Speciation(redirected from Species barrier)
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The process by which new species of organisms evolve from preexisting species. It is part of the whole process of organic evolution. The modern period of its study began with the publication of Charles Darwin's and Alfred Russell Wallace's Theory of Evolution by Natural Selection in 1858, and Darwin's On the Origin of Species in 1859.
Belief in the fixity of species was almost universal before the middle of the nineteenth century. Then it was gradually realized that all species continuously change, or evolve; however, the causative mechanism remained to be discovered. Darwin proposed a mechanism. He argued that (1) within any species population there is always some heritable variation; the individuals differ among themselves in structure, physiology, and behavior; and (2) natural selection acts upon this variation by eliminating the less fit. Thus if two members of an animal population differ from each other in their ability to find a mate, obtain food, escape from predators, resist the ravages of parasites and pathogens, or survive the rigors of the climate, the more successful will be more likely than the less successful to leave descendants. The more successful is said to have greater fitness, to be better adapted, or to be selectively favored. Likewise among plants: one plant individual is fitter than another if its heritable characteristics make it more successful than the other in obtaining light, water, and nutrients, in protecting itself from herbivores and disease organisms, or in surviving adverse climatic conditions. Over the course of time, as the fitter members of a population leave more descendants than the less fit, their characteristics become more common.
This is the process of natural selection, which tends to preserve the well adapted at the expense of the ill adapted in a variable population. The genetic variability that must exist if natural selection is to act is generated by genetic mutations in the broad sense, including chromosomal rearrangements together with point mutations. See Genetics, Mutation
If two separate populations of a species live in separate regions, exposed to different environments, natural selection will cause each population to accumulate characters adapting it to its own environment. The two populations will thus diverge from each other and, given time, will become so different that they are no longer interfertile. At this point, speciation has occurred: two species have come into existence in the place of one. This mode of speciation, speciation by splitting, is probably the most common mode. Two other modes are hybrid speciation and phyletic speciation; many biologists do not regard the latter as true speciation.
Many students of evolution are of the opinion that most groups of organisms evolve in accordance with the punctuated equilibrium model rather than by phyletic gradualism. There are two chief arguments for this view. First, it is clear from the fossil record that many species persist without perceptible change over long stretches of time and then suddenly make large quantum jumps to radically new forms. Second, phyletic gradualism seems to be too slow a process to account for the tremendous proliferation of species needed to supply the vast array of living forms that have come into existence since life first appeared on Earth. See Animal evolution, Population genetics, Species concept
the process of the formation of new species. From the teachings of C. Darwin on the origin of species and from all subsequent development of the biological sciences, it necessarily follows that species change with time, acquiring new characteristics and properties, and are differentiated in such a way that two or more new species are formed from a single one. Natural selection is the sole guiding factor in speciation. The process of speciation in nature can be ob-served only on very rare occasions; speciation encompasses periods of time significantly exceeding the life span of several generations of humans or else it occurs narrowly and localy in individual populations of the “old” species and is over-looked by investigators. For these reasons, only theoretical assumptions about the mechanism of speciation in nature are possible. There are two major directions for creating and developing such propositons: (1) the comparative study of various phases and levels of intraspecific differentiation (geographic and ecological forms) and (2) the logical development of assumptions on the possible mechanisms of speciation derived from various branches of genetics, including cytogenetics (the change of karyotypes and frequency of genes in populations, the value of genotypes in selection, isolation mechanisms, and so forth).
During Darwin’s time and in the subsequent period of the development of the classical theory of evolution, speciation was considered to occur primarily geographically. It was proposed that geographic subspecies were “stages” of speciation. This was supported by the existence of geographic vicarious species, closely related in their morphophysiological characteristics and occupying different, but similar in physicogeographic conditions, areas of distribution (allopatry). Further evidence of this was the abundance of endemic species given territorial isolation (on islands, for ex-ample). It was thought that the significant overlapping of areas of distribution of different species or the distribution of one species within the area of distribution of another species (sympatry) was a secondary phenomenon, linked with the penetration of already established species in a general terri-tory. However, data are already gathered testifying to the possibility that new species arise under sympatric conditions as well.
Speciation requires natural conditions of isolation barriers, which prevent crossbreeding, formation of transitional hybrid zones, and the leveling off of the achieved differences between the original and new forms. In addition to various forms of geographic isolation (territorial-mechanical), there are also various forms of biological isolation, which can be divided into three major groups: ecological-ethological, morphophysiological, and particularly genetic. Biological isolation leads to decreased probability of encounter between individuals of different sexes during the reproductive period, decreased sexual attraction and effective mating, and a drop in the viability or fertility of the hybrids formed as a result of crossbreeding.
In allopatric speciation, some form of territorial-mechanical isolation (a break in the area of distribution) usu-ally appears first. For example, in the Palearctic Region, when the glaciers advanced during the Quarternary Period, many species of animals and plants were pushed to the south, into the so-called glacial refuges, where groups of populations long separated by the advance of the glaciers existed in isolation. During this period they were able to develop some form or other of biological isolation. As a result, after resettling in a new place to the north (with the end of the Ice Age) and a second meeting, these forms no longer crossbred under natural conditions, since they had been transformed into distinctly different species.
In sympatric speciation, a more or less biologically isolated group of individuals first forms within the bounds of a species population. If this group acquires some qualities that in certain conditions of the environment give an advantage in selection, then this group can be the start of a new sympatric species. In such forms, which are still not absolutely isolated within the original species, it is possible, for instance, to accelerate the accumulation of chromosome rearrangements, which would lead to absolute genetic isolation and the formation of new species, still morphophysiologically very close to the old one (within the limits of twin species). In plants, as a result of self-pollination, there may frequently arise groups of tetraploid individuals within an original species; when they crossbreed with the original diploid individuals, this yields sterile triploid hybrids. If such forms are not rejected by selection, they can form the beginning of new species not crossbreeding with the original. This is supported by the frequent encountering among plants of series of diploid species within a genus.
Since the process of speciation is extremely prolonged, it is worthwhile to await the appearance in nature of beginning, still not fully formed, species. Thus, within the boundaries of the Polar Circle area of distribution gulls (herring gull and lesser black-backed gull) form a continuous chain of sub-species leading from one to the other; the extreme links of this chain in the Baltic-White Sea region behave like distinctly different species.
The processes of speciation are not always causally linked with the appearances of new adaptations (for instance, the appearance of tetraploids in plants); such adaptations may accompany geographic speciation if some form of ecological isolation occurs first. In the majority of cases, evidently, new adaptations occur more easily after some degree of isolation of the new species, when the leveling of differences between forms stops, interspecific competition begins, and the possibility of forming new ecological niches accelerates. Adaptive and nonadaptive changes in a species over time can be traced within the limits of a generically related group (“phratry”) of species. All the processes of speciation in nature are complex and occur under the pressure of various elementary evolutionary factors, acting differently on the mixed genotypic composition of populations.
REFERENCESDarwin, C. “Proiskhozhdenie vidov putem estestvennogo otbora .…” Soch., vol. 3. Moscow-Leningrad, 1939.
Simpson, D. Tempy i formy evoliutsii. Moscow, 1948. (Translated from English.)
Zavadskii, K. M. Vid i vidoobrazovanie. Leningrad, 1968.
Mayr, E. Zoologicheskii vid ievoliutsiia. Moscow, 1968. (Translated from English.)
Timofeev-Resovskii, N. V., N. N. Vorontsov, and A. V. lablokov. Kratkii ocherk teorii evoliutsii. Moscow, 1969.
N. V. TIMOFEEV-RESOVSKII, N. V. GLOTOV, and V. I. IVANOV