Explain how internal fertilisation increases the likelihood of reproductive success in terrestrial animals compared with external fertilisation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Internal fertilisation increases reproductive success on land because it removes the need for a water medium – gametes meet inside the body, so the lack of water on land is no longer a barrier.
It protects gametes and the zygote from desiccation; the female’s reproductive tract keeps them moist.
Because sperm are delivered directly to the egg, fewer sperm are required and the chance of fertilisation is higher.
Thus internal fertilisation allows terrestrial animals to reproduce in dry habitats and colonise a wider range of environments, independent of an aquatic environment.
It protects gametes and the zygote from desiccation; the female’s reproductive tract keeps them moist.
Because sperm are delivered directly to the egg, fewer sperm are required and the chance of fertilisation is higher.
Thus internal fertilisation allows terrestrial animals to reproduce in dry habitats and colonise a wider range of environments, independent of an aquatic environment.
Examiner tips
- Use the word ‘internal fertilisation’ and contrast with ‘external fertilisation’; mention water, desiccation, sperm delivery and habitat range.
- Show the logical chain: water requirement → protection from drying → fewer gametes → wider habitat colonisation.
Common mistakes
- Confusing external fertilisation with internal; not mentioning water requirement.
- Lacking the point about fewer gametes or habitat range; using vague terms like ‘better chance’ without explanation.
Mark scheme (4 marks)
- External fertilisation requires water as a medium for gametes to meet, which is unavailable or unreliable on land, whereas internal fertilisation occurs inside the body so does not depend on an aquatic environment.
- Internal fertilisation protects gametes / the zygote from desiccation (drying out) within the reproductive tract of the female.
- Fewer gametes need to be produced because the probability of a sperm reaching an egg is much higher / sperm are delivered directly close to the egg.
- Internal fertilisation allows direct transfer of sperm into the female, enabling mating to occur independently of an aquatic habitat, so terrestrial species can colonise a wider range of environments / reproduce in dry conditions.
Key terms in this question
internal fertilisation · external fertilisation · reproductive success
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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