Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 11. Means of Transformation of Organs and Functions

11.3. Substitution of Organs and Functions

THE PRINCIPLE OF organ substitution, proposed by N. Kleinberg in 1886, consists in replacing an ancestral organ in descendants with a new one while maintaining its Location and function, alongside the reduction of the previous organ. An example of such evolutionary transformations is the replacement of the notochord with cartilaginous or bony vertebrae.

D.M. Fedotov in 1925 established the principle of physiological substitution, whereby the function of a certain ancestral organ is replaced by a similar function in descendants, but the organ performing it has a different Structure and location. The ancestral Organs and their Functions are reduced, while the new function is merely similar to the previous one. An example of this substitution is The Development of Lungs in terrestrial vertebrates, which functionally replaced the gills of fish.

A.M. Severtsov distinguished the principle of functional substitution, which involves not only A change in the Location and Structure of an organ, but to some extent in the function itself. Thus, the reduction of limbs in snakes is associated with the fact that the functions of walking and running are replaced by an analogous function — crawling. There are cases where the function in descendants is not similar to the superseded ancestral function, but is comparable. An example of such changes is found in subterranean species, where the function of Vision is replaced by the functions of smell and Touch, while the eyes undergo significant reduction. An even more complex ADAPTATION TO ENVIRONMENTAL conditions should be considered the variant of substitution in which the descendant's function has nothing in common with that of the ancestral forms, yet is biologically equivalent to it. The most striking example is the turtle's shell, which allowed this animal to transition from active to passive defense, which is biologically equivalent for the species' development.

According to A.M. Severtsov, the principle of functional substitution underlies the reduction and degeneration of organs.

The cause of reduction is always some substantial change in the living conditions of ancestral forms, necessitating adaptive changes in certain organs. If descendants develop a new organ with a new function better suited to the novel conditions, a substitution process takes place. Therefore, the DEVELOPMENT OF NEW adaptations is primary, rather than reduction (the post-adaptation pathway of forming adaptations). As a result of such transformations, the function of certain organs or their parts, and even their very existence, becomes pointless, which leads to their subsequent degeneration.