Fundamentals of Evolution - O.P. Korzh - 2006

Part I. MICROEVOLUTION

Chapter 9. The Species as a Principal Form of Biological Organization

9.2. Modern Concepts of Species

Life is inherently discrete, and this property can be considered one of the most fundamental laws of nature. Discreteness manifests itself not only in the fact that life on our planet is represented by individual organisms (individuals), but also in the way these individuals are grouped into species. Therefore, the species as a natural phenomenon serves as the primary structural unit of the living world.

Unfortunately, an optimal definition of a species has yet to be proposed, although the number of attempts may soon rival the number of species themselves. Most of these attempts have been made by systematists, primarily to single out traits suitable for practical Classification. Often, the proposed Structure/97.html">Definitions rely on recognizing the composite nature of a species: it is viewed as an aggregate of certain individuals, populations, ecotypes, subspecies, or even traits, generations, and so on. At the same time, only a few characteristics (usually superficial ones) of a given species are taken into account, but their greatest flaw is viewing the species as a mere aggregation of forms, thereby losing sight of its integrity.

The opposing tendency—that is, defining a species as a group of organisms united by a common type of Organization (morphological, physiological, genetic, etc.)—also encounters a major problem: completely monomorphic species do not exist in nature. According to Charles Darwin, The amount of difference deemed sufficient to distinguish two forms as separate species is almost impossible to define. Therefore, functional definitions of species cannot be universal.

According to Ernst Mayr, a species is a group of interbreeding natural populations that are reproductively isolated from other such groups. The biological species concept itself is based on three main pillars: 1) species are defined not by differences, but by distinctness; 2) species consist of populations rather than isolated individuals; and 3) species are more adequately defined by reproductive isolation than by sterility in crossing individuals (referring not only to genetic isolation, but to other Types of isolation as well).

We will define a species as a genetically isolated, stable, yet potentially evolving form of existence of living nature, which can be considered both a stage and the primary unit of the evolutionary process. In other words, organisms are united into a single species based on a set of criteria and the community of their evolutionary destiny.

The entire history of species research demonstrates that there is no single organismal trait that can be used as a universal absolute criterion. For instance, the very first criterion employed by humans is the morphological one. It offers no objective way to determine what degree of structural difference qualifies as specific to a species. Since the time of Carl Linnaeus, the search for essential distinctions has failed to yield the desired result, because the exact same trait has different taxonomic significance in different systematic groups. A striking example is the vast morphological differences across entire correlative systems of traits in domestic animals, which are granted breed status, whereas species-level differences in other groups are significantly smaller (at least an order of magnitude).

Consequently, the same trait in different groups acquires a fundamentally different taxonomic significance or, conversely, loses it entirely, becoming merely an example of intraspecific variation. Furthermore, according to Ernst Mayr, any trait—ranging from microscopic features, such as Chromosome structure, to macroscopic ones—may serve as a species criterion in some groups, while in others it may indicate a genus, a family, or have no systematic significance at all.

When used in isolation, the geographical criterion can lead to the classification of any spatially separated population as a distinct species. Moreover, this offers no guarantee that a group of organisms sharing all other indicators is not internally fragmented. In general, this criterion fails to distinguish between morphologically distinct species living in the same area, yet it may lump together spatially separated organisms that are identical in other respects.

Only when ranges are significantly isolated can the geographical distinctness of a species be considered an essential feature. A geographical range is a mandatory characteristic of a species, but it cannot serve as its absolute and sole criterion.

The biochemical criterion exhibits a nearly identical limitation. There are no specific species-specific substances, immunological reactions, or the like. Biochemical traits carry the same weight as morphological, geographical, or any other traits. Therefore, amidst the current infatuation with biochemical, molecular-biological, or similar studies of species structure, one must remember that a species cannot be reduced to a Genetic Code. When biochemical properties of organisms are used independently, The problem of the sufficiency of differences between individual organismal groups arises; The Essence of the species is lost, and its boundaries can be established only arbitrarily.

The same holds true for any other species criteria. For example, the ecological criterion cannot even distinguish contrasting trait variations within a single population. Nor does the genetic criterion possess absolute significance, given sexual isolation (the physiological criterion). This is because ecologically or ethologically isolated species can interbreed under artificial conditions, producing fertile offspring. Therefore, determining the systematic status of a group of organisms requires relying on the entire complex of known criteria.

When determining the universality of the species as a whole, two questions arise: 1) Is the species as ancient as life itself, or did it emerge later?; 2) Does the species exist in all modern groups of organisms? Regarding the time of The Emergence of the species as a biological phenomenon, there is no direct evidence. It is believed that The formation of this structure dates back to the time when protobionts acquired the capacity for precise self-reproduction. This is supported by the profound uniformity and conservatism of structures and biochemical processes in living organisms, indicating their great antiquity. From the very beginning, the species has been tied to the Stability of the genetic apparatus (primarily referring to Replication processes). Therefore, it can be assumed that the species emerged only with the transition of protobionts to stable self-reproduction.

