🎲 It produces variation
The offspring are not identical to each other or to their parents. Each one has a different mix of genes.
Reproduction » Pros and Cons of Sexual and Asexual Reproduction
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.6.1.3 (biology only)
- The advantages of sexual reproduction
- The advantages of asexual reproduction
- Which organisms use both methods, and why
- How to weigh up the pros and cons for an organism you have not met before
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Unlock This CourseWhy does it matter which method is used?
You already know that sexual reproduction needs two parents and makes offspring that are different from each other, while asexual reproduction needs one parent and makes clones. Neither method is "better" every time. Each one wins in different situations. This lesson is about why.
Key terms:
- Variation: the differences between individuals of the same species.
- Selective breeding: humans choosing which plants or animals to breed together to get the characteristics they want.
Advantages of sexual reproduction
🌿 It helps survival when things change
If the environment changes, for example a new disease arrives or the climate gets warmer, some individuals may be better suited to the new conditions. Variation gives a survival advantage by natural selection.
There is a third advantage. Humans can speed up natural selection by selective breeding. A farmer picks the plants or animals with the best characteristics and breeds from them. This can increase food production. It only works because sexual reproduction makes variation to choose from.
Remember
All three advantages come from one idea: sexual reproduction mixes genes, so the offspring are different.
Advantages of asexual reproduction
☝ Only one parent needed
There is no need to find a mate. A single organism on its own can reproduce.
⚡ Saves time and energy
No energy is spent finding a mate. It is also faster than sexual reproduction.
Asexual reproduction can also produce many identical offspring when conditions are favourable. If the parent is well suited to its habitat, then so are all its clones. A population can grow very quickly.
The downsides
Each method has disadvantages. They are the other side of the advantages above.
🔄 Sexual reproduction
Two parents are needed, so a mate must be found. It uses more time and energy, and it is slower than asexual reproduction.
📑 Asexual reproduction
There is no variation, so all the offspring are clones. If the environment changes, or a disease arrives that one of them cannot survive, then none of them may survive.
Organisms that use both methods
Some organisms reproduce by both methods, depending on the circumstances. These are the three examples you need to know.
- Malarial parasites: they reproduce asexually in the human host, but sexually in the mosquito.
- Many fungi: they reproduce asexually by making spores, but also reproduce sexually to give variation.
- Many plants: they make seeds sexually, but also reproduce asexually. Strawberry plants send out runners, and daffodils reproduce by bulb division.
Using both gets the best of each. Asexual reproduction can build up numbers fast when conditions are good. Sexual reproduction brings in variation so that some offspring can cope when conditions change.
Key terms:
- Spore: a tiny reproductive cell made by fungi. A fungus can make very large numbers of them.
- Bulb division: a bulb, such as a daffodil bulb, splitting to form new bulbs that grow into new plants.
How much do you need to know?
The spec says you only need to know reproduction in the organisms above. But you must be able to explain the advantages and disadvantages for any organism if you are given the information.
Applying it to an organism you have not met
In the exam you may be given information about an unfamiliar organism. Use this method:
- Read how the organism lives. Is it alone? Is its habitat stable or changing?
- Decide what is in its favour: speed and numbers (asexual) or variation (sexual).
- Use the right words: one parent, no mate, quick, identical offspring, or variation, survival, natural selection.
Worked example
A small flatworm lives in a pond. It can split in two to make a new worm, and in autumn it also makes eggs after mating. Why is it useful to have both?
Splitting in two needs one parent, so the worm does not need to find a mate and numbers can grow quickly in good conditions. Mating in autumn gives variation, so some offspring may survive if the pond changes.
Common mistakes
1. Saying asexual reproduction is "better" or "worse" without saying when. Always give the situation.
2. Saying variation helps an individual survive. It helps the species, because some individuals may be suited to the change.
3. Mixing up the malarial parasite: asexual in the human, sexual in the mosquito.
4. Saying that asexual offspring are "similar". They are genetically identical.
Exam-style question
Many fungi can reproduce asexually by making spores. They can also reproduce sexually.
(a) Give two advantages of reproducing asexually. [2 marks]
(b) Explain why reproducing sexually is an advantage if the environment changes. [3 marks]
Model answer
(a) Only one parent is needed / no need to find a mate (1). It is faster / more time and energy efficient / many identical offspring can be made quickly (1).
(b) Sexual reproduction produces variation in the offspring (1). Some may be better suited to the changed environment (1). They survive by natural selection and pass on their genes (1).
Exam tip
For a 3-mark "explain" question, link three steps: sexual reproduction, so variation, so survival advantage when the environment changes.