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

It has been found for a number of morphological species that they actually consist of a complex of two or more species that can fundamentally not be differentiated on the basis of morphological traits. In some cases, they can be differentiated on the basis of ethological and ecological traits and in some cases on the basis of the geographic occurrence. However, sometimes the main or only indication of the existence of cryptic species (sibling species) is the existence of a reproduction barrier between their members. This can be manifested either in direct study of reproduction of the members of the particular “species” or in study of the genotype composition of the population. In the latter case, a certain combination of alleles does not occur in the population, although its occurrence would have been expected on the basis of the frequency of the individual alleles in the gene pool of the population. It is understandable that the absence, for example, of heterozygotes in a particular locus does not necessarily indicate the existence of cryptic species. Some heterozygotes could, for some reason, not be viable and there could be a strong selection pressure against them in natural populations. However, if the absence is related to a greater number of genotypes and a greater number of loci, the existence of cryptic species becomes a very probable explanation of the given phenomenon.
            The existence of cryptic species constitutes a complication, not only from the viewpoint of the theoretical definition of a species, but from a factual standpoint. Cryptic species generally have overlapping ecological niches, so that it is a mystery how they can occur in nature over prolonged periods and with such a high frequency. It is quite possible that they could correspond to newly emerging species that will also differ morphologically in the future. However, it is also possible that this is only a transitory state that will end either with the disappearance of one of the almost identical species or renewal of gene flow between the given species.