The hybridization of two different species frequently yields crosses that have phenotype traits different from both the original species and even exhibit better viability in some habitats. If crossing occurs repeatedly between these species, interspecific crosses can have great importance in the particular ecosystems. In subsequent filial generations or on recrossing with the parent species, however, both their viability and their fertility are reduced as a result of irregularity in the separation of chromosomes derived from two different species. If we ignore the possibility of transition to a purely asexual means of reproduction, there are two basic ways in which hybridization can lead to the formation of a new fully fledged species.
The first means of hybridization speciationis called recombination speciation. The individual recombinants derived from crossing of hybrids of the F1-generation may occasionally contain individuals that are normally fertile and have different ecological requirements than the original species. If sufficiently strong reproduction barriers are also created between these individuals and the original species, they can form the basis for the emergence of a new species.
Hybrid polyploidization is another means of hybridization speciation. The emergence of fertile individuals through hybrid polyploidization, i.e. the emergence of a fertile alopolyploid, is even easier than its formation by polyploidization of a nonhybrid individual, i.e. than the formation of a fertile autopolyploid. In autopolyploids, the double chromosome set is derived from a single species. For example, in autotetraploids, all the chromosomes are present in the cell in four copies and, in meiosis, tetravalents can be formed instead of bivalents. The presence of these structures can seriously disturb the progress of meiosis and thus reduce the fertility of the polyploid. In contrast, with alopolyploids, the two original sets are derived from two different species so that the relevant homeological chromosomes mostly do not pair together and, rather than tetravalents, twice as many regular bivalents are formed during meiosis. As a result, alopolyploids can be fully fertile.
Polyploidization mostly occurs in that the first (reduction) division does not occur during meiosis, yielding diploid gametes. Tetraploids are only rarely formed by the meeting of two rare diploid gametes. Mostly a diploid gamete first encounters a haploid gamete and a triploid is formed. This then forms a triploid gamete as a consequence of a disorder in reduction division. A tetraploid individual is formed only by fusion of a triploid gamete with a haploid gamete. Thus, the triploid stage is a frequent intermediate step in the evolution of a new species by polyploidization; this has two haploid chromosome sets from one species and the third from the other species. Once again, it is necessary to bear in mind that this type of speciation comes into consideration primarily for species without chromosomal sex determination through differentiated sex chromosomes, where the ratio of the gene dose located on the sex chromosomes and on the autosomes must be retained, and thus primarily for some taxons of plants and fish. Hybridization speciation can be of relatively great importance for these taxons. According to some estimates, up to 11% of all species diversity in plants exists as a result of hybridization (Barraclough & Nee 2001).