Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 13. Features of Phylogenesis

13.3. Rules of Group Evolution

According to L. Dollo's law of irreversibility (1893), evolution is an irreversible process, and an Organism cannot return to a previous state characteristic of its ancestors. Thus, secondary aquatic vertebrates do not transform back into fish; instead, they acquire new adaptations to the aquatic environment at their own level of Organization. In doing so, they develop structures that merely simulate the earlier, more primitive state to a certain extent (the flippers and tails of cetaceans or ichthyosaurs have a completely different Structure compared to fish).

Some scientists disagree with this rule, arguing in favor of the Cyclical Nature of the evolutionary process and the possibility of returning to a previous state. Indeed, reverse Mutations can restore an ancestral trait, but even at the microevolutionary level, reconstructing the entire genotype completely is impossible (only clones share an identical genotype). At the same time, reversion to the so-called wild type under the reconstruction of initial environmental conditions indicates the possibility of reverse development at the microevolutionary level. Acknowledging the reversibility of individual traits does not, however, refute the irreversibility of the evolutionary process as a whole.

Ch. Depéret's rule of progressive specialization (1876) states that a group that embarks on a path of specialization continues to evolve solely through the deepening of that specialization. At the same time, According to the author of this rule, specialization predominantly affects individual Organs (associated with adaptation to specific environmental conditions) and does not affect the organism as a whole. For instance, in birds specialized for flight, subsequent evolution involves The formation of various flight modes (active, soaring) due to the deepening specialization of wing structure in different species.

Sometimes the law of phylogenetic increase is added to this rule, according to which every phylogenetic branch exhibits a trend toward an increase in the body size of organisms (development proceeds from smaller representatives to larger ones). Upon reaching critical sizes and disrupting vital proportions, the given group becomes extinct.

According to E. Cope's rule of the origin from unspecialized ancestors (1896), new major groups originate from relatively unspecialