Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 11. Modes of Transformation of Organs and Functions

11.2. Modes of Organogenesis

Among the numerous ways in which Organs and Functions transform (modes of Organogenesis), The most significant are the following.

Enhancement of the main function is considered one of the primary mechanisms of organ transformation. In many Multicellular Organisms, progressive evolutionary shifts occur precisely along this path, with the enhancement of a function in descendants being accompanied by specific structural reorganizations of the organ. This can be achieved both by altering The Structure of the organ and by increasing the number of homologous components. For example, the intensification of Heart activity in vertebrates occurs primarily through the restructuring of the organ itself, which ultimately enabled the complete Separation of the SYSTEMIC AND PULMONARY circulatory systems in mammals and birds. Birds are characterized by a significant increase in the number of Muscle fibers in the myocardium due to its extraordinarily intensive workload (Fig. 11.2). Similarly, The Development of the pectoral Muscles in birds is accompanied not only by an increase in their biomass, but also by the restructuring of the ribcage, resulting in The formation of a keel that improves the attachment and functioning of these muscles. Thus, structural reorganization and an increase in the number of components very often occur simultaneously in evolution. The intensification of functions can affect either the organ as a whole or its individual parts, leading to Changes in the proportions of body parts in descendants. An example is the enhancement of the running function in ungulates through limb restructuring, accompanied by the intensive development of certain digits and the reduction of others.

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Fig. 11.2. Diagram of The Heart structure and aortic arches in various vertebrate classes (after E. Hadorn, R. Wehner, 1989):

1 - fish; 2 - amphibian larvae; 3 - tailed amphibians after metamorphosis; 4 - reptiles; 5 - birds; 6 - mammals (roman numerals indicate aortic arches)

The weakening of the main function is also recognized as a fairly widespread phenomenon. This is because any organ whose primary function diminishes undergoes reduction (Fig. 11.3), a process common among living organisms. At the same time, it should be understood that this process is much more complex and rarely occurs in isolation. For instance, the weakening of Vision in subterranean dwellers leads to eye reduction. However, in most cases, the degeneration of one organ is linked to the intensive development of another that functionally replaces it (a phenomenon known as substitution, or replacement).

Fig. 11.3. Limb reduction in serpentiform lizards (after A.M. Severtsov, 1945):

1 - Skeleton of the forelimb; 2 - skeleton of the hindlimb of Seps tridactylus; 3 - skeleton of the hindlimb of Ophiodes striatus (roman numerals indicate digits)

According to A.M. Severtsov, the basis for such transformations lies in profound changes in the environmental conditions of certain organisms and, consequently, the DEVELOPMENT OF NEW adaptations. Only thereafter does a substantial reduction of the redundant organ occur, whose function is then taken over by another, more progressive one. Thus, the weakening of The primary function of the Hair coat in cetaceans should be viewed strictly in parallel with the active development of a thick layer of subcutaneous fat, which is responsible for thermoregulation in these animals.

Polymerization of organs involves an increase in the number of homogeneous organs or structures. This is a fairly widespread evolutionary pathway that primarily serves to enhance the main function (or other functions as well). Examples of such transformations can be observed in many invertebrates (Fig. 11.4) and in plants exhibiting repeated homogeneous structures. These may include petals, stamens, or other floral components, animal body segments, and so forth. Among vertebrates, snakes, for instance, have an increased number of vertebrae, whereas certain arboreal mammals that cling to branches exhibit an increased number of caudal vertebrae.

Fig. 11.4. An example of organ polymerization (after C. Nitecki et al., 1991).

In stone centipedes (1, 2), the number of segment-bearing limbs is 15, whereas in Geophilus (3) it ranges from 49 to 57.

Oligomerization of organs and concentration of functions refer to a reduction in the number of homogeneous organs or structures associated with functional intensification. This process is most clearly visible in the evolution of The Nervous system in invertebrates (Fig. 11.5). The ventral nerve cord of Annelids undergoes significant modification in insects, primarily through the reduction of individual ganglia, and in some cases (flies and other highly advanced forms) through the fusion of ganglia to form large neural agglomerations. The Evolution of the avian skeleton proceeds via the fusion of individual elements, with homodynamous vertebrae coalescing in the lumbar, pelvic, and caudal regions. Another example of the differentiation, specialization, and loss of a portion of homologous and homodynamous organs is the reduction in the number of digits in ungulates.

