Directions of evolution according to Severtsov

Aromorphosis: reaching a new level

Aromorphosis is a major adaptation of a universal nature. It is this pathway that allows organisms to radically change their lifestyle and successfully colonize entirely new habitats. The main consequence of aromorphosis is a significant increase in the overall level of organization of an animal or plant.

Due to such global restructuring during evolution, the largest taxonomic groups arise — phyla and classes.

Examples of major aromorphoses:

• General biological: the appearance of a defined nucleus in eukaryotes, the emergence of multicellularity, the capacity for photosynthesis, and the transition to sexual reproduction.
• In vertebrates: the formation of lungs for breathing atmospheric air, the appearance of terrestrial-type limbs, and the development of warm-bloodedness.
• In the mammalian lineage: the appearance of a hair coat, the transition to viviparity, and feeding the young with milk.

The formation of a new large taxon is a prolonged process, taking an average of 10–20 million years. Transitional (intermediate) forms at this stage are extremely rarely discovered by paleontologists. The mechanism of such changes is complex: initially, organisms may change the function of an organ while preserving its old structure. Subsequently, profound structural rearrangements occur (most often affecting the skeleton) that involve precisely those body parts directly responsible for colonizing the new environment. Interestingly, new systematic traits may finally form only after the adaptive transition has been completed.

Idioadaptation: fine-tuning to the environment

Idioadaptation is the appearance of minor, highly specific adaptations that help organisms survive in specific, narrow environmental conditions. Unlike aromorphosis, idioadaptation represents adaptation within a single adaptive zone and scale.

The most important feature of this direction is that it does not increase the overall level of organization of the organism. Idioadaptations serve as markers for distinguishing smaller taxonomic groups: species, genera, families, and orders.

Striking examples include differences in the structure of attachment organs in parasitic tapeworms (cestodes). Thus, the pork tapeworm anchors in the intestine using hooks and suckers, whereas the broad tapeworm uses specialized slits called bothria for the same purpose. Another example is the enormous diversity in the structural details of respiratory organs among various representatives within the classes of amphibians and reptiles, reflecting their adaptation to specific microhabitats.

General degeneration and coenogenesis

In addition to increasing or maintaining the level of organization, evolution can proceed along the path of secondary simplification. This direction is called general degeneration.

It is accompanied by the loss of entire organ systems or complex structures. Most commonly, degeneration is observed in species that have transitioned to a sessile (immobile) or parasitic lifestyle. A classic example is the complete loss of sensory organs and the active locomotion apparatus in many endoparasites. It is important to understand that despite the apparent structural simplification, general degeneration invariably increases the level of biological fitness of the species, meaning it is a form of true biological progress.

A special direction of evolution is coenogenesis (embryoadaptation). These are specialized adaptations that arise exclusively in embryos or larvae to survive in their specific environment, but completely disappear in adult organisms.

Examples of coenogenesis:

• The presence of a sharp stylet in nematode larvae (plant parasites), necessary for piercing plant tissues and migrating within the host.
• The formation of the amnion in higher vertebrates, which enabled them to reproduce on land, becoming independent of the aquatic environment.
• The development of provisional (temporary) organs in the human embryo: the amnion, chorion, allantois, and placenta.

A.N. Severtsov's law

All described directions of evolution do not exist in isolation; they consistently succeed one another. A.N. Severtsov formulated the rule of alternation of phases of the evolutionary process, which was named A.N. Severtsov's law.

According to this law, evolution proceeds according to the following scenario:

1. First, aromorphosis occurs — a sharp qualitative leap (e.g., the appearance of the notochord) that elevates the overall organization of the group to a fundamentally new, higher level.
2. Following the leap, a long period of idioadaptations begins — the group actively colonizes available ecological niches at the achieved level, acquiring particular adaptations.
3. In some cases, the evolutionary branch may enter the phase of general degeneration if the species chooses the path of parasitism or a sedentary lifestyle, which requires structural simplification.

Graphically, this process is often depicted as a stepped curve: aromorphosis symbolizes a steep upward climb, idioadaptation represents broad development on a horizontal plateau (at the same level of organization), and degeneration looks like a descent that, nonetheless, leads to the prosperity of the species.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of formation of new morphological traits after the completion of the adaptive transition.
• Structural features of cestode attachment organs (bothria and hooks) as a classic example of idioadaptation.
• Human provisional organs (allantois, chorion, placenta) in the context of the doctrine of coenogenesis.
• Reasons for the extreme rarity of finding intermediate forms during the formation of major taxa.

Other topics in the section "Pathways to Biological Progress"

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