics; populations occupy environments and contain variation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What evidence is central to the biological species concept?
What you will learn
- Discuss the biological species concept using interbreeding and fertile offspring
- Compare morphological and ecological species concepts
Build the explanation
The biological species concept groups organisms that can interbreed to produce fertile offspring. The morphological concept groups organisms by shared observable structural features. The ecological concept groups organisms by their ecological niche, including the resources and conditions they use. Each concept can be useful, but they do not always classify the same organisms in the same way.
Use evidence within scope
The biological concept is difficult to apply to fossils and asexual organisms and can be complicated by hybridisation. Similar appearance does not alone prove interbreeding; ecological similarity does not prove that organisms form one reproductive species. State which concept is being used.
Pause and explain
An asexual organism reproduces successfully but cannot interbreed. Why is the biological species concept limited here?
Worked example
Two populations look similar and occupy similar habitats, but no interbreeding evidence is available. Which concept can group them from the stated evidence?
Show the worked solution
- Shared appearance supports morphological comparison.
- Similar niche supports ecological comparison.
Answer The evidence supports morphological and ecological comparisons, but it does not establish the biological species concept because interbreeding and offspring fertility have not been tested.
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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.
Fossil remains share several structural features. Which species concept can be applied most directly?
Give me a hint
Identify the required evidence and sequence the reasoning.
Compare my reasoning
- The fossil morphology can be compared.
- Reproductive compatibility cannot usually be tested directly.
The morphological species concept can be applied directly from preserved structural features; the biological species concept cannot be tested from the fossils alone.
Look for these in your work
- Relevant evidence selected
- Reasoning linked to conclusion
Two organisms occupy the same niche but have visibly different structures. Explain what the ecological concept can and cannot establish.
Give me a hint
Use the supplied example or data to justify a conclusion.
Compare my reasoning
- A similar role can support ecological grouping.
- It does not prove shared morphology or interbreeding.
The ecological species concept can group them by niche, but it does not demonstrate that they look alike or can interbreed to produce fertile offspring.
Look for these in your work
- Concept applied
- Limit or assumption stated
An asexual organism reproduces successfully but cannot interbreed. Why is the biological species concept limited here?
Give me a hint
Apply the concept in a new context and qualify its limits.
Compare my reasoning
- Its definition depends on interbreeding.
- Asexual reproduction does not test reproductive compatibility.
The concept relies on interbreeding and fertile offspring, so it is not directly useful for classifying an organism that reproduces asexually.
Look for these in your work
- New context handled
- Conclusion appropriately bounded
Common mistakes
- Treating one definition as suitable for every organism
- Claiming that similar appearance proves fertile interbreeding
What can you explain now?
What evidence is central to the biological species concept?
Compare with the explanation
Interbreeding that produces fertile offspring
The definition concerns reproductive compatibility, not only appearance.
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Make it stick
What evidence distinguishes biological, morphological and ecological species concepts?
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