Phylogenetic Transformations of Organ Functions

Principles of the emergence of new organs

The appearance of fundamentally new structures in the process of phylogenesis relies on the principle of differentiation. However, this mechanism does not work "from scratch." The basic condition for the formation of a new organ is the multifunctionality of existing anatomical structures. If an organ is initially capable of performing several tasks, during the course of evolution it can develop in new directions, ultimately leading to global changes in its functioning.

Change and expansion of functions

These two mechanisms demonstrate how the hierarchy of tasks within a single structure changes:

1. Change of function is characterized by the loss of the biological significance of an organ's primary task. At the same time, a function that was previously only secondary comes to the forefront. This process is always accompanied by morphological restructuring.

Examples:

• The transition of vertebrate ancestors to a jaw apparatus led to the transformation of placoid scales into fully developed teeth.
• The swim bladder, characteristic of lobe-finned fish, evolved into lungs — an organ for breathing atmospheric air.
• The emergence of mammary glands in mammals. They are homologous to ordinary sweat glands. The ancestors of mammals (as well as modern egg-laying monotremes, such as the platypus and echidna) featured focal accumulations of sweat glands whose function eventually changed dramatically.

1. Expansion of functions implies that an organ retains its original tasks while acquiring additional ones, thereby increasing its functional repertoire.

Example: The human upper limb (arm). Initially, it served exclusively for locomotion. In the course of evolution, the ability to grasp objects was added to this, and ultimately the limb became a complex organ of labor.

Enhancement and activation

Transformations can relate to the degree of intensity of an organ's work and its transition from a passive state to an active one.

• Enhancement (intensification) of functions. This occurs due to a significant increase in the number of primary functional units. Visually, this is often manifested by an increase in the size of the organ and a profound restructuring of its tissues at the histological level. Classic examples: the evolutionary development of the forebrain in vertebrates and the complexification of mammary glands by increasing the number of their lobes.

• Activation of functions. In this case, organs that were previously passive structures become active elements. A striking example is the formation of antagonistic limb muscles in terrestrial vertebrates and humans (for example, thigh muscles and the hamstring). This made it possible to create a fully functional active apparatus for flexion and extension.

Substitution

Substitution represents a radical evolutionary pathway in which one organ completely yields its place to another. The new structure takes over the functions of the predecessor.

What happens to the old organ? It either disappears without a trace in the process of phylogenesis or is reduced to a vestigial state.

Classic examples of substitution:

• Support function: the primary notochord is replaced by the more complex vertebral column.
• Excretory system: in vertebrates, the pronephros eventually gives way to the mesonephros.

Clinical significance: impaired embryonic development

The study of phylogenetic mechanisms helps to understand the nature of severe developmental pathologies of the brain, in particular, disorders of neuronal migration.

The rarest anomaly of this kind is periventricular (nodular) heterotopia (PNH).

The essence of the pathology is that during the formation of the nervous system, islands of gray matter cells fail to reach the cortex. They halt within the bulk of white fibers, localizing directly beneath the ependyma (near the ventricles). Despite such a serious anomaly, the intelligence of such patients may remain at a high level, but highly specific difficulties often manifest, such as problems with learning to read.

Class and exam questions

What else to study on this topic

• Morphological relationship between placoid scales and teeth in gnathostomes
• Histogenesis and phylogenesis of mammary and sweat glands
• Evolution of the excretory system: change of kidney types in vertebrates
• Mechanisms of neuronal migration and the pathogenesis of gray matter heterotopia

Other topics in the section "General patterns of organismal phylogeny"

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