The evolutionary process is characterized by irreversibility. The position on the irreversibility of evolution was first formulated in 1893 by the Belgian paleontologist L. Dollo (1857-1931). The essence of this provision, later calleddollo law, is thatthat organisms, moving into their former habitat, do not completely return to the previous state of morphophysiological organization. Thus, the gills and fins of fish, lost by their tetrapod descendants, were never restored in reptiles and mammals that re-assimilated the aquatic lifestyle (the caudal fin and flippers of ichthyosaurs and cetaceans only superficially resemble the fins of fish, with a profound difference in their internal structure). The irreversibility of evolution is a statistical pattern arising from the improbability of a complete return to the previous (ancestral) state of a set of processes realized in the genotype and phenotype of a particular group of organisms.

The evolution of structures is based on the processes of differentiation and integration. Principle of differentiation established in 1851 by the French zoologist A. Milne-Edwars (1800-1863),principle of integration first described by G. Spencer (1820-1903).Morphophysiological differentiation is the development in the process of evolution from one structure of several different-quality structures that perform different, narrower (private) functions. As an example of differentiation, one can cite the division of the originally simply arranged digestive tube (performing a single general function of splitting and absorbing cleavage products) into sections (mouth, pharynx, esophagus, stomach, small and large intestines), in which food is already subjected to specific influences (mechanical processing , chemical breakdown, absorption, etc.).

The process of integration is inextricably linked with differentiation. Integration is the expedient association and coordination of the actions of different parts of an integral living system. If differentiation leads to an increase in the degree of subordination of parts (organs) to the body as an integral system, then integration is manifested in the unification of organs into functionally unified systems that provide one of the aspects of the life of the organism.

The basis of the morphofunctional transformations of individual organs ismultifunctionality (polyfunctionality) - the performance by this body of several functions at the same time, among which one can usually single out the main (main) function and a number of secondary ones. For example, the main function of the swim bladder of ray-finned fish is hydrostatic (regulation of buoyancy by changing volume). Along with this, it is also used as a baroreceptor, signaling the depth of immersion, as an apparatus for transforming sound vibrations, which increases the sensitivity of the hearing organ, and in primitive ray-finned (lobe-finned) it performs the function of a respiratory organ.

Most often, the evolution of organs proceeds in a waychange of functions, in which one of the secondary functions of the organ under the influence of the changed relations of the organism with the external environment becomes more important (main) than the former main function. So, in the ancestors of vertebrates, skin scales in the area of ​​the closing edges of the jaws were transformed into teeth that perform another main function: instead of mechanical protection, the function of capturing, holding and crushing food. In plants, for example, the petals of the corolla of a flower evolved from leaves that changed the function of photosynthesis to the function of attracting insects for pollination. The change of functions was first described by the German zoologist Anton Dorn (1840-1909). The ability to change functions is based on the multifunctionality of organs. When a function is changed, the direction of evolutionary transformations changes accordingly, sincenatural selection improves the structure of an organ, primarily in relation to its main function.

The transformation of the functions of organs in phylogenesis can also occur in a way of expanding functions. The expansion of functions is

Rice. 160. Evolution of the Vertebrate Heart:

1 - two-chambered heart of fish; 2 - three-chambered heart of amphibians; 3 - a three-chambered heart of reptiles with an incomplete septum in the ventricle; 4 - four-chambered heart of mammals; P - atrium; G - stomach

in the acquisition by an organ (structure) in the course of evolution of new functions while maintaining existing ones. The expansion of functions was first described in 1912 by the German zoologist L. Plate (1862-1937). For example, in warm-blooded animals, the circulatory system is involved in the regulation of heat exchange with the environment; in mammals, it also acquires the function of providing immunity. L. Plate described another way of evolutionary transformations of organs- intensification, or strengthening of functions. Its essence liesin the increase in the course of evolution of the number of functional units and the corresponding complication of the structure of the organ. As examples, we can consider the complication of the structure of the heart (two-chamber, three-chamber, four-chamber) and the intensification of its functions (Fig. 160), the complication of the structure of the brain and the intensification of the functions of the central nervous system.

Two methods of transformation during the evolution of organs (substitution of functions, separation of functions) were described in 1931 by A.N. Severtsov (1866-1936). Substitution of functions (heterotopic substitution) is the loss in the course of evolution of one of the functions and its replacement by another, biologically equivalent, but already performed by another organ located in a different place. For example, the function of moving through the limbs is replaced in snakes by moving with the help of bending of the spine (crawling), breathing with the help of gills (extracting O 2 from water) in terrestrial vertebrates is replaced by gas exchange in the lungs. The division of functions is accompanied by the division of the body into independent departments. So, a single unpaired fin was divided in fish (Fig. 161) into dorsal and anal (rudders), as well as caudal (propulsion).

Rice. 161. Scheme of division of a single unpaired fin in fish

The second type of substitution- substitution of organs (homotopic substitution), described by the German zoologist N. Kleinenberg (1842-1897). Its essence isin the replacement of one organ by another, carrying a similar function and occupying the same position (the notochord, for example, is replaced by a cartilaginous skeleton, and the latter by a bony spine).

