1.8.7Inquiry Process
Speciation in Plants — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 32 of 160
Part 1 of 6Polyploidy
Polyploid
- A plant with more than two sets of chromosomes, caused by errors in cell division such as failed separation in meiosis.
Autopolyploidy
- The chromosome sets all come from one species, as when a diploid becomes tetraploid.
Allopolyploidy
- The sets come from two different species that first hybridise and then double their chromosome number.
Fertility
- A sterile hybrid becomes fertile after doubling because each chromosome now has a homologous partner for meiosis.
Isolation
- The new polyploid cannot breed fertilely with its parents, so it is reproductively isolated at once.
Part 2 of 6Examples
Plants can form new species rapidly by changes in chromosome number.
Examples
Bread wheat
- A hexaploid with 42 chromosomes (six sets of 7), arising from hybridisation of three grass species.
Cord grass
- Spartina anglica arose in Britain in the 1800s from a sterile hybrid whose chromosomes doubled.
Crop plants
- Many crops such as potatoes, bananas and strawberries are polyploid, often larger or more vigorous.
Frequency
- Polyploidy is common in flowering plants but rare in animals.
Part 3 of 6Allopolyploid Sequence
1
Two related species cross-pollinate and produce a hybrid with one set from each.
2
The hybrid is sterile because chromosomes cannot pair at meiosis.
3
A cell division error doubles the chromosome number in the hybrid.
4
The doubled plant has pairs and so makes viable gametes by meiosis.
5
It can self-pollinate to form a population which is a new species.
| Step | Chromosomes (wheat) |
|---|---|
| Emmer wheat gamete | 14 |
| Aegilops gamete | 7 |
| Sterile hybrid | 21 |
| After doubling | 42 |
Part 4 of 6Why Plants Differ from Animals
Why Plants Differ from Animals
Self-fertilisation
- Many plants can self-pollinate, so a single polyploid plant can found a new population.
Asexual reproduction
- Plants that spread by runners or cuttings can survive even if a hybrid is sterile.
Developmental tolerance
- Plants tolerate extra chromosome sets better than most animals, where imbalance often disrupts development.
Crop breeding
- Breeders use colchicine to double chromosome numbers and create larger or fertile hybrids on purpose.
Part 5 of 6Worked Example
Question
- Emmer wheat is tetraploid with 28 chromosomes and Aegilops tauschii is diploid with 14.
- Calculate the chromosome number of their sterile hybrid and of the fertile bread wheat formed after doubling.
- Emmer wheat gamete
- 28 ÷ 2 = 14 chromosomes.
- Aegilops gamete: 14 ÷ 2 = 7 chromosomes.
- Hybrid14 + 7 = 21 chromosomes, which are not in homologous pairs, so it is sterile.
- After chromosome doubling21 × 2 = 42 chromosomes, all in pairs, so the plant is fertile.
Answer
- The hybrid has 21 chromosomes and is sterile;
- the doubled bread wheat has 42 chromosomes (hexaploid) and is fertile.
Part 6 of 6Exam Checklist & Tips
1.8.7 – Key Points for the Exam
- Polyploidy Having more than two sets of chromosomes.
- Allopolyploid A polyploid with chromosome sets from different species.
- Autopolyploid A polyploid with all sets from one species.
- Hybrid Offspring of parents of different species.
Common Mistakes
- Saying polyploid species take thousands of generations to form; they can form in one generation.
- Adding diploid numbers rather than haploid numbers for the hybrid.
- Thinking polyploidy is common in mammals.
- Forgetting that hybrid sterility comes from unpaired chromosomes.
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