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

Buffon’s concept of a species is apparently the oldest definition of a species as a natural taxon (Mayr 1982). According to this concept, the criterion of belonging to the same species is the ability to productively reproduce, i.e. the ability of a pair of individuals of the opposite sex to produce fertile progeny. As soon as the members of two different forms become capable of fertile reproduction, it is necessary to consider them to be members of the same species, even if their phenotypes are very different and they do not cross in nature – for example because their areas of occurrence do not meet. It is apparent that this method of distinguishing species can be employed only for sexually reproducing organisms.