Hybrid speciation

There is one type of sympatric speciation which is uncontroversial and well known to exist: hybrid speciation which regularly occurs in plants.

Interspecies hybrids are usually sterile because the chromosome pairs, which consist of one chromosome from one species and another chromosome from the second species, do not segregate regularly at meiosis.

When a hybrid species evolves, this sterility can be overcome by polyploidy: the chromosome numbers are doubled. Each chromosome pair at meiosis contains two chromosomes from one species, and regular segregation is restored. Polyploidization is encouraged by applying the chemical colchicine in the commercial production of new species. Many popular species of garden flowers such as these tulips (opposite) have been created like this. Hybrid speciation can also occur naturally at a low rate. In this case, a new hybrid species may evolve. Some hybrid species also evolve without polyploidy; the initial sterility of the hybrid is overcome by some other genetic means.

Polyploid hybrids are interfertile among themselves, but reproductively isolated (by the mismatch in chromosome numbers) from the parental species; they are therefore well defined new species.