Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 13. Features of Phylogeny Realization

13.2. Directions of Phylogeny

To better understand the mechanisms of evolution in supra-species groups and to account for time as a developmental factor, G. Simpson proposed METABOLISM/2.html">THE CONCEPT OF the adaptive zone. This refers to the complex of environmental conditions within which the evolution of a given taxon takes place and which determines the directions of this evolution. Just as an ecological niche is characterized by the traits of the species occupying it, an adaptive zone is characterized by the taxon developing within it.

Despite certain differences in the views of various evolutionists, There is a growing consensus today regarding the mechanisms governing The Development of major taxa under specific environmental conditions along two main pathways: arogenesis—the development of a group accompanied by the expansion of its adaptive zone and entry into new natural zones due to the acquisition of significant new adaptations; and allogenesis—the development of a group within the confines of a single adaptive zone through minor deviations and the subsequent divergence of organisms into different ecological niches (Fig. 13.9).

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Fig. 13.9. Diagram of evolutionary transformations (after A.M. Severtsov, cited in A.V. Konstantinov, 1975):

aromorphoses (a) — ascent to a higher level; idioadaptations (b) — development within a single adaptive zone; specializations (c) — dead-end branches; regression (d) — transition to a lower level

It should be noted at the outset that arogenesis and allogenesis are viewed as corresponding pathways in the development of certain taxonomic groups. The similarly sounding terms "aromorphosis" and "allomorphosis" (some scientists consider them synonyms for the preceding terms) should be regarded strictly as phenomena involving the reorganization of individual Organism structures that may facilitate a group's entry onto a specific developmental pathway.

According to paleontological data, development via orogenesis is rare in nature: a relatively small number of groups of organisms pass from one natural zone to another. This transition, referred to as orogenesis, occurs relatively quickly and is accompanied by the extinction of most intermediate forms (the brief existence of transitional forms complicates the

search for paleontological evidence of these transformations). At the same time, this form ensures the possibility for the subsequent Progressive development of the organisms in question; almost all major taxonomic groups originated precisely through arogenesis. It can be achieved in two ways: via aromorphosis (major, fundamental adaptations characteristic of rather large systematic groups) and through morphophysiological degeneration (the regression of certain structures).

An example of groups achieving arogenesis through aromorphic organizational transformations is The formation of most major systematic groups (phyla, classes, and sometimes even orders). As a result of aromorphic transformations, an organism gains advantages of a general character, enabling it to move beyond the habitat of its ancestors. Aromorphoses can be viewed as critical nodal points in the development of certain groups of organisms, from which new paths of the evolutionary process originate. Such development gives rise to new relationships between the environment and the organism, accompanied by corresponding adaptations and coadaptations.

Thus, the transition from filter feeding in jawless fish to predation in fish, along with a significant expansion of their adaptive zone, is associated with the formation of jaws, a reduction in the external Skeleton, an increase in body size, and enhanced locomotor activity—all of which required corresponding Changes in the

Structure of The Nervous system and behavior (morphic traits of their Organization). Furthermore, the formation of jaws is widely recognized as one of the most crucial Prerequisites for the subsequent progressive development of all vertebrates, allowing them to shift from filtering to predation. The conquest of land by vertebrates and the subsequent development of terrestrial traits constitute a whole series of aromorphic transformations (discussed in more detail in the next chapter).

The process of general degeneration (A.M. Severtsov and I.I. Schmalhausen propose the alternative term "catamorphosis") is driven by a simplification of environmental conditions, with the organism gaining advantages only in extremely simple habitats. A whole series of Organs becomes redundant and undergoes reduction, along with all functionally related parts. Degeneration is a process of breakdown and disintegration in which all existing dependencies lose their meaning, even culminating in their phylogenetic elimination.

General regression and partial organ reduction are most often based on underdevelopment resulting from the loss of adaptive significance. However, the organism remains integral, albeit noticeably simplified, when associated with a simple environment.

