IB Biology HLEvolution & SpeciationPaper 1 & 2~11 min read
Speciation in Plants
Everywhere else in this topic, speciation takes thousands of generations. In plants it can happen in one. A mistake during meiosis produces a seedling that cannot breed with its own parents — and that, by definition, makes it a new species.
📚 What you need to know
Speciation is usually slow, because allele changes accumulate slowly. In some plants it can happen within a single generation: abrupt or instant speciation.
This is possible because plant cells stay viable when they are polyploid — carrying more than two sets of chromosomes (3n triploid, 4n tetraploid).
Autopolyploidy is gaining extra sets from within a single species. Allopolyploidy is gaining them from two different species.
Extra sets arise from chromosome nondisjunction during meiosis, which produces a diploid (2n) gamete.
A 2n gamete plus a normal n gamete gives a 3n zygote; a 2n gamete plus another 2n gamete gives a 4n zygote.
A 4n plant cannot produce fertile offspring with the 2n parent population, so it is a new species.
Examples: Persicaria (smartweeds) and Fallopia (knotweeds).
Why plants can do this and animals cannot
In most situations speciation is a slow process, because it depends on the slow rate at which allele changes accumulate. Plants break that rule. In some plant species speciation happens within a single generation, which is known as abrupt or instant speciation.
The reason is that plant cells are able to remain viable even when they are polyploid. An animal embryo with the wrong number of chromosome sets usually fails; a plant with extra sets often grows perfectly well.
Term
Sets of chromosomes
Where you normally see it
Haploid (n)
One
Gametes
Diploid (2n)
Two
Normal body cells
Triploid (3n)
Three
Polyploid, and infertile
Tetraploid (4n)
Four
Polyploid, and often fertile
Polyploidy can arise when an individual gains more than two sets of chromosomes from within a single species, which is autopolyploidy, or from two different species, which is allopolyploidy.
🧠
Auto or allo?
Auto means self, as in automatic or autobiography — the sets come from the plant’s own species. Allo means other — the sets come from another species. Same prefixes as allopatric speciation.
Why polyploid plants do well
Polyploid varieties of plant appear to be successful, and this is thought to be because of advantages such as these.
Polyploidy may allow hybrids that would otherwise be infertile to carry out meiosis, because the additional chromosomes give every chromosome a partner to pair with.
Polyploid plants are often larger and more vigorous than their diploid parents.
Having more copies of each gene reduces the impact of negative mutations, because harmful alleles are masked by the extra healthy copies.
The two routes to a polyploid
Both routes depend on the same accident: chromosome nondisjunction, the failure of chromosomes to separate during meiosis. When homologous pairs do not separate correctly, one daughter cell may contain two sets of chromosomes — a diploid gamete.
The pink arrows are meiosis in both panels. The only thing that changes is whether the two 2n gametes came from the same species or from two.
Autopolyploidy in detail
Autopolyploid 4n individuals can arise within a 2n plant population.
During meiosis, the separation of homologous pairs does not occur correctly, so one daughter cell ends up with two sets of chromosomes. This failure is chromosome nondisjunction.
The resulting diploid (2n) gamete can then fuse with a normal n gamete to produce a 3n zygote.
Or it can fuse with another diploid gamete to produce a 4n zygote.
Allopolyploidy in detail
To generate allopolyploidy, the diploid gametes from individuals of different species fuse together to produce a polyploid zygote. Individuals from two different species breeding together is hybridisation, and the resulting zygote is a polyploid hybrid.
This is where polyploidy quietly rescues hybrids. An ordinary hybrid is usually infertile because its chromosomes have no partners to pair with. Give it a full extra set from each parent species and suddenly every chromosome has a match, so meiosis works and the hybrid can reproduce.
Why the new plant is instantly a new species
This is the step that makes it speciation rather than just an odd plant.
Any 4n individual in the population will produce 2n gametes.
The surrounding 2n plants produce normal n gametes.
A 2n gamete fusing with an n gamete produces a 3n zygote, and an individual developing from a 3n zygote is infertile — three sets cannot be divided evenly into pairs at meiosis.