The earliest factual data on fossil remains of specific species date back to Proterozoic deposits (quartzites of South Australia, Europe, etc.). Studies of ancient fauna and flora from these deposits (Bacteria, blue-green Algae, hydrozoans, jellyfish, Annelids, etc.) indicate that even one and a half billion years ago, the species as a biological phenomenon in living organisms was no different from modern ones. According to V.I. Vernadsky, a complex array of living forms was established at the very dawn of life: the formation of individual organisms occurred alongside the Formation of primary species of biocenoses and the biosphere.

Regarding the distribution of species among modern organisms, the majority of scientists believe that all contemporary organismal groups are organized into species; in this sense, the species is a universal phenomenon of living nature. At the same time, other views exist, such as the hypothesized existence of "presubcellular or pre-species forms of life" (Viruses), or the restriction of THE SPECIES CONCEPT exclusively to organisms with sexual reproduction.

The universality of the species is also manifested in its general features, which are inherent to almost All living organisms. First, a species is a supra-individual entity, meaning it consists of A large number of individuals, with its population size acting as a qualitative characteristic. Second, a species possesses a unified hereditary basis, expressed in the Specificity of the DNA–RNA–protein system, similarities in metabolic pathways and morphogenesis, individual Morphology, and the intraspecific interactions of populations. Thus, the species acts as a relatively homogeneous entity and acquires the status of a distinct unit.

Properties of a species include the capacity for self-reproduction, the preservation of its qualitative definiteness during reproduction, and its distinctness from other similar entities. A species necessarily occupies specific boundaries of distribution in nature (its range) and represents a discrete link in the cycling of matter.

Despite its isomorphism, a species always possesses an internal structure and a certain degree of differentiation. The population serves as the fundamental unit of this system. Finally, due to its capacity for evolutionary change, a species has a temporary existence that takes the form of a phylogenetic branch. At the same time, it is sufficiently stable and lacks any pre-programmed lifespan.

Thus, only those groups of organisms that exhibit the aforementioned traits can be considered a species. It should be noted, however, that the expression of individual traits may vary in level across different species, and new private characteristics may emerge in each specific case.

As a whole, the species can be conceptualized as one of the primary and fundamental forms of organization of living matter.

THE PRINCIPLE OF complex life organization dictates that the Earth's living cover simultaneously comprises a vast number of subordinate systems of varying organization. A key feature of this organization is the emergence of novel qualitative properties upon the integration of two components, which cannot be explained solely by The properties of the parts in isolation. Relative to life, organization manifests as the spatio-temporal ordering of various elements. Today, the existence of several hierarchical series of interrelated life elements at different Levels of organization is widely recognized (individual - population - ecosystem - biosphere; individual - population - species).

The existence of relatively independent, universal, and stable living systems interconnected by reticulate and hierarchical relationships underlies the existence and Evolution of the organic world. Data from biogeochemistry, paleontology, and other sciences indicate that the Organism, species, biocenosis, and biosphere as a whole are primary forms of life. In other words, living matter began to self-organize from its very inception.

A species is a special, highly individualized form of life organization capable, on the one hand, of existing for an indefinitely long period and, on the other, of evolving relatively independently. The absence of intrinsic limits to a species' lifespan fundamentally distinguishes it from an organism (let alone pre-organismic structures) and a biocenosis, bringing it closer to The Biosphere as a whole. According to modern evolutionary concepts, biological structures that have reached the species level of organization automatically acquire an intrinsic capacity for an indefinite

duration of existence. However, this capacity can only be realized if the general direction of natural Selection is aimed at preserving the already existing adaptive norm of reaction, which in natural conditions is never fully realized.

The current level of knowledge suggests that the evolutionary process occurs almost exclusively at the population-species level of life organization. Any subpopulation structures, despite the presence of a mutation process, are incapable of independent evolutionary transformations. The evolution of an organism is not an independent and species-unrelated process. In nature, only species evolve, yet the most visible changes are those that have occurred in an individual—a lower form of life organization compared to a species.

Regarding the capacity of biocenoses and the biosphere as a whole for evolutionary changes, scientists do not yet share a consensus on this important issue (which is discussed In the second chapter). We will only note here that successional changes make a biocenosis an unstable structure, while Floristic and faunistic Changes in the COMPOSITION OF THE biosphere over geological time, as some scientists believe, do not alter its foundational organizational principles (essential components of matter and energy cycling, such as producers, consumers, and decomposers, remain mandatory).

A species is a qualitative stage of evolution, yet it is understood in various ways. Some scientists, addressing this problem, have noted that a significant number of species have not evolved since their appearance, existing for a long time unchanged or becoming extinct. Therefore, they have questioned the postulate of a species as a stage (phase) of evolution. E. Mayr proposed viewing a species not as a stage, but as the result of evolution: in his view, THE ORIGIN OF a species is the final step of speciation. We, however, will consider a species as a nodal stage of evolution, since it is simultaneously the product of speciation and can serve as the basis for the subsequent evolutionary process.