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

The morphology or numbers of chromosomes in the karyotype change by chromosome mutation. In sexual reproduction, especially during meiosis, the new and old karyotypes need not be compatible, i.e. all the pairs of homologous chromosomes need not be capable of forming regular bivalents. As a consequence, meiosis need not occur successfully or can lead to the formation of aneuploid germinal cells. This results in partial or complete sterility of hybrids, which form an effective postzygotic reproduction barrier. As soon as homozygotes with the new karyotype appear in the population, they can cross together without any problems. The formation of a new species by chromosome mutation is usually termed chromosome speciation.s 

The importance of chromosome speciation is a frequent subject of discussions. They are often based particularly on the fact that most even very closely related species differ in their karyotype. It is sometimes estimated that 90-98% of speciation is accompanied by a karyotype change (White 1978). A great many biologists conclude from this that chromosome mutations have a fundamental, possibly key importance in speciation. However, a number of biologists object that the karyotype differences between related species could be caused by the fact that this trait mutates quite frequently and the mutations can spread rapidly by meiotic drive within a panmictic population. It is sufficient if a certain chromosome variant, for example a chromosome formed by the fusion of two other chromosomes, has a greater chance of being transferred to the oocyte in the cells of the heterozygote than to the polar body, and it will spread very effectively in the panmictic population even if it will reduce fertility, and thus the fitness of its host, to a certain degree. New chromosome mutations can spread very rapidly within one species, so that all the members of the species are mostly karyotypically uniform. However, as soon as the gene pool is divided into several parts through any type of speciation, the new chromosome mutations can no longer cross the borders between the gene pools and a different mutation is fixed in each of them - the types become karyotype differentiated. Thus, interspecific differences in karyotypes can be rather a consequence than a cause of branching speciation.

The main difficulty with chromosome speciation (at least if this were to occur sympatrically in a panmictic population) is that, immediately after their formation in the population, mutants occur with low frequency and thus reproduce almost exclusively with individuals with the original, incompatible karyotype. Thus, they are at a considerable selection disadvantage compared to the original form as the members of this form, to the contrary, encounter almost only the much more common individuals with compatible karyotype. Thus, there is only a relatively low chance in a panmictic population that a chromosome mutation could lead to the formation of a new species. In contrast, the situation is far more favorable in a spatially structured population, for example in immobile organisms, such as plants. Mutant individuals have a good chance of always reproducing with their neighbors, which will often be their relatives and thus carriers of the same mutation. Thus, they need not be at a great disadvantage compared to members of the original form. Thus, the population of mutants can gradually spread from a certain place within the range of the original species. Low-mobility organisms, in whose karyotype a certain type of chromosome mutation frequently occurs, can thus very easily and frequently undergo stasipatric speciation and form complexes of mutually neighboring, geographically separated and phenotypically rather similar or identical species in a certain territory (King 1993).