Basics of Evolution - O.P. Korzh - 2006

Part I. MICROEVOLUTION

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

9.1. History of the Concept of "Species"

The Study of species and speciation is one of the fundamental problems in modern biology. Although the term "species" was introduced into biology by Aristotle, it did not receive a satisfactory definition until 2,000 years later.

As humanity encountered the wilderness, it faced The Challenge of exploring it, which led to the study of individual PLANT AND ANIMAL species. This fostered the accumulation of baseline knowledge regarding certain properties of species, which were referred to by the then more common term "breed" (порода). This term already encompassed three future criteria of a species:

1) morphological, i.e., the similarity of a specific group of individuals;

2) physiological, i.e., The ability to reproduce fertile offspring;

3) genetic, i.e., the preservation of parental traits by descendants.

Aristotle is considered the founder of Taxonomy, classifying organisms based on a complex of traits. He introduced the term "species" into biology from logic, where it denoted a multiplicity of phenomena subordinate to a higher category—the genus. Thus, another characteristic of the species was revealed—its substantive nature: the generic category forms merely the foundation, without accounting for the specific traits inherent to the immediate species as a member of that genus. In all other respects, the term remained undefined and was used to characterize groups of organisms of any category, provided they were part of a larger taxonomic group (genus). Consequently, for nearly two millennia, biology was dominated by an initial undifferentiated view of taxa, making the scientific investigation of species out of the question.

Issues concerning the species were first addressed in the works of J. Ray, published in the late 17th century. He was the first to single out the species as a biological phenomenon and define it, highlighting several specific features. In his view, a species is the smallest assemblage of organisms that are nearly morphologically identical, interbreed, and produce offspring that retain this similarity. The core essence of a species thus became the constancy of form across generations—that is, the derivation of like from like.

During the first half of the 18th century, the species was not yet clearly outlined as a stable taxonomic unit, and biological research was primarily focused on gathering baseline data. The systematization and synthesis of all prior experience were carried out by C. Linnaeus, who demonstrated the universality of this phenomenon and highlighted its significance as a structural unit of living matter.

According to Linnaeus, a species is a universal, concrete, and qualitatively delimited phenomenon of nature. The scientist was able to isolate species across all genera of plants and animals, proving their general biological distribution—namely, that species constitute The Structure of the organic world. It was following his work that the species became the fundamental unit of taxonomy.

Thus, by the late 18th century, two additional vital traits of the species were discovered: its stability and discreteness. Observation established that a species preserves its morphological features both in time (traits are passed down from generation to generation) and in space (amid geographical shifts in environmental conditions). Its discreteness lay in the fact that a species proved to be a biologically isolated entity. This segregation from similar entities was manifested both in morphological differences and in the cross-incompatibility of individuals (or the sterility of offspring), which enabled the formulation of the morphological and physiological criteria of a species.

The accelerated advancement of knowledge about species at the beginning of the 19th century brought to the forefront another biological problem: the correlation between species stability and Variability. All views on this central issue at the time can be divided into at least three groups. The first, formulated by Linnaeus, recognized the species as real and immutable. Similar views were held by G. Cuvier, the founder of paleontology and catastrophism. Noting significant differences between ancient and modern faunas, he explained this not through the ability of species to undergo gradual evolutionary change, but via periodic catastrophes resulting in the extinction of "unsuccessful" forms, which were then replaced by new species. Subsequently, followers of G. Cuvier added that after each successive catastrophe, the Creator initiated the act of creation anew.

J.B. Lamarck was an exponent of the second system of views on the species problem. Until the late 18th century, he acknowledged the existence of real species, but after formulating his pioneering evolutionary theory, his views changed radically. According to J.B. Lamarck, the notion of permanent species was erroneous, as only individuals exist in nature: nature presents us solely with individuals descended from one another. As for species, their constancy is relative, and their immutability is temporary. Thus, having discovered yet another trait of species—the relativity of stability—Lamarck failed to balance organisms' capacity for evolutionary change with the objective reality of species, considering them to be nothing more than artificial assemblages of individuals useful merely for taxonomic bookkeeping.

The third system of views combined the two preceding ones into a thesis formulated by É. Geoffroy Saint-Hilaire: "Species change." Another proponent of this viewpoint was K.F. Roulier. He believed that species are not imaginary groupings of individuals sharing the highest similarity across a complex of traits, but real phenomena of nature. In the scientist's view, we have equal grounds to assume or deny the immutability of species. While denying species immutability, the adherents of this system failed to provide a well-founded explanation for the causes of such changes (their proposed explanations were quite naive).

Only C. Darwin, who formulated the mechanisms of species modification, succeeded in convincing the majority of scientists of their variability. According to his views, species originate at some point and exist temporarily, as sooner or later they either become extinct or transform into new ones. The scientist was convinced that not all incipient species evolve into true species—a significant number of them become extinct or remain as subdivisions of the ancestral group for an indefinitely long period.

Thus, the differentiation of a species is a natural consequence of group evolution through adaptation to diverse environmental conditions and, simultaneously, a prerequisite for its further evolution. Species formation, development, subsequent divergence, and decline began to be viewed as stages in a gradual process of adaptive evolution. Thus, a new general biological evolutionary Concept of the species was formulated, and the science of species extended far beyond the boundaries of taxonomy.

Paradoxically, a new wave of challenges regarding METABOLISM/2.html">THE CONCEPT OF species was triggered by the rapid growth of genetics in the early 20th century. This subsequent crisis stemmed from the discovery of yet another characteristic of the species: as a highly complex system encompassing a vast array of minor forms. This intricate internal structure clashed with established views of the species as a simple, relatively homogeneous entity—an elementary, indivisible unit of life.

Consequently, two opposing concepts of the species emerged: as an extremely complex system of minor forms that is nonetheless compositionally homogeneous, and as a group of races. This duality was most pronounced in the views of neo-jordanists (early 20th-century geneticists and breeders such as Johannsen, de Vries, Lotsy, et al.). They recognized as true species only minor, hereditarily stable forms that do not segregate further, obtained as the End products of the experimental genetic dissection of natural populations. To isolate such a species, it was sufficient to find differences in at least a single trait. De Vries proposed calling such species jordanons (named after A. Jordan, a 19th-century botanist who recognized only ideal monotypes as species). A jordanon was conceived as the lowest constant taxonomic unit based on the independence of hereditary units determined by Mendelian laws during crossing.

Conventional species accepted in taxonomy were named linneons (after Linnaeus) and were defined as artificial mixtures of forms that do not actually exist in reality but are convenient for scientific nomenclature. Operating on the premise of the species as an absolutely homogeneous entity, neo-jordanists dissected polymorphic populations into nearly monokaryotic/monozygotic forms. As a consequence, they began to ignore the most crucial attribute of the species—its capacity for independent existence and reproduction.

It was not until the mid-20th century that the biological species concept gradually took shape, emphasizing the dual Biological Significance of species: on the one hand, reproductive isolation, and on the other, a shared Gene pool. Today, the biological concept of the polytypic species is universally accepted.