Barriers to hybridization prevent gene flow before or after fertilization, maintaining species boundaries between potentially interbreeding populations over evolutionary time.
Reproductive barriers prevent gene flow and can act before or after fertilization. Prezygotic barriers prevent mating or fertilization, so no zygote forms: geographic separation, different habitats, different breeding times and different courtship behaviours are examples. Postzygotic barriers act after fertilization: the hybrid may be non-viable or may survive but be infertile, as in a mule.
Polyploidy is more than two complete chromosome sets and can produce abrupt sympatric speciation in plants. Autopolyploidy arises when chromosome number doubles within one species, often after meiotic failure; a fertile tetraploid can no longer produce fertile offspring with the original diploid population. Allopolyploidy begins with hybridization between two species followed by chromosome doubling, which can restore pairing and fertility in the hybrid.
If the new polyploid is viable and fertile but reproductively isolated from the parent population, it is a new species. Knotweeds/smartweeds in the genus Persicaria provide the named example. The scoring chain is chromosome change → incompatible meiosis with parent → reproductive isolation → speciation.
HL speciation questions usually ask you to choose the right layer. Route layer: allopatric means geographic isolation; sympatric means no spatial separation. Niche layer: adaptive radiation produces many related species from one ancestor as they occupy different niches, as in Darwin’s finches. Barrier layer: prezygotic barriers act before fertilization, postzygotic barriers act after hybrid formation. Chromosome layer: plant polyploidy can abruptly create reproductive isolation and fertile new lineages.