Selection intraspecific and interspecific

The term natural selection is sometimes erroneously extended to include two completely different phenomena, intraspecies andinterspecies selection, also denoted(especially by ecologists) as interspecies competition. Another evolutionary mechanism – species selection – is based not on mutual ecological competition between the members of various species, but rather on the competition between entire evolutionary lines, and follows from the existence of differences in the rates of speciation and extinction of various taxons (Stanley 1975). This mechanism was first described relatively recently and will be discussed in part IV.8.4 and also in the part concerned with macroevolution (XXVI.3). All the phenomena described to date were related to intraspecies selection and Darwinism as a whole can, with certain simplifications, be understood as the theory of emergence and gradual development of modern organisms through the mechanism of intraspecies selection.

Intraspecies selection andinterspecies competition are two phenomena that are incomparable in their biological importance. While intraspecies selection is capable of gradually forming and improving various useful biological structures, organs, macromolecules and patterns of behaviour, interspecies competition functions only as a one-step process capable, in the final analysis, of  deciding which of the mutually competing species is better at the given time and place. Species with worse parameters mostly do not have a chance to evolutionarily adapt to the competition and are usually immediately eliminated on an evolutionary time scale.  If two competing species have only partly overlapping niches and only partly overlapping areas of occurrence, the weaker species need not be completely eliminated; however, there can be a drastic change in its niche and, because of interspecies competition, it can successfully survive only in certain, strictly limited types of biotopes or only in those parts of its original area of occurrence in which the other species is not present. Thus, it can gain time for the relevant evolutionary changes and could, in time, eventually expand back into its original biotope or to other parts of its original area of occurrence. However, in this case, the relevant evolutionary changes accumulate through the classical mechanism of intraspecies selection.

While intraspecies selection is apparently the most important factor in biological evolution, interspecies competition, similar, e.g., to genetic drift and sexual selection, is only an important factor in evolution, affecting some of its properties and determining some properties of the organisms. Its main importance apparently lies in “niche reduction”. The fact that each kind of organism is limited to only a relatively narrow niche forces it to specialize, to select only a specialized life strategy and to improve this strategy as much as possible through intraspecies selection If there were no interspecies competition, e.g. if only one kind of organism were to live on the Earth, this would probably be an unspecialized species, capable of living under various conditions and utilizing various resources. Its individual organs and life functions would probably not be as well adapted to the environment as those of contemporary, mostly highly specialized species. If the survival of a member of a certain species is dependent on how fast it can run, the evolution of the motor system will occur much more rapidly for this species (and will advance much further) than if its survival were determined by a number of various factors or even by chance.

Natural selectioncould theoretically occur even at higher levels than the population or species. Consequently, it is possible that entire flora and fauna communities could compete together or, on a cosmic scale, entire biospheres of various planets. Ecological data indicate that competition between communities does actually occur and sequences of succession stages have been described, which regularly alternate in a certain biotope. However, from the standpoint of evolutionary biology, competition at a higher level than intraspecies is a rare phenomenon. The main factor responsible for minimal effectiveness of selection at a community level consists in low heritability of the properties of communities. If selection actually occurs at the level of ecological communities, then everything that was said of group selection is also true here (to an elevated degree).

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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