A coral reef can release millions of gametes into open water during one spawning event, while a mammal produces far fewer gametes and protects fertilisation inside the body. Both strategies can maintain continuity of species, but they solve the problem of successful fertilisation in very different ways.
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Imagine two species. Species A releases thousands of gametes into seawater. Species B produces far fewer gametes, but fertilisation occurs inside the female body.
Which species do you think is "more successful" at reproduction? Before learning the formal biology, explain what factors you would need to know before deciding. Is producing more gametes automatically better?
Wrong: Natural selection means organisms change because they want or need to.
Right: Natural selection acts on random genetic variations; organisms do not consciously adapt.
Core Content
Animal reproduction is not just about producing offspring. It also has to solve a chromosome-number problem. Gametes are haploid, so continuity across generations requires their fusion to restore the diploid state.
Fertilisation is the fusion of a male and female gamete. Each gamete is haploid, meaning it contains one set of chromosomes. When the gametes fuse, the resulting zygote is diploid. This is critical because it restores the species' normal chromosome number in the next generation.
That means animal reproductive systems must do more than simply bring sperm and egg into contact. They must do so in a way that maximises the chance of successful fertilisation and survival of the resulting zygote or embryo.
External fertilisation works by scale. Because many gametes are lost to the environment, animals using this strategy often compensate by releasing very large numbers of gametes at the same time.
In external fertilisation, sperm and eggs are released into the external environment and fuse outside the body. This strategy is most common in aquatic animals such as many fish, amphibians and marine invertebrates because water prevents gametes from drying out and allows sperm to swim toward eggs.
Coral spawning is a classic example. Success depends on releasing many gametes simultaneously. Frogs also commonly use external fertilisation, with eggs and sperm released into water where the zygote forms externally.
Internal fertilisation works by control. By placing sperm closer to the egg inside the female reproductive tract, animals can use fewer gametes while still maintaining a relatively high chance of fertilisation.
In internal fertilisation, sperm are transferred into the female reproductive tract and fertilisation occurs inside the body. This strategy is common in reptiles, birds, mammals and some fish.
Birds, reptiles and mammals all rely on internal fertilisation, but what happens after fertilisation differs. Some lay eggs, while others retain developing embryos internally. That is why fertilisation method must be kept separate from mode of development.
Fertilisation strategy reflects environmental constraints and trade-offs. Water availability, risk of gamete loss, parental investment and embryo protection all influence which approach is favoured.
| Animal group | Typical fertilisation strategy | Why it suits the group |
|---|---|---|
| Many bony fish | External | Aquatic habitat supports gamete release; many offspring can be produced at once. |
| Many frogs | External | Eggs and sperm are released in water; fertilisation depends on moisture and timing. |
| Reptiles | Internal | Allows fertilisation on land and reduces risk of desiccation. |
| Birds | Internal | Supports terrestrial reproduction even though development may later occur in an egg. |
| Mammals | Internal | Protected fertilisation and usually high parental investment in fewer offspring. |
The important HSC distinction is that fertilisation strategy and developmental strategy are not the same thing. Birds and reptiles have internal fertilisation even though many develop externally in eggs. Mammals also have internal fertilisation, but most keep development inside the uterus.
Fusion of haploid gametes to form a diploid zygote, restoring chromosome number.
Occurs outside the body, usually in water; many gametes, lower per-gamete success.
Occurs inside the female body; fewer gametes, greater protection and higher per-gamete success.
"Internal fertilisation is advantageous in terrestrial environments because gametes are protected from desiccation and the probability of successful fertilisation per gamete is higher."
Activities
Decide whether each case is better explained by external or internal fertilisation, then justify using environment, gamete number or protection.
Use full biological reasoning. Avoid writing only "external = bad" or "internal = better".
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
1. What is fertilisation?
2. Which feature is most strongly associated with external fertilisation?
3. Why is internal fertilisation advantageous in terrestrial environments?
4. Which statement correctly separates fertilisation method from developmental strategy?
5. A student claims, "External fertilisation is less successful because internal fertilisation is always superior." What is the best response?
6. Define fertilisation and explain why it is important for continuity of species in sexually reproducing animals. 3 marks
7. Compare external and internal fertilisation in terms of environment, gamete number, protection and reproductive success. 4 marks
8. Coral spawning and mammalian reproduction use very different fertilisation strategies. Evaluate which strategy is more effective for continuity of species, using evidence from environmental conditions and reproductive trade-offs. 5 marks
You should now be able to explain why "more gametes" is not automatically better. External fertilisation can be highly successful in water when large numbers and timing compensate for loss, while internal fertilisation improves protection and efficiency when water is limited and parental investment is higher.
1. External fertilisation, because the gametes are released into seawater and fertilisation occurs outside the body.
2. Internal fertilisation, because reproduction succeeds in a dry habitat and gametes must be protected from desiccation.
3. This suggests external fertilisation, because success depends on water remaining available for gamete survival and fusion.
4. Internal fertilisation, because the sperm and egg fuse inside the female before the egg is laid.
Fish spawning versus mammals: Fish using external fertilisation often release very large numbers of gametes because many are lost in the environment. Mammals using internal fertilisation usually produce fewer gametes per reproductive event because protection inside the reproductive tract increases the chance of success.
Internal fertilisation and terrestrial life: It protects gametes from drying out and increases the chance of sperm reaching the egg, so reproduction is less dependent on open water.
When external fertilisation can still succeed: In aquatic environments with synchronised spawning and very large gamete release, such as many marine invertebrates and fish.
1. C - Fertilisation is the fusion of haploid gametes to form a diploid zygote.
2. A - External fertilisation is associated with large gamete numbers and release into water.
3. D - Internal fertilisation protects gametes and improves fertilisation success on land.
4. B - Birds use internal fertilisation even though the embryo later develops in an egg outside the mother's body.
5. B - External fertilisation is not inferior by default; it can be highly effective in aquatic contexts where many gametes are released simultaneously.
Q6 (3 marks): Fertilisation is the fusion of haploid sperm and egg cells to form a diploid zygote [1]. It is important because it restores the normal chromosome number of the species in the next generation [1]. This allows hereditary information from the parents to be combined and passed on, supporting continuity of species [1].
Q7 (4 marks): External fertilisation occurs outside the body, usually in an aquatic environment, and often involves release of many gametes because the probability of any one sperm reaching any one egg is relatively low [1]. Internal fertilisation occurs inside the female reproductive tract, so gametes are better protected and fewer are usually needed [1]. External fertilisation has lower protection and is more affected by environmental loss [1], while internal fertilisation usually has higher per-gamete success and supports reproduction in terrestrial environments [1].
Q8 (5 marks): Neither strategy is universally more effective; effectiveness depends on context [1]. Coral spawning is effective in aquatic environments because huge numbers of synchronised gametes can be released into water, allowing enough fertilisations to occur despite high loss [1]. Mammalian internal fertilisation is effective in terrestrial environments because gametes are protected, fertilisation success per gamete is higher, and parental investment can increase offspring survival [1]. External fertilisation is better suited to environments where water supports gamete movement and many offspring can be produced [1], whereas internal fertilisation is better suited to protected reproduction with fewer offspring and greater parental investment [1].
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