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

It has been estimated that more than 90% of speciation events are accompanied by the formation of a modified karyotype in a daughter species (White 1978). It is probable that, in the case of allopatric speciation, meiotic drive is responsible for this phenomenon or, to be more exact, the fact that the karyotype of the species changes much faster through the effect of meiotic drive than new species are formed. Speciation events only conserve differences in the gene pool of the two populations existing at the given instant and simultaneously create a barrier capable of preventing spreading of chromosome mutations from one population to another. Thus, the karyotypes of two daughter species can diverge. As this divergence occurs through the effect of relatively fast meiotic drive, divergence of karyotypes occurs faster than divergence of phenotypes, which change through the action of the slower processes of genetic drift and selection.

            However, in the case of sympatric speciation, it is assumed that meiotic drive could sometimes participate directly in the creation of species barriers and that it could thus actively contribute to the origin of new species. If Robertsonian translocations gradually spread from various areas in the  area occupied by a given species, this entire area can disintegrate into a number of separate areas, where individuals of a different chromosomal race will live in each of them. The boundaries between these areas can be very sharp, especially if two races are next to one another, whose karyotypes contain two different Robertsonian translocations, in which the same acrocentric chromosome, as one of the pair of fused chromosomes, is present (see Fig. XXI.12). Because of the common branch, the two different metacentric chromosomes participate in creation of a chromosome tetrade during meiosis in  hybrids between the two races. Subsequently, disorders occur in the transfer of the chromosomes to the fields of the meiotic spindle and a substantial percentage of nonfunctional gametes is formed. In species in which more intense sperm competition occurs, because the female is often rapidly fertilized by several males in succession, the amount and quality of the sperm in the ejaculate or spermatheca decide the paternity of the individual embryos to a substantial degree. In this case, the heterozygote sons of a male that penetrated into the area of occurrence of a different chromosomal race have substantially reduced biological fitness. This can form a relatively effective barrier against spreading of metacentric chromosomes from the area of one chromosome race into the area of another. Because of the existence of crossing-over, such a barrier need not prevent the flux of the individual genes (or, to be more exact, alleles) between sympatric or parapatric (adjacent) populations of the two chromosome races.  However, reduced fertility of heterozygotes can create very strong selection pressure for the formation of specific recognition mechanisms, capable of preventing mutual crossing of the members of two different chromosome races. If the genes affecting this recognition occur in the area of chromosomes in which crossing-over does not occur for some reason, for example, in the inversion area, it is highly probable that meiotic drive will lead to the formation of two separate species.