New plant species can form in a single generation when polyploidy gives an individual extra complete sets of chromosomes. The chromosome-number difference causes immediate reproductive isolation from the original population, satisfying a key requirement for speciation.
Polyploidy is the possession of more than two complete chromosome sets. It can result from non-disjunction during meiosis, which produces unreduced diploid gametes rather than haploid gametes.
If an unreduced diploid gamete fuses with another unreduced gamete, the offspring may be tetraploid. A tetraploid plant can undergo meiosis successfully because its chromosomes have homologous partners. However, crossing it with a diploid parent usually produces triploid offspring, whose chromosomes cannot divide evenly during meiosis; these offspring are often sterile. The tetraploid is therefore reproductively isolated from the diploid population but may reproduce by self-pollination or with another tetraploid.
Polyploidy can also restore fertility after hybridization between two species. Suppose species A contributes chromosome set A and species B contributes set B:
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Hybridization produces an AB hybrid.
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Chromosomes from A and B are not homologous, so they cannot pair correctly during meiosis; the hybrid is sterile.
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Chromosome doubling produces AABB.
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Each chromosome now has a homologous partner, allowing meiosis and fertile gamete production.
Process | Chromosome outcome | Effect on speciation |
|---|---|---|
Autopolyploidy | Extra sets from one species | Isolates the polyploid from its diploid parent population |
Hybridization followed by polyploidy | Chromosome sets from two species are doubled | Restores hybrid fertility and creates an allopolyploid lineage |
“Instantly” means that reproductive isolation can arise in one generation, notnot that a stable population is guaranteed immediately. The new polyploid must still reproduce and establish a viable population. The IB syllabus uses plants in the genus Persicaria as an example of abrupt speciation through hybridization and polyploidy.
Exam technique: For an explain question, link chromosome doubling → successful homologous pairing → fertile meiosis → reproductive isolation from diploid parents. Avoid merely stating that polyploidy “creates variation”; isolation and fertility are the essential marking points.
