A student states: 'Sexual reproduction in animals always involves both internal fertilisation and the production of large numbers of offspring.' Evaluate this statement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sexual reproduction in animals involves the fusion of male and female gametes. Gametes are haploid cells produced by meiosis. The resulting zygote is a diploid cell that develops into a new organism.
Model answer (5 marks)
Sexual reproduction in animals does not always involve internal fertilisation – many species, such as fish and amphibians, fertilise externally.
It also does not always produce large numbers of offspring – mammals, for example, fertilise internally and usually have few, well‑developed young.
Thus the statement is partly correct (internal fertilisation occurs in some animals) but incorrect in its generalisation about fertilisation and offspring numbers.
It also does not always produce large numbers of offspring – mammals, for example, fertilise internally and usually have few, well‑developed young.
Thus the statement is partly correct (internal fertilisation occurs in some animals) but incorrect in its generalisation about fertilisation and offspring numbers.
Examiner tips
- Identify the two key points to evaluate – fertilisation type and offspring numbers.
- Show contrast between internal and external fertilisation with examples.
- Explain why external fertilisers produce many offspring and internal ones fewer.
- Use the exact terminology: internal/external fertilisation, large/small numbers of offspring.
Common mistakes
- Assuming all animals use internal fertilisation. Misstating that all animals produce large numbers of offspring. Failing to give examples to support each point.
Mark scheme (5 marks)
- Fertilisation is the fusion of a male gamete and a female gamete / haploid cells fuse to form a diploid zygote
- The student is correct that internal fertilisation occurs in some animals (e.g. mammals / birds / reptiles)
- The student is incorrect because some animals use external fertilisation (e.g. fish / amphibians / frogs)
- Animals that use external fertilisation tend to produce large numbers of offspring / gametes because many are lost / not fertilised
- Animals that use internal fertilisation typically produce fewer offspring because the chance of fertilisation / survival is greater / offspring are protected inside the body
Key terms in this question
fertilisation · internal fertilisation
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
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