So the 4n plant is unable to breed with the original population to produce fertile offspring.
A population that cannot produce fertile offspring with its parent population is a new species. Speciation has taken place.
Nothing here needed thousands of years. One faulty meiosis was enough to close the door between the new plant and its own parents.
Real examples
Persicaria — the smartweeds
The plant genus Persicaria contains a range of ploidy types: P. foliosa is diploid (2n), P. japonica is tetraploid (4n) and P. puritanorum is hexaploid (6n).
It is thought that the tetraploid species arose by allopolyploidy between two diploid species, and that the hexaploid species arose from a hybridisation event between a diploid and a tetraploid species.
Fallopia — the knotweeds
Japanese knotweed is octoploid (8n).
Giant knotweed is tetraploid (4n).
Bohemian knotweed is hexaploid (6n), and is a polyploid hybrid of the other two. Japanese and giant knotweed would have undergone normal meiosis in this case, producing 4n and 2n gametes.
Japanese knotweed is famously invasive, and its polyploid nature is thought to help its vigorous growth. Bohemian knotweed is thought to be even more vigorous — a neat illustration of the idea that extra gene copies make for a tougher plant.
You do not need the Latin. In an exam it is perfectly acceptable to write “Japanese knotweed” rather than Fallopia japonica. Learn the ploidy numbers and the relationships, not the binomial names.
Worked examples
WE 1
Speciation in one generation
Explain how a tetraploid (4n) plant arising in a diploid (2n) population can be considered a new species. (4 marks)
Point 1: the gametes
The 4n plant produces 2n gametes, while the original population produces n gametes.
Point 2: the offspring
A 2n gamete fusing with an n gamete gives a 3n zygote.
Point 3: why that matters
A 3n individual is infertile, because three sets of chromosomes cannot pair up evenly during meiosis.
Point 4: the definition
The 4n plant therefore cannot produce fertile offspring with the parent population, which is the definition of a separate species.
No fertile offspring with the parents = a new species, immediatelyquote the species definition in the last line; it converts a description into an explanation
WE 2
Auto or allo?
Distinguish between autopolyploidy and allopolyploidy. (2 marks)
Point 1: autopolyploidy
Extra sets of chromosomes come from within a single species, usually after nondisjunction produces a 2n gamete.
Point 2: allopolyploidy
Diploid gametes from two different species fuse, so the polyploid is a hybrid of the two.
Auto = one species. Allo = two species.“distinguish” means both sides must be stated, not just one
WE 3
Why polyploids succeed
Suggest two reasons why polyploid plant varieties are often successful. (2 marks)
Reason 1: size and vigour
Polyploid plants are often larger and more vigorous than their diploid parents.
Reason 2: masked mutations
Extra copies of each gene mean harmful alleles are masked, so a negative mutation has less impact.
Bigger plants, and a spare copy of every genea third option: polyploidy can let an otherwise infertile hybrid carry out meiosis
💡 Exam tips
Use the term abrupt or instant speciation when a new plant species appears in one generation.
Name chromosome nondisjunction as the cause of the 2n gamete.
Do the arithmetic in your answer: 2n + n = 3n, and 2n + 2n = 4n.
Explain infertility through pairing at meiosis, not just by saying the number is odd.
Finish with the species definition — cannot produce fertile offspring with the parent population.
Latin names are not required; common names are fine.
⚠ Common mistakes
Saying polyploid plants are always infertile. 3n plants are infertile; 4n plants usually breed happily with each other.
Mixing up auto and allo. Auto = self = one species; allo = other = two species.
Calling nondisjunction a mutation of a gene. It is a failure of chromosomes to separate.
Forgetting to say why 3n is infertile. Three sets cannot pair up in meiosis.
Saying this happens in animals too. The point of this section is that plant cells stay viable when polyploid.
That completes Evolution & Speciation. The seven notes tell one continuous story: heritable variation appears at random, selection sorts it, gene flow decides whether the population stays whole, and once two groups can no longer produce fertile offspring together, the split is permanent.
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