Of the four characteristic properties of the products of biological evolution on the Earth, i.e. complexity, organization, usefulness and diversity (see Chap. I), the existence and origin of diversity probably represent the most complicated problem.In nature, organisms form natural, frequently very sharply defined groups – species.The similarity is substantially greater amongst the individuals of a single species than amongst the individuals belonging to various species.In addition, groups of organisms form a clearly hierarchically ordered system, in which the individual species of the lowest order (species) form common higher groups (higher taxons), whose members have some shared properties; these higher groups again form higher groups of an even higher order, etc.Simultaneously, there are quite sharp boundaries between the individual groups; there are only a minimum number of taxons whose members would exhibit a random combination of the properties of organisms belonging, for example, to two other taxons.

At his time, Darwin was of the opinion that he had resolved both the problem of the existence of species and also the existence of hierarchical structures in the system of higher taxons, when he discovered that the individual organisms were formed gradually one from another during biological evolution.It follows directly from the mechanism of evolution, to be more exact from the mechanism of cladogenesis, that the individual taxons can gradually “grow – bud out” from one another and thus one group can inherit traits from another group or subgroup, but cannot be formed “de novo” or from two or more taxons at once and thus acquire a combination of the traits characteristic for various, mutually unrelated taxons.Thus, Darwin entitled his most important book “On the Origin of the Species by Means of Natural Selection”.Only over time did it become apparent that, while the mechanism of cladogenesis can explain the existence of hierarchical structures in the system of higher taxons and the absence of higher taxons whose members would exhibit a random combination of the traits of the other existing taxons, it cannot, in itself, explain the existence of the lowest taxon, i.e. the existence of a biological species.At this lowest level, we would tend, on the basis of Darwin’s original theory, to expect that there will exist more or less continuous transitions between the individual species and not the clearly defined boundaries that can be observed in nature.It can be stated that it was not until the 20th century that Darwin’s theory was supplemented by a part concerned with the aspect of the formation and existence of distinct species.The aspect of the existence of species will form the subject of this chapter and the aspect of the formation of the species will be discussed in the next chapter.The subject of the formation of higher taxons will be discussed in Chapter XXIII, devoted to phylogenetics.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.