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

A phylogenetic species is most frequently defined as the basic (smallest possible) set of a population between which the ancestor – progeny relationship exists and which can be distinguished from other such sets on the basis of a diagnostic trait (Nixon & Wheller 1990). This definition encompasses both the requirement on monophyly of the given set in the population (otherwise some of them would have ancestors outside of the population set) and also the requirement on the presence of a phenotypic trait, on the basis of which it is possible to differentiate its members from the members of other species. This trait can, of course, be a certain combination of individual traits that, in themselves, i.e. in other combinations, can also exist in other species. Simultaneously, the trait need not be present in all the individuals in the population and can occur, for example, only in the members of one sex or only at a certain stage in development, e.g. in larvae or in adults. The definition of a phylogenetic species does not permit a decision to be made on whether a certain population is a species. The main contribution of the concept of a phylogenetic species is, however, that it allows delimitation of what is certainly not a species. Thus, higher taxons cannot be species as this is not the smallest possible set in the population fulfilling both criteria, and even a certain phenotype variant cannot be a species, for example all albino individuals, because they split off repeatedly, independently of mutual relatedness and do not, together, form a monophyly. Even a monophyletic group in the population, which would not differ in any trait from other similar groups in the population, cannot be a phylogenetic species. Thus, according to the concept of a phylogenetic species, the formation of a new species requires anagenesis – an evolutionary change in phenotype.