A.N. Severtsov's theory of evolution
The foundations for understanding the main pathways and directions of organic evolution were laid in the theory of A.N. Severtsov (1925). He proposed a clear distinction between progressive and regressive development of living organisms, highlighting two completely different aspects: morphophysiological and biological.
Types of evolutionary progress according to Severtsov:
- Morphophysiological progress is the structural complexification of an organism. Changes in structure and life processes proceed from simple to complex forms, which implies an increase in the general level of organization.
- Biological progress is an increase in fitness. It reflects the ultimate success of a species in the struggle for existence, regardless of whether its structure became more complex or simplified.
Morphophysiological regression is also distinguished — this is a simplification of organization, which at the physical level is expressed in the reduction (disappearance or underdevelopment) of certain organs .
Criteria and examples of biological progress
You can tell that a group of organisms is in a state of biological progress (i.e., thriving) by three main criteria:
- Continuous growth in the number of individuals within populations.
- Expansion of the habitat range (conquest of new territories).
- Increase in the rate of intraspecific differentiation (active formation of new intraspecific groups).
Modern thriving groups: In the animal kingdom, striking examples of biological progress include insects, teleost fish, birds, and mammals. In the plant kingdom, angiosperms are considered the absolute leaders in evolutionary success.
Human evolution is also a clear example of biological progress. Human numbers have grown rapidly throughout history: from about 5 million in the Neolithic era to 20–40 million by the time the first cities emerged. During the Roman Empire (early AD), the population reached 200 million. By 1600, 500 million people lived on Earth, by 1800 — 1 billion, and currently the population exceeds 6.5 billion. Intraspecific differentiation in humans is manifested in their ethnic structure: today there are about 3,000 peoples existing in the forms of nations, nationalities, tribal groups, and individual tribes.
Pathways of Biological Progress
The evolutionary success and wide distribution of a group of organisms can be achieved through completely different pathways of evolutionary changes. Three main pathways are distinguished:
- Arogenesis is progressive evolution carried out through aromorphoses. Aromorphoses are major morphophysiological changes that raise the general level of organization of living beings.
- Allogenesis is evolution proceeding via idioadaptations. This is the acquisition of specific adaptations to particular environmental conditions. At the same time, the general level of organization does not rise; species simply adapt to their narrow niches.
- Catagenesis is evolution by general degeneration (morphophysiological regression). This is a significant simplification of organization. Organisms often choose this path when transitioning to a sedentary or parasitic lifestyle. Paradoxically, the simplification of structure allows the group to ideally adapt to new specific conditions and widely spread.
Examples of catagenesis (general degeneration):
- Complete loss of the digestive system in tapeworms.
- Loss of wings in certain parasitic insects (lice and fleas).
- Reduction (degeneration) of the tail in the course of human evolution.
Briefly: answers to topical questions
What specific aromorphoses led to the emergence of the class Mammalia?
The emergence of the class Mammalia (Mammalia) is associated with aromorphoses — major universal adaptations that raise the general level of organization and cause the formation of large taxonomic groups.
Aromorphoses in mammalian evolution include:
- hair coat;
- viviparity;
- milk feeding.
Mammals are also characterized by warm-bloodedness.
What circulatory system aromorphoses are characteristic of the progressive evolution of birds and mammals?
The progressive evolution of the circulatory system in birds and mammals is characterized by:
| Sign | Birds | Mammals |
|---|---|---|
| Heart structure | Four-chambered heart | Four-chambered heart |
| Blood separation / circulatory loops | Complete separation of arterial and venous blood | Complete separation of loops |
| Aortic arch | Only the right aortic arch is preserved | Only the left aortic arch is preserved |
| Erythrocytes | No data in sources | Anucleated |
For birds, progress is a complete septum in the ventricle and a four-chambered heart. For mammals, the similarity with birds consists in a four-chambered heart and complete separation of circulatory loops.
To memorize the pathways of evolution, use the association with movement directions: Arogenesis — a step up (complexification), Allogenesis — a step to the side (adaptation at the same level), Catagenesis — a step down (simplification, degeneration). The outcome of any of these steps is evolutionary success (biological progress).
Class and exam questions
What is the difference between biological and morphophysiological progress?
Morphophysiological progress is the complexification of an organism's structure and the elevation of its level of organization. Biological progress is the global evolutionary success of a species (increase in population size, range expansion), which can be achieved through either structural complexification or simplification.
Can degeneration lead to the prosperity of a species?
Yes, general degeneration (catagenesis) is one of the pathways to achieve biological progress. The loss of unnecessary organs, for example, upon transition to parasitism, allows a species to save energy, better adapt to the environment, and increase its population.
By what criteria can it be determined that a species is in a state of biological progress?
The criteria are: a constant increase in the number of individuals, colonization of new habitats (range expansion), and active formation of new intraspecific forms.
What else to study on this topic
- Mechanisms of aromorphosis formation
- Examples of idioadaptations in plants and animals
- Intraspecific differentiation and speciation
- A.N. Severtsov's evolutionary theory in detail
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