To consider the transformation of the structure as memorized, it is sufficient to preserve its consequences during one elementary link in development (three successive states of the system). In a long process of development, these elementary links overlap each other: the states of the system that are part of one link can form, with the states of the links next to them, the previous and the next, four different types of combinations, such that each of them remains an elementary link in development.

Structure, by definition, has holistic properties. This means that any memorized new formation is mediated by all the former properties of the structure that were not forgotten during the selection operation, and all preserved properties of the structure turn out to be mediated by the new formation. In other words, the new formation in the process of memorization becomes an integral aspect (and not part) of this choice - the result of a synthesis of the old and the new. Preservation of the memorized during one elementary link of development affects the outcome of at least two acts of choice that are not included in this elementary link of development and following it. These considerations are invariant for any elementary link in development and for a sequence of overlapping (by one or two states of the system) links of arbitrary length. Hence the conclusion follows: the consequences of this act of remembering will not be equal to zero, and the given aspect of memory that has arisen as a result of this act will not be completely forgotten after any predetermined finite number of elementary links of development (changes of states). The sequence of such elementary steps of development is the own internal time of the developing object

The opposite is tantamount to a break in continuity between the past and the future. Complete oblivion of the consequences of only one act of memorization is tantamount to the destruction of the choice corresponding to it - the state of the structure in the past development of the system. Consequently, the existence of a developing structure at any given moment of time is determined not only by the succession of all the previous states taken together, but also by each of them separately, which is a link between the previous and subsequent states.

Thus, we have obtained a formal conclusion of L. Dollo's position on the indestructibility of the past. Other aspects of the irreversibility of evolution, arising from the rules of phylogenesis considered earlier, are less powerful and only additionally confirm the conclusion about the indestructibility of the past. However, let's list them. Destabilization, without which the emergence of the new is impossible, is associated with at least partial oblivion of the past. If oblivion affects unique information that is not stored after it in the system in any copy, then due to the principle of irreplaceability of completely lost information by A. A. Lyapunov, such a loss is fundamentally irreversible. Since the amount of novelty available for acquisition by the system is proportional to the amount forgotten by it, the more new the system has received, the more irreversible, other things being equal, the development process that led to this will turn out to be.

Any move in the direction of progressive specialization is safer, and therefore more likely, than any change in the direction of specialization. Reversing the direction of development is no exception, although the possibility of extensive use of past experience facilitates a change in the direction of specialization. Therefore, the phenomena of neoteny and fetalization are widespread in phylogenesis. However, the use of previous experience testifies not to the reversibility of development, but to its continuity. You can use only not forgotten experience. The rules of key aromorphosis, monophyly, parallelism, homological and heterological series by E. D. Cope make one think about the scale of the successive transmission of latent potencies and doubt the validity of the statement about the complete irreducibility of completely atrophied organs. It suffices to recall examples of atavisms, among which there are signs that are completely lost in the norm. Many striking examples of this kind (for example, in the bone skull of a person) are cited by A.P. Bystrov. As for the reversibility of evolution, it certainly does not exist, since in these cases there is no reversion even of the structure of the skull, let alone of the entire organization.

L. Dollo and his followers believed that the irreversibility of evolution is based on a very low probability of a strictly step-by-step reverse process - involution. The formal conclusion we have obtained testifies to the deterministic nature of L. Dollo's law. Indeed, if development is a stochastic process, then the development trajectory that has already been realized is the only irreplaceable reality of its kind, the embodiment of which is the current structure of the developing system. This makes it possible to give the status of a law to the irreversibility of development, highlighting it among other empirical rules of phylogenesis.

There are three most famous formulations of L. Dollo's law. The first belongs to C. Darwin: a species that once disappeared cannot appear again, even if completely identical conditions would repeat again - organic and inorganic. Two other formulations belong to L. Dollo: 1) the organism cannot return (at least partially) to the previous state, which was carried out in the series of its ancestors; 2) the organism never returns exactly to its previous state, even if it finds itself in conditions of existence identical to those through which it passed; but due to the indestructibility of the past, he always retains a trace of the intermediate stages that he passed through.

