One of the key issues in the theory of evolution is the problem of evolutionary progress. This concept expresses the general tendency of living systems to complicate organization in the course of evolution. Despite the fact that phenomena of the reverse order are also observed - simplification - or stabilization of systems at the same level of complexity, the direction of the evolutionary process of some large groups of organisms demonstrates the development from simple to complex.
A great contribution to the development of the theme of progressive evolution was made by A. N. Severtsov (1866–1936), one of the founders of the evolutionary morphology of animals.
Development of ideas about the progress of living systems
The most important merit of A. N. Severtsov is the distinction between the concepts of biological and morphophysiological progress.
Biological progress refers to the success achieved by any group of organisms. It may appearin many forms such as:
- increasing the degree of adaptation of the group to environmental conditions;
- population growth;
- active speciation within a group;
- expansion of the area occupied by the group;
- increase in the number of subordinate groups (for example, the number of units in the class of mammals).
Accordingly, a decrease in these parameters characterizes failure - a biological regression of a group of organisms.
Morphophysiological progress is a narrower concept. This term refers to the improvement of the organization, expressed in the complication of the structure and functions of the body. The delimitation of concepts related to progress made it possible to get closer to understanding how and why morphophysiological progress ensures biological prosperity.
The concept of aromorphosis
The term was also proposed by A. N. Severtsov. Aromorphosis is a progressive change that leads to a complication of the organization of living systems. Progressive evolution is like a series of such changes. Aromorphoses, thus, can be considered separate stages of morphophysiological progress (arogenesis).
Aromorphosis is a major adaptive acquisition that increases vitality and leads a group of animals or plants to new opportunities, such as a change in habitat. As a result of the accumulation of aromorphoses, as a rule, high-ranking taxa arise, such as a new class or type of organisms.
Complication of the structure (morphology) only together with functional acquisitions can be considered aromorphosis. It is necessarily associated with changes in the system of regulation of certain functions of a living system.
Main features of the process of arogenesis
Morphophysiological progress is characterized by changes in the set of features that determine the degree of complexity of living systems.
- The level of homeostasis increases - the ability to maintain the stability of the internal environment of the body (for example, a constant body temperature in warm-blooded animals, s alt composition, and so on). The ability to maintain the sustainability of development in changing external conditions also increases - homeoresis. This indicates the improvement of regulatory systems.
- The level of energy exchange between the organism and the external environment is growing. For example, warm-blooded animals have fast metabolisms.
- The amount of information is growing, the ways of processing it are becoming more complicated. So, with the complication of the genome, the amount of genetic information increases. The progressive evolution of vertebrates is accompanied by the process of cephalization - the growth and complication of the brain.
Thus, morphophysiological progress, affecting all of the above indicators, allows a living system to increase independence from the external environment.
Genetic foundations of evolutionary transformations
The material that undergoes transformations in the course of evolution is the gene pool of a population of organisms. Its main properties are the genetic diversity of individuals and hereditary variability. The main driverstheir factors are the recombination of genetic material during transmission to offspring and mutations. The latter can be repeated and accumulated.
Natural selection reinforces beneficial mutations in the gene pool and discards harmful ones. Neutral mutations accumulate in the gene pool, and when conditions change, they can become both harmful and beneficial and also undergo selection.
By contacting, populations exchange genes, thanks to which the genetic unity of the species is preserved. It is violated in the case of various options for isolating populations - all of them contribute to the process of speciation.
One of the most important outcomes of selection action is adaptive acquisition. Some of them turn out to be very large and significant under certain conditions - these are aromorphoses.
Examples of aromorphic changes
In unicellular organisms, examples of aromorphosis are such major evolutionary events as the formation of cells with mitochondria (the latter were independent organisms in the early stages of life development), the emergence of sexual reproduction, the appearance of eukaryotic cells.
The largest aromorphosis in the animal kingdom was the emergence of true multicellularity (multi-tissue). In chordates and vertebrates, examples of such major structural and functional rearrangements of organisms are: the formation of the cerebral hemispheres, the jaw apparatus (with the transformation of the anterior gill arches), the appearance of the amnion in the ancestors of higher tetrapods and warm-bloodedness in the ancestors of mammals andbirds (independently in both groups).
Plants also show many examples of morphophysiological progress: tissue formation, leaf and root development, dried pollen in gymnosperms, and flower in angiosperms.
