Explain why two organisms that can successfully interbreed and produce fertile offspring are classified as the same species, and describe two limitations of using this definition to classify all living organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Two organisms that can interbreed and produce fertile offspring are classified as the same species because they share a common gene pool and can pass genes to the same next generation. Fertile offspring indicates that the two parents are genetically compatible enough to be considered one species.
Limitations:
1. The definition cannot be applied to organisms that reproduce asexually, as there is no interbreeding.
2. It cannot be applied to extinct or fossil organisms, because interbreeding cannot be observed.
Edge cases such as mules (horse × donkey) show that some different species can interbreed but produce infertile offspring, highlighting that the definition is not always clear-cut.
Limitations:
1. The definition cannot be applied to organisms that reproduce asexually, as there is no interbreeding.
2. It cannot be applied to extinct or fossil organisms, because interbreeding cannot be observed.
Edge cases such as mules (horse × donkey) show that some different species can interbreed but produce infertile offspring, highlighting that the definition is not always clear-cut.
Examiner tips
- Use the phrase "share a common gene pool" and "fertile offspring" to hit the key points. Include the two specific limitations. Mention the mule example only if time allows to show awareness of edge cases.
Common mistakes
- Writing that interbreeding alone proves same species without mentioning fertility. Failing to state the two limitations, or giving a limitation that is not in the mark scheme. Using vague terms like "similar" instead of "genetically compatible".
Mark scheme (5 marks)
- Organisms that interbreed and produce fertile offspring share the same gene pool / can pass genes to the same next generation
- Fertile offspring means the two parent organisms are sufficiently genetically compatible to be the same species
- Limitation 1: the definition cannot be applied to organisms that reproduce asexually
- Limitation 2: the definition cannot be applied to extinct / fossil organisms (cannot observe interbreeding)
- Some organisms from different species can interbreed but produce infertile offspring (e.g. mules from horse and donkey), showing the definition has edge cases / is not always clear-cut
Key terms in this question
species · fertile offspring · interbreed
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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