Almost all objections to the strict application of the principle of irreversibility concerned the formulations of L. Dollo, and not C. Darwin. D. N. Sobolev believed that the facts of despecialization contradict the irreversibility of evolution. Obviously here we are talking about the reversibility of the level of specialization to certain limits, and not development. It is no coincidence that in this connection Sobolev writes about the reversibility of ontogeny. In unicellular organisms, we are talking about despecialization, while in multicellular organisms, it is about continuity. There is no true reversibility of ontogeny. This is also indicated by the laws of K. Baer and the equifinality of ontogenesis. When D. N. Sobolev writes about the widespread reverse evolutionary development and the return of the group to the starting point of development, it is always about the phenomena of iterative evolution, neoteny or fetalization. Obviously, the very fact of such "reversibility" can be established morphologically only because of its incompleteness. True, D.N. Sobolev, being a paleontologist, at least in principle could judge the reversibility of identical direct and reversible phylogenetic series, using their gradualness and independently (more precisely, quite independently, especially when it comes to ammonites) this scale of geological time. However, he never relies on a purely stratigraphic succession, and all forms of the "reverse" series receive taxonomic names, and most importantly, diagnoses different from those of the direct series. In the case of sufficiently complete reversibility, the position of V.P. Makridin, who questioned the principle of irreversibility in the formulation of C. Darwin, would be more consistent. Moreover, assuming that reversibility can be of jump-identical direct and reverse phylogenetic series, true reversibility and reject the hypothesis of long-term persistence of the ancestral form along with progressively developing descendants. Moreover, neontologists, a number of whom made similar judgments, could not empirically detect complete reversibility.

Some of the considerations similar to those described were also expressed by P. P. Sushkin. It is no coincidence that he believed that evolution is reversible insofar as ontogeny is similar to phylogeny. Here again, the continuity observed during hypogenesis is meant, and not true reversibility. Being a good embryologist, P. P. Sushkin undoubtedly meant incomplete reversibility. but main feature position of P. P. Sushkin, as well as many other researchers, is the opposition of the strict irreversibility of the organization of the whole organism to the possible reversibility of individual, especially simple signs. Even for relatively simple combinations of simple features, P. P. Sushkin considered reversibility less likely. This position, where the reversibility of an individual, arbitrarily selected from the whole feature, the probability of reversibility of which as a member of the ensemble decreases, can appear, obviously has an epistemological character. The law of irreversibility in this case acquires a probabilistic character not as a result of the probabilistic nature of the irreversibility of evolution itself as an ontological phenomenon, but due to the probabilistic nature of any historical reconstructions and, in general, any empirical judgments based on limited material. Such an approach to the problem of reversibility is quite justified, since evolution, like any reality, is not available for a specific study in the fullness and diversity of its manifestations. We are fully aware that a fundamental solution to the problem of the irreversibility of development leaves many epistemological problems (reversibility at a certain level).

In the latter case, we are not talking about evolution itself and its irreversibility, but about the possibilities, methods and limits of its cognition empirically. It is the ontological side that has always been the main one in discussions around Dollo's law.

L. Dollo himself felt the incompleteness of the purely empirical justification of his law, he even tried to give him a theoretical deductive justification. Rightly believing it to be non-rigorous, P. P. Sushkin strove to remain on the basis of reliable empirical facts. This was the basis of his criticism of Dollo's law. But at the same time, P.P. Sushkin did not notice that he was no longer discussing evolution, but our ability to judge it on the basis of empirical data. Indeed, this ability turned out to be limited, which is in good agreement with the opinion of L. Dollo about the incompleteness of the empirical justification of the law.

If we turn to the ontological aspect of the meaning of the term "feature", namely in this sense we use it when we try to judge evolution itself (and not the ways of its cognition), we immediately find the opposition of the part to the whole in the approach of P. P. Sushkin. Obviously, speaking of reversibility, it is supposed to compare two states of a feature. The identification of features means the establishment of their homology. For this, homology criteria are used. In this case, we are interested in the criterion of the position of a feature as a part or aspect of the whole (meron) in its relationship (connection) to other features of the same whole. Without using this criterion, it is impossible to establish the place of the meron in the archetype, and therefore, to compare the corresponding features as merons. In fact, R. Owen wrote about this. Such an attitude towards a trait is in good agreement with the idea of ​​the organism as a whole, in which each part or aspect is mediated by the whole. Consequently, the opposition of an individual feature to an ensemble of features or to the whole (quite acceptable as a methodological device) in the ontological approach not only contradicts the principle of integrity, but is illegal from a purely morphological point of view: without being able to compare features as such (establish their homology), we cannot judge about their identity, and hence reversibility. From this point of view, the first of L. Dollo's formulations of the law of irreversibility, which met with the greatest number of objections, turns out to be correct.

Thus, the main objections to the law of irreversibility of evolution are reduced to the following.

1. Repetition (usually far from complete) of signs, based on which the opponents themselves see continuity, and not reversibility. This is evidenced by the interpretation of these cases as iterative evolution, neoteny, fetalization or atavisms.

2. The reversibility of the level of specialization, and not of phylogenesis as such.

3. Repetition of an organization or part of it only at a given, arbitrarily chosen level of consideration (for example, at a given level of knowledge). In fact, the repetition is not complete, which usually follows from an analysis of the technique used by the opponent. Usually the very fact of reversibility cannot be empirically established if it is sufficiently complete.

4. Statement of the impossibility or extreme difficulty of empirically demonstrating the irreversibility of each individual feature (which is especially important for a paleontologist with his always defective material).

Thus, the first three objections are based on methodological miscalculations at the level of interpretation of empirical data; the latter is based on a mixture of the ontological and epistemological aspects of the problem. Without belittling the relevance and complexity of the latter, we believe that they are not directly related to evolution.

To content:

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