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

Darwin’s fundamental work “On the Origin of the Species by Means of Natural Selection” was concerned to a major degree with the aspect of anagenesis, i.e. changes in phenotype traits of organisms as a result of natural selection. However, it is apparent from his diaries and correspondence that Darwin attributed equal weight to another idea, which is discussed in the book only very briefly, i.e. the origin of new species during evolution by branching speciation (Gould 2002). Using modern terminology, we would say that Darwin assumed that disruptive selection acts constantly on populations and species, resulting in a trend towards a consistent increase in the number of biological species. Variability is constantly generated in each species, where the individuals at the most distant ends of the phenotype spectrum compete the least and are thus automatically at an advantage compared to variants located close to the centre of the phenotype spectrum. Thus, they leave more progeny and, once again, among this progeny, the most distant forms prosper best. It can be concluded from analysis of Darwin’s texts that he supposed that a similar mechanism could also be active at an interspecific level. The species with most distant phenotypes create the greatest number of daughter species; this will, of course, not lead to a greater number of species (greater diversity), but there will be greater phenotype differences amongst them (greater disparity) (Gould 2002).

            The mechanism proposed by Darwin, which assumed the action of intraspecific disruptive selection, seems to be erroneous from the modern point of view. The fact that species form sharply defined groups of similar individuals and the results of direct observations of the mortality of individuals in natural populations both indicate that normalizing selection tends to act on the population in nature with incomparably greater frequency and places individuals with extreme rather than central phenotype at a disadvantage. It is similarly apparent that this speciation mechanism could act only in species without sexual reproduction. In a sexually reproducing population, gene flow in the population would constantly blur the differences between the extreme phenotype variants. Thus, a functioning model would also have to include the emergence of reproductive isolation barriers, functioning, for example, on the basis of positive assortative reproduction, i.e. preferential reproduction with individuals with similar phenotype. Modern models of ecological speciation include these mechanisms and simultaneously assume that several mutually opposing selection pressures, whose existence is connected primarily with the heterogeneity of the environment, act simultaneously on the population.

The situation is somewhat more favorable for the Darwin’s mechanism based on interspecific competition. To the very least, this mechanism could be equally valid for asexual and sexual species. It is quite possible that disruptive selection acting at the interspecific level could provide an advantage for species that utilize unusual niches and thus contribute to a gradual increase in the phenotype variability of organisms in nature, i.e. disparity, and also the total number of species that can coexist at a single moment in natural ecosystems, i.e. diversity in the narrow sense of the word.