Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 14. Progressive Evolution

14.6. Concepts of Specialization and Regressive Evolution

According to A.M. Severtsov, evolution can proceed just as freely in progressive as in regressive directions. Moreover, There is a distinct correlation between progressive and regressive transformations of Organization and Changes in the living conditions of organisms. Unfortunately, due insufficient attention given to problems of regression to date, our understanding of the regressive development of organisms remains rather sparse.

Almost all organisms that evolve As a result of significant environmental changes exhibit both progressive modifications of certain Organs and regressive transformation of structures specifically adapted to previous conditions. These are replaced by new structures that ensure the normal existence of organisms in altered environmental conditions (A.M. Severtsov refers to such organ replacements as progressive substitution). For instance, modern birds, whose ancestors possessed Teeth, have now lost them, yet the function of the jaws remains; a horny beak has developed, whose sharp edges have taken over the function of teeth. Similarly, the horny scales that covered the bodies of mammalian ancestors gradually atrophied and were replaced by Hair, which, alongside protective Functions, also assumed thermoregulatory and other roles. For some time, transitional forms existed in which hair was located in groups among the scales (Fig. 14.3).

Class="center">

Fig. 14.3. Reconstruction of the scaly covering of mammalian ancestors. Dots indicate hair follicles located in groups among the scales (after Wiedersheim, cited in A.M. Severtsov, 1945)

As these Examples demonstrate, the reduction of certain structures does not diminish the overall level of ORGANIZATION OF THE respective organisms: the atrophy of organs is offset by The Development of new ones with analogous functions. In this case, The phenomenon of reduction should be considered beneficial, as the Organism rids itself of redundant organs that would otherwise hinder the functioning of new structures. For example, the Development of the spine while retaining the notoord becomes problematic—it is necessary to "free up" organizational and functional space for The formation of new, efficient structures.

The Progressive development of new adaptive properties in an organism is directly linked to the regression of preceding adaptations.

Furthermore, reduction prevents the excessive complexification of an organism's organization that would otherwise result solely from progressive development and the continuous accumulation of new adaptations. Had humans retained gills, fish scales, fins, the bony armor of stegocephalians, a full coat of hair, and the tail of mammalian ancestors, the viability of the human organism would hardly be high. Therefore, the progressive development of organisms without the elimination of obsolete traits appears impossible.

Nevertheless, phenomena of both partial and general regression must not be confused with processes of degeneration, which ultimately lead to the extinction of respective organism groups. Today, the majority of scientists recognize the absence of any biological predisposition in organisms toward natural extinction; rather, extinction processes are caused in most cases by a mismatch between adaptations and specific environmental conditions, or by the inability to develop appropriate adaptations in response to rapid environmental changes. Morphological regression (even taking general degeneration into account) serves as the natural foundation for biological progress, at the very least.

Recently, special attention has been paid to The Theory of biological regression (S.A. Severtsov), the primary objective of which is to find out why a progressive group sooner or later embarks on the path of regression and goes extinct. The extinction of organisms is considered a routine phenomenon, with one of the most critical causes being significant shifts in living conditions (the principles of humanity's negative impact on organisms are no different from climate changes or other environmental parameters driven by natural successions). A decline in the population size of a particular species creates optimal conditions for the surviving individuals, thereby eliminating The Need for dispersal (due to the continuous deterioration of living conditions, the species vanishes within its range).

If climatic characteristics shift in a specific direction (predominantly north-south), population numbers decrease in one part of the range and increase in another, creating the impression of migration (striking examples include the shifts of flora and fauna during the advance and retreat of glaciers).

At the same time, the extinction of individual species cannot be equated with that of supraspecific taxa. Given the significant distribution of major taxonomic categories (genera, families, orders) and the presence of forms adapted to diverse environmental conditions, explaining their disappearance solely through successional processes is nearly impossible. According to S.A. Severtsov, modern odd-toed ungulates should be regarded as a biologically regressive branch. In terms of both morphophysiological and mental characteristics, the horse family can be considered one of the most highly developed representatives of ungulates. However, the existence of a single extant genus compared to 12 extinct ones, the fragmentation of distribution ranges, and the fact that nearly all species are on the verge of extinction indicate the biological regression of this group. The only environments where equids (aside from certain zebras) still feel more or less at home are deserts and high-altitude regions, where competitors and large predators are virtually absent. In other territories (barring the African savanna), they have lost ground to even-toed ungulates, which possess complex stomachs and more efficient adaptations. The colonization of diverse habitats by more progressive groups can lead to the displacement of more primitive forms (it is in this sense that we can speak of adaptive and non-adaptive forms).

We mentioned earlier that one of the causes for the extinction of certain groups is their excessively high specialization. Like progress or regression, specialization is a relatively relative concept, as all organisms are specialized to one degree or another (only the degree of this specialization varies significantly). Specialization can be considered a consequence of adaptation, which also varies widely. Completely unspecialized organisms cannot exist, as such creatures would be incapable of surviving under any conditions.

If an organism proves to be adapted only to highly specific environmental conditions, it can be considered narrowly specialized. Such an organism has a higher probability of extinction if environmental conditions change. One cause of narrow specialization is the predominance of interspecific Struggle for Existence, which leads closely related forms to diverge into different (almost invariably narrow) ecological niches. Specialization is inherently tied to self-limitation, which ultimately leads to the formation of evolutionary dead ends.

An example of this phenomenon is the evolution of sabertooth cats, which includes the largest mammalian predator, Andrewsarchus mongoliensis. This species possessed specialized adaptations: highly developed upper canine teeth, which allowed the animals to kill large but sluggish prey—giant forms of ungulates, rodents, and insectivores. The gradual disappearance of large prey caused the extinction of these specialized predators, as they were unable to switch to hunting smaller or more agile animals.

Thus, progress, regression, and specialization are relative and interrelated concepts. The exact same phenomenon can simultaneously produce both progressive and regressive consequences for a given organism, and is invariably accompanied by a certain level of specialization.

Food for Thought

The issues raised in this topic further emphasize our low level of awareness regarding the mechanisms of natural processes. Consequently, Interference in the biosphere's functioning—conducted without understanding or clear, explicit criteria for progressive and regressive development—could accelerate humanity's transition into a state of biological regression. The most frightening aspect, as sociologists and political scientists suggest, is that we will believe we are marching toward a "bright future," remaining entirely incapable of correctly assessing our actual condition.