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Plants
Hybrid speciation occurs when two divergent lineages(e.g.,species) with independent evolutionary histories come into contact and interbreed. Hybridization can result in speciation when hybrid populations become isolated from the parental lineages, leading to divergence from the parent populations.

Polyploid Hybrid Speciation

In cases where the first-generation hybrids are viable but infertile, fertility can be restored by whole genome duplication (Polyploidy), resulting in reproductive isolation and polyploid speciation. Polyploid speciation is commonly observed in plants because their nature allows them to support genome duplications. Polyploids are considered a new species because the occurrence of a whole genome duplication imposes post-zygotic barriers, which enable reproductive isolation between parent populations and hybrid offspring. Polyploids can arise through single step mutations or through triploid bridges. In single step mutations, allopolyploids are the result of unreduced gametes in crosses between divergent lineages. The F1 hybrids produced from these mutations are infertile due to failure of bivalent pairing of chromosomes and segregation into gametes which leads to the production of unreduced gametes by single division meiosis, which results in unreduced(diploid) diploid gametes. Triploid bridges occur in low frequencies in populations and are produced when unreduced gametes combine with normal(1N) gametes to produce a triploid offspring that can function as a bridge to the formation of tetraploids. In both paths, the polyploid hybrids are reproductively isolated from the parents due to a difference in ploidy. Polyploids manage to remain in populations because they generally experience less inbreeding depression and have higher self-fertility.

Homoploid Hybrid Speciation

Homoploid(diploid) speciation is another result of hybridization, but unlike polyploid speciation, it is observed less commonly because the hybrids are not characterized by a genome duplication and isolation must develop through other mechanisms. In homoploid speciation, the hybrids remain diploid. Studies on diploid hybrid populations of Louisiana irises show how these populations occur in Hybrid zones created by disturbances and ecotones (Anderson 1949). The existence of these novel niches allows for the persistence of hybrid lineages. For example, established sunflower (Helianthus) hybrid species represent transgressive phenotypes and display genomic divergence separating them from the parent species.