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

Another role of sexual reproduction as a source of evolutionary potential lies in its contribution to the emergence of the phenomenon of the existence of species.The fact that organisms are organized in nature in individual, mutually delimited species is far from being a matter of fact.To the contrary, it seems that the emergence of species could have occurred only with the emergence of sexual reproduction.Where sexual reproduction does not occur, species in the true sense of the word (not only in the sense of the best-known contemporary definition of the term biospecies, see XX.3.3) do not exist.Here, organisms frequently form a sort of network of more or less similar and more or less related genealogical lines.This situation is encountered, for example, in some groups of plants (blackberries, dandelions) and in a great many groups of microorganisms.Where individual species can be sufficiently exactly differentiated on the basis of morphological or other traits, we sooner or later encounter some type of sexual reproduction.As will be shown in Section XX.2.2.4.1, sexual reproduction and the associated constant mixing of genes in the common gene pool is the most important mechanism of species cohesion (Mayr 1963), i.e. the mechanism that maintains the characteristics of members of individual species within a certain narrowly defined framework over the long term (see XX.2.2.4).