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 the organization during evolution. Despite the fact that there are also phenomena of the reverse order - simplifications - or stabilization of systems at the same level of complexity, the orientation of the evolutionary process of some large groups of organisms demonstrates 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 evolutionary animal morphology.
The 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 can manifest in many forms, such as:
- increasing the degree of adaptation of the group to environmental conditions;
- population growth;
- active speciation within the group;
- expansion of the area occupied by the group;
- an increase in the number of subordinate groups (for example, the number of units in the class of mammals).
Accordingly, a decrease in indicators according to these parameters characterizes failure - biological regression of a group of organisms.
Morphophysiological progress is a narrower concept. This term denotes the improvement of the organization, expressed in the complication of the structure and functions of the body. The distinction between concepts related to progress has brought us closer to understanding how and why morphophysiological progress provides biological prosperity.
The concept of aromorphosis
The term was also proposed by A. N. Severtsov. Aromorphosis is called progressive change, which leads to a complication of the organization of living systems. Progressive evolution is a series of similar changes. Aromorphoses, therefore, can be considered separate stages of morphophysiological progress (arogenesis).
Aromorphosis is a large adaptive acquisition that increases vitality and leads a group of animals or plants to new opportunities, for example, to 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 organism.
The complication of the structure (morphology) only in conjunction 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.
The main features of the process of arogenesis
Morphophysiological progress is characterized by changes in the complex of characters that determine the degree of complexity of living systems.
- The level of homeostasis rises - the ability to maintain stability of the internal environment of the body (for example, a constant body temperature in warm-blooded, salt composition, and so on). Also, the ability to maintain development stability in changing external conditions - homeoresis, is increasing. This indicates the improvement of regulatory systems.
- The level of the body’s energy exchange with the environment is growing. For example, warm-blooded animals are characterized by an accelerated metabolism.
- The amount of information is increasing, and methods for processing it are becoming more complicated. So, with the complexity 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 the living system to increase independence from the external environment.
Genetic basis of evolutionary transformations
The material that undergoes transformation during evolution is the gene pool of a population of organisms. Its main properties are the genetic diversity of individuals and hereditary variation. Their main driving factors are the recombination of genetic material during transmission to offspring and mutations. The latter can be repeated and accumulate.
Natural selection fixes useful mutations in the gene pool and rejects harmful ones. Neutral mutations accumulate in the gene pool, and when conditions change, they can become both harmful and useful and also be subjected to selection.
In contact, populations exchange genes, which preserves the genetic unity of the species. It is violated in the case of different variants of isolation of populations - all of them contribute to the process of speciation.
One of the most important results of the selection process is adaptive acquisitions. Some of them turn out to be very large under certain conditions, significant - these are aromorphoses.
Examples of aromorphic changes
In unicellular organisms, such large evolutionary events as the formation of cells with mitochondria (the latter in the early stages of life development were independent organisms), the occurrence of sexual reproduction, and the appearance of eukaryotic cells can serve as examples of aromorphosis .
The largest aromorphosis in the animal kingdom was the emergence of true multicellularity (multifacetedness). In chordates and vertebrates, examples of such large structural and functional rearrangements of organisms are: the formation of hemispheres of the brain, jaw apparatus (with the transformation of the anterior gill arches), the appearance of an amnion in the ancestors of higher tetrapods and warm-blooded ancestors of mammals and birds (independently in both groups).
Plants also show many examples of morphophysiological progress: tissue formation, the appearance of the leaf and root system, dry pollen in gymnosperms and flower in angiosperms.
The components of the evolutionary process
In addition to aromorphoses, A. N. Severtsov singled out such types of changes as idioadaptations (allomorphoses) and morphophysiological regression (catagenesis, general degeneration).
Idiadaptations are local adaptations to specific conditions. Idioadaptations include, for example, the appearance of a protective coloration or the specialization of limbs in animals, modification of shoots in plants.
If, thanks to aromorphoses, the largest taxa have formed (kingdom, type, class), then idioadaptations are responsible for the formation of taxa of a lower rank - orders, families, and lower. Idiadaptations are expressed in changes in body shape, in reduction, or in enhanced 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 only possible to assess the scale and quality of change after the fact, when it is already known what role it played in further evolution.
As regards regression, this is a simplification of the general organization of living systems. This process can lead to the loss of some signs that are useless for certain groups of organisms in the new conditions. They will be rejected by selection. Thus, a chord was reduced in tunicans; in parasitic and semi-parasitic plants (mistletoe), the root system is reduced.
Evolution Factors and Biological Progress
All these phenomena - morphophysiological regression and progress, idioadaptations - influence the evolutionary fates of living systems.
So, structural and functional degeneration is associated, as a rule, with a transition to a less active lifestyle (parasitic, sedentary). A group of organisms falls into conditions when selection will encourage mutations that lead to the loss of traits that are unnecessary and harmful in these new conditions. With a successful combination of circumstances, regressive changes can lead the group to success, that is, ensure biological progress.
Isoadaptations also contribute to success, because, although they are of a fundamental nature, they give the group the opportunity 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 provides species biological prosperity.
Arogenesis Limiting Factors
The specific adaptations of many groups of organisms (especially higher ones), as their organization becomes more complex, may impose restrictions on further arogenesis, channeling it in a certain direction and changing the nature of the process itself. This is already evident at the genetic level: the complexity of the genome is largely due to an increase in the number of regulatory mechanisms that chemically affect mutagenesis.
The methods of evolution of higher organisms are different from those of primitive living systems. For example, bacteria evolve mainly biochemically, and during the development of adaptation, selection discards 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 changing living conditions. This tendency was most clearly manifested in the process of anthropogenesis.
Reasons for the progressive nature of evolution
We can clearly see a tendency toward a complication of organization in individual groups — most clearly in vertebrates or in vascular plants. If we remember the kinship of all life on Earth, then the origins of the line of morphophysiological progress can be found at the very early 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-organization of chemical systems with the extraction and conversion of energy from the external 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 complexity and is able to increase it.
An increase in complexity could become not only possible, but also necessary for early life, when still primitive organisms, on the one hand, competed for external resources, and on the other, entered into symbiotic relations, which increased the energy efficiency of consumption of these resources. Then, obviously, the aforementioned tendency toward complication was laid down in the biochemical, including hereditary, properties of living systems.
The indirect confirmation of this point of view may be the presence of parallelisms in the evolutionary lines of different groups of organisms. It is not for nothing that 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 are far from always comparable with significant changes in ecological conditions. Therefore, to some extent, the processes of arogenesis depend on the properties inherent in the organisms themselves.
Upon 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 is “ahead” sharply, demonstrating yet another example of a progressive morphophysiological leap.