Components of the evolutionary process
In addition to aromorphosis, A. N. Severtsov singled out such types of changes as idioadaptation (allomorphosis) and morphophysiological regression (catagenesis, general degeneration).
Idioadaptations are local adaptations to specific conditions. Idioadaptations include, for example, the appearance of protective coloration or specialization of limbs in animals, modification of shoots in plants.
If due to aromorphoses the largest taxa (kingdom, phylum, class) were formed, then idioadaptations are responsible for the formation of taxa of a lower rank - orders, families and below. Idioadaptations are expressed in changes in the shape of the body, in reduction or in the increased development of individual organs, while aromorphoses manifest themselves in the formation of qualitatively new structures.
To draw a clear line between idioadaptation and aromorphosis can be difficult. After all, it is possible to assess the scale and quality of change only after the fact, when it is already known what role it played in further evolution.
As for regression, it is a simplification of the general organization of living systems. This process can lead to the loss of some features that are useless for certain groups.organisms under new conditions. They will be culled by selection. So, in the tunicates, the chord was reduced; in parasitic and semi-parasitic plants (mistletoe) the root system is reduced.
Factors of evolution and biological progress
All these phenomena - morphophysiological regression and progress, idioadaptation - influence the evolutionary fate of living systems.
Thus, structural and functional degeneration is associated, as a rule, with the transition to a less active lifestyle (parasitic, sedentary). A group of organisms finds itself in conditions where selection will encourage mutations that lead to the loss of traits that are redundant and harmful in these new conditions. With the right combination of circumstances, regressive changes can lead the group to success, that is, to ensure biological progress.
Idio adaptations also contribute to success, because, although they are fundamental, they enable the group to succeed in specific conditions.
As for aromorphoses, they play a leading role in achieving biological progress, as they are large-scale adaptive acquisitions and allow the wide development of new habitats. As a result of aromorphic changes in the group, there is a massive and fairly rapid increase in diversity, active speciation with specialization in the local conditions of the new environment - adaptive radiation. This explains why morphophysiological progress ensures the biological flourishing of species.
Factors limiting arogenesis
Specific adaptations of many groups of organisms (especially higher ones), as their organization becomes more complex, can impose restrictions on further arogenesis, channeling it in a certain direction and changing the nature of the process itself. This is already manifested at the genetic level: the complication of the genome is largely associated with an increase in the number of regulatory mechanisms that chemically affect mutagenesis.
The ways of evolution of higher organisms are different from those of primitive living systems. For example, bacteria evolve mainly biochemically, and in the course of developing adaptations, selection culls a huge number of individuals. In eukaryotes, adaptive changes are already largely associated with morphological transformations. As for higher animals, due to the high degree of cephalization, adaptive changes in behavior become characteristic of them. To some extent, this reduces the need for morphological changes when living conditions change. This trend was most clearly manifested in the process of anthropogenesis.
Reasons for the progressive nature of evolution
We can clearly see the trend towards more complex organization in certain groups - most clearly in vertebrates or vascular plants. If we keep in mind the relationship of all life on Earth, then the origins of the line of morphophysiological progress can be found at the earliest stages of the formation of life. It is logical to assume that this tendency is inherent in the properties of living matter.
From the point of view of the thermodynamic approach, life can be defined as an autocatalytic process of self-organizationchemical systems with the extraction and conversion of energy from the environment. The theory of self-organizing systems tells us that as soon as the complexity of such primary self-organization reaches a certain level, the system automatically maintains the complexity and is able to increase it.
The increase in complexity could become not only possible, but also necessary for early life, when even primitive organisms, on the one hand, competed for external resources, and on the other, entered into symbiotic relationships, which increased the energy efficiency of consuming these resources. Then, obviously, the aforementioned tendency to complication was incorporated into the biochemical, including hereditary, properties of living systems.
An indirect confirmation of this point of view can be the presence of parallelisms in the evolutionary lines of different groups of organisms. No wonder they say, for example, not about the “appearance of mammals”, but about the “mammalization of theriodonts”, thereby emphasizing that several related groups participated in the process.
It is known that key aromorphoses can not always be compared with significant changes in environmental conditions. Therefore, to some extent, the processes of arogenesis depend on the properties inherent in the organisms themselves.
After reaching a certain level of complexity, related groups of plants or animals are able to undergo similar aromorphoses almost simultaneously, after which, as a rule, the group that has accumulated the most successful combination of changes abruptly “breaks ahead”,demonstrating yet another example of a progressive morphophysiological leap.
