Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Polyploid individuals are readily formed in some taxons and a new species can be formed by polyploidization speciation from these individuals under favorable circumstances. In species with differentiated sex chromosomes, the balance between autosomes and the sex chromosomes is frequently disturbed in polyploid individuals, so that polyploidization, for example by doubling of a chromosome set, yields individuals with serious disorders in the development of the sex organs and thus reduced or even zero fitness. In species without differentiated sex chromosomes, especially in plants, these individuals are frequently fertile and can lead to the emergence of a new species (Rieseberg 2001). It has been estimated that polyploidization speciation is responsible for 2-4% of all speciation in vascular plants (about 7% amongst ferns) (Otto & Whitton 2000).

Crossing between tetraploid and diploid individuals then yields triploid individuals, which are often infertile. During meiosis, part of the chromosomes do not find unoccupied homologous partners with which they could form bivalents during meiosis and thus participate in the formation of trivalents or even remain unpaired as free univalents. Especially univalents are unevenly distributed amongst the daughter cells or their presence can completely block the completion of meiosis. Thus, reproductive barriers exist between polyploids and diploids, which can lead to the emergence of a new species under favorable circumstances. This speciation is greatly assisted by the fact that, due to the larger size of cells of polyploidy plants, the phenotype traits of polyploids and diploids can differ substantially (Otto & Whitton 2000). This can contribute to differentiation of the ecological niches of the two forms and facilitate their prolonged or even permanent sympatric or microallopatric coexistence in a single territory.