General degeneration, which leads a group into a new adaptive zone, is characteristic mainly of parasitic (cestodes being the most striking example of such a pathway of transformation) or sessile (tunicates) forms. Despite the loss of many Organ Systems, these organisms occupy a fundamentally new (albeit quite simple) adaptive zone where they face no significant competitors and are capable of progressive development, as will be discussed further. At the same time, we must understand that development via general degeneration is accompanied by significant specialization of the organisms, which may lead them into an evolutionary dead end regarding the potential for future transformations.

Some scientists believe that such constraints can be lifted through a transition to neoteny and paedogenesis (larval reproduction and the loss of the adult form). As early as the beginning of the 20th century, hypotheses emerged suggesting that nearly all major systematic categories arose via fetalization (hypomorphosis, neoteny, paedogenesis—other terms may also be used). In this case, the regression of overly specialized organisms and the termination of their life cycle at the larval stage can indeed remove all constraints characteristic of the corresponding adult form. Verifying the reality of such transformations is difficult not only in phylogeny but even in ontogeny: given significant differences between the adult and larval forms, they can only be correlated if the entire Life Cycle of the organism is known (the axolotl is a clear example). Nonetheless, such assumptions can currently be considered only as working hypotheses on equal footing with others.

Allogenesis is realized through idioadaptations (adaptations of the same grade without significant alteration of the general ORGANIZATION OF THE organisms, determining specialization within a specific adaptive zone or a part of it) and continues as long as that adaptive zone exists. In this process, the organism's transformations occur within the framework of preserving the previous general character of limited adaptation (the organism undergoes neither significant complexification nor fundamental simplification of its organization). Some organs undergo further differentiation, while others lose their significance and are reduced.

Allogenesis is related to the degree of specialization of each form to specific conditions within that adaptive zone. Sometimes this specialization goes so far that a significant narrowing of the adaptive zone occurs (known as telomorphosis according to I.I. Schmalhausen), thereby closing off opportunities for the further progressive Development of the group. It is believed that the specialization of only isolated organs does not substantially affect the ability of organisms to undergo subsequent evolutionary transformations.

Examples of allogenesis include the well-known phenomena of adaptive radiation: Darwin's finches, Hawaiian honeycreepers, and numerous other forms. The key point here is that the ADAPTATION OF ORGANISMS to different ecological niches is more or less similar, while the general level of organization remains virtually unchanged. For instance, the reduction of eyes in the mole has almost no effect on its level of organization (there are no fundamental differences from the organization of other insectivores) and can be regarded as an idioadaptation to a subterranean lifestyle where eyes would actually be a hindrance.

In addition to the aforementioned processes, I.I. Schmalhausen proposed distinguishing epimorphosis (a higher grade of aromorphosis, characteristic only of humans); telomorphosis (specialization); hypermorphosis (overdevelopment of certain structures leading to a loss of adaptability and plasticity in organisms); catamorphosis (general degeneration); and hypomorphosis (a special case of catamorphosis involving organismal underdevelopment). In our view, such a detailed subdivision of the phylogenetic processes of organismal groups is unnecessary, especially considering that some degree of specialization accompanies any direction of development. Evolutionarily, the most important aspects remain The ability to develop within a single adaptive zone (allogenesis) and with an transition into a new one (arogenesis).

Extinction is one of the regular processes accompanying the phylogenetic transformations of the living world. According to paleontologists, modern species account for only 2–5% of the total number of species that have ever arisen on Earth. It is believed that one of the reasons for this state of affairs may be extreme specialization and the inability of organisms to change in response to new environmental conditions. Extinction frees up living space for the emergence and spread of new groups. At the same time, A large number of relict forms indicate that this process is not mandatory and point to the potential for a very long existence of any group of living organisms. Thus, what remains evolutionarily important is not only the adaptation of organisms to specific conditions, but also their capacity for further transformation, as discussed in the first chapter. The laws, or rules, of group evolution provide relevant insights into the causes of various changes in different groups of organisms.