Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 9. Species as One of the Main Forms of Life Organization

9.3. Modern Problems of the Species Concept

Even the universally accepted biological species concept does not fully satisfy biologists. Modern genetic and evolutionary Structure/97.html">Definitions of a species are applicable only to organisms that reproduce sexually through cross-Fertilization. Regarding agamic and parthenogenetic forms, as well as organisms that reproduce via self-fertilization, The Use of this term requires certain clarifications. Since a species is recognized today not as a genetically closed system, but as a genetically stable one, THE SPECIES CONCEPT can also be applied to these groups of living organisms. In these cases, a species should be defined as a group of phenotypically similar individuals that share a related genotype, occupy a common geographic range, and are bound by a common evolutionary destiny.

A large number of plants are capable of Vegetative Reproduction, and some of them (primarily artificially obtained forms) lose The ability to sexual reproduction altogether. Certain species are apomicts (where seed formation requires pollen to land on the pistil and begin germinating, but fertilization does not occur) or reproduce via parthenogenesis. The latter mode of reproduction (without fertilization) is quite common among animals. Moreover, in forms with facultative parthenogenesis (rotifers, certain crustaceans, aphids, etc.), the ability for normal reproduction is retained, as males appear before the onset of unfavorable environmental conditions and fertilize females in the usual way. However, agamic and obligately parthenogenetic forms no longer fit the biological definition of a species, because two individuals never participate in creating the hereditary complex of a third; that is, each individual has only one parent.

The consequence of agamic and parthenogenetic reproduction is that all descendants of each individual are genetically almost identical to their parent, excluding The formation of Mutations. In other words, all descendants of a single individual over any number of generations are connected to each other by their origin and traits to a much greater extent than to any other individual. The question arises: is this group a distinct species or merely a variety? This could only be answered by relying on the criterion of interbreeding, but it is completely absent in these forms.

Therefore, for the aforementioned forms, the most important factors in defining species are the morphological Features of the individuals and the commonality of their evolutionary destiny.

A whole range of new problems arises when studying species in paleontology. This is explained by the fact that a species is considered not only in space, as in neontology (the present day), but also in time. The interbreeding criterion becomes completely inapplicable for paleontological forms, regardless of the reproductive mode of ancient organisms. Therefore, the main criteria become morphological and reconstructive-biological (taking into account fossilized tracks, traces of activity, etc.). However, even in this case, the description of a species requires a large number of studied organisms; otherwise, scientists cannot form the necessary Concept of the individual Variability of certain traits.

All modern organisms are interconnected to some degree by common ancestors, meaning that individual natural groups can be represented as the terminal branches of a single evolutionary tree. Therefore, the distinctness of certain forms arises primarily due to the incompleteness of paleontological data and the elusiveness of all possible transitional forms (with the exception of certain groups of organisms).

The latter complicates the understanding of the evolutionary pathways of some groups, but significantly simplifies their Classification. The vast majority of fossil forms were initially described based only on individual (often incomplete) specimens, so determining their species affiliation did not cause particular difficulties. However, subsequent studies revealed the presence of significant chronological variants. Meanwhile, The system of Classification and Nomenclature, aimed at establishing groups of organisms, becomes completely unsuitable for entities that are isolated in certain locations or time periods, yet connected by a continuous series of transitions in any other place or period.

Taking into account significant changes over time in both ranges, intraspecific structure, and even the morphological traits of organisms belonging to the same evolutionary branch, METABOLISM/2.html">THE CONCEPT OF "species" in paleontology has been abandoned in favor of generic names or the term phratry to designate a segment of the Phylogenetic Tree corresponding to the formation time of a single species (the neontological analog of a species).

Another problem concerning species is the inequivalence of certain species regarding their establishment and definition. Some species are established based on only a few individuals, which provides no insight into their distribution, individual variability, or other purely species-specific characteristics. This is especially true for fossil forms of organisms.

The situation is different with modern widespread species. To study them, mandatory annual censuses of game animals are conducted, respective services monitor the outbreaks of mass reproduction of certain pests, and special monitoring activities are carried out to study the state of populations of rare and endangered species. All of this equips scientists with knowledge not only about the Distribution of a species, but also about its structure and dynamic processes, allowing them to assess the consequences of various impacts. A similar problem arises when defining species based only on certain traits, such as morphological ones. Therefore, we cannot equate agamic forms with true biological species that reproduce sexually, because in this case, the delineation of species boundaries does not rely on the interbreeding criterion and can become rather conventional.

It must be taken into account that natural species have certain differences from those recognized in Taxonomy. This is one of the reasons why a complete natural classification cannot be created, as all existing forms would need to be assigned a clearly defined status within it. In reality, species have a complex structure with a certain number of intermediate, transitional forms, etc. Special problems arise due to the prolonged Nature of the speciation process, when a certain group has already separated from the parent species but has not yet acquired final species independence.

The problem regarding the content of this term arises when comparing species from different taxonomic groups. This concerns not only differences in intraspecific structure (number of subspecies, population size, sex and age structure, etc.), range size, ecological features, but also The Significance of individual species. For instance, while the mosquito Aedes flavescens is one of many Representatives of the genus Aedes, some genera are represented by a single modern species. The white rhinoceros (Ceratotherium simum), for example, is the only modern representative of the genus Ceratotherium. The platypus (Ornithorhynchus anatinus) is generally the sole modern representative of the entire family Ornithorhynchidae, whereas the tuatara (Sphenodon punctatus) belongs even to the subclass Lepidosauria of the Class Reptilia. Obviously, we cannot consider these species equivalent.

All these issues cannot be resolved simultaneously, and some of them require constant, close attention from specialists. These problems have acquired particular relevance recently, as nature conservation activities aimed at preserving the species diversity of life forms become one of the priority directions.

Food for Thought

The problem of species definition cannot have a single unambiguous solution, just like a significant number of other biological problems. For instance, if scientists are still debating whether Viruses should be considered alive, can we compare their species with the species of any other group of organisms? The continuity and relentless nature of the evolutionary process will provide naturalist biologists with work for as long as humanity exists. That is, we will never have a completely comprehensive picture of the species composition of living organisms on planet Earth, as the latter is in a state of constant reorganization. Even less should we ever hope to fully control this process.