Fig. 11.5. The process of oligomerization illustrated by changes in the STRUCTURE OF THE nervous system in crustaceans (after V.F. Natali, 1963):

1 - Branchiopoda; 2 - Leptostraca; 3 - crabs; 4 - Copepoda

A decrease in the number of functions occurs during the specialization of specific organs, structures, or the Organism as a whole. For instance, the adaptation of vertebrates to flight through the transformation of limbs into wings (pterosaurs, birds, bats) leads to the loss of most of the functions previously performed by those limbs. Often, a reduction in the number of functions is linked to the enhancement of the primary function. Thus, the transformation of limbs in ungulates diminishes their secondary functions, while the shortening of the neck in cetaceans reduces the secondary Functions of the oral apparatus (Fig. 11.6).

Fig. 11.6. Body outline and skeleton of the bowhead whale (Balaena mysticetus) (after A.M. Severtsov, 1945)

An increase in the number of functions can occur even without significant Anatomical and morphological changes in the corresponding organ. A clear example is the human hand, which, while remaining morphologically close to the generalized limb structure, develops an extraordinary range of functions. The multiplication of functions can also happen while preserving the primary function of the organ—for instance, through the formation of pappi or tufts by the protective coats of certain seeds (Fig. 11.7), or in the gills of lamellibranch Mollusks, which, alongside Respiration, perform transport functions (delivering food to the Mouth), and in some forms are even used as a brood pouch.

Fig. 11.7. Examples of anemophily (after S.S. Morozyuk et al., 1991):

1 - dandelion; 2 - poplar; 3 - willow; 4 - cotton; 5 - elm; 6 - maple; 7 - linden; 8 - ash

The distribution of functions and organs occurs when homologous homodynamic organs specialize to perform specific tasks out of their entire former functional spectrum, which is accompanied by corresponding structural rearrangements of the organs themselves. A striking example is mammalian dentition (Fig. 11.8). Unlike ancestral forms, mammals retain only short marginal rows of Teeth on the jaws, which develop into four distinct types in adults. The anterior teeth—incisors—have a simple conical or chisel-like shape (adapted for biting). Behind them are the canines (one on each side)—large, pointed, and deeply rooted (used predominantly by predators to attack prey; the most prominent example being saber-toothed cats). The premolars (often bicuspid), featuring a grinding surface on the crown, and the molars take over the masticatory function, thereby acquiring a complex crown structure.

Fig. 11.8. Specialization of teeth in mammals (after B.O. Kuznetsov, 1975):

1 - incisors; 2 - canines; 3 - premolars; 4 — molars

The shift of function is regarded as one of the primary mechanisms of organ evolution. Such evolutionary transformation is achieved by diminishing the primary function while enhancing a secondary one (Fig. 11.9). It is grounded in THE PRINCIPLE OF functional expansion of a particular organ, and this shift drives significant subsequent structural modifications of the latter. For instance, at the onset of vertebrate evolution, the primary function of The Stomach was to store food until it could be received by the intestine, along with partial physical Processing (as seen in elasmobranch Fishes). Later on, the function of chemical Digestion using digestive Enzymes was added, which became paramount in higher vertebrates. Conversely, in birds, the lower part of the stomach transformed into a muscular organ with a hard internal lining, whose main function is the physical processing of food (illustrating yet another shift in function). Plants provide a multitude of examples as well (the formation of petals and sepals from leaves, tubers, ROOT crops, etc.).

Fig. 11.9. Transformation of the primary insect ovipositor (1) into a bee sting (2) (after V.I. Lebedev, N.G. Bilash, 1991)

Other pathways of organ and functional transformation are also known, such as the fixation of developmental phases, which involves the stabilization of a specific stage in performing a certain function to amplify it (the evolution of digitigrade locomotion in animals serves as an example). However, even the transformation modes mentioned above are largely interdependent and interconnected. Therefore, there is no need to single out an endless number of principles; instead, an individualized approach should be taken when studying the phylogeny of specific biological forms.