4.7.1Additional lessons
Sexual and Asexual Reproduction — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 55 of 101
Part 1 of 5Asexual Reproduction
Asexual reproduction is efficient in a stable environment where parental genes already suit the habitat.
One parent
- A single organism produces offspring without gametes, so no partner or fertilisation is needed.
Mitosis
- Offspring form by mitosis, so they are genetically identical to the parent and to each other, forming a clone.
Methods
- Examples include binary fission in bacteria, budding in yeast and Hydra, and vegetative propagation in plants such as strawberry runners.
Speed
- Populations can grow very quickly because every individual can reproduce and no time is spent finding a mate.
Part 2 of 5Sexual Reproduction
Gametes
- Specialised haploid sex cells are produced by meiosis in each parent.
Fertilisation
- A male and female gamete fuse to form a diploid zygote that develops into the offspring.
Variation
- Offspring differ from both parents and from each other, because of crossing over, independent assortment and random fertilisation.
Cost
- It needs two parents, takes more time and energy, and each parent passes on only half of its genes.
| Feature | Asexual | Sexual |
|---|---|---|
| Parents | One | Usually two |
| Cell division | Mitosis | Meiosis and fertilisation |
| Offspring | Identical clones | Genetically varied |
| Rate of increase | Rapid | Slower |
Part 3 of 5Evolutionary Significance
Asexual advantage
- Successful genotypes are copied exactly, which is useful in a constant environment.
Asexual disadvantage
- A new disease or a change in conditions may kill all members of a clone because none are different.
Sexual advantage
- Variation gives some offspring a chance of surviving environmental change, so natural selection can act.
Mixed strategies
- Many organisms, including aphids, fungi and some plants, use both methods depending on conditions.
Part 4 of 5Comparing Methods
Gamete number
- Sexual species often produce many gametes, because few will meet and fuse.
Parental care
- Species with few offspring often invest more care in each one.
Habitat
- Stable habitats favour asexual reproduction, whereas changing habitats favour sexual reproduction.
Part 5 of 5Exam Checklist & Tips
4.7.1 – Key Points for the Exam
- Clone A group of genetically identical organisms descended from one parent.
- Gamete A haploid sex cell that fuses with another in fertilisation.
- Zygote The diploid cell formed when two gametes fuse.
- Vegetative propagation Asexual reproduction in plants from parts such as runners, tubers or bulbs.
Common Mistakes
- Saying that asexual reproduction involves meiosis.
- Stating that asexual offspring are never different from the parent; mutations can still occur.
- Claiming that sexual reproduction is always better; clones are faster and use less energy.
- Describing gametes as diploid.
Last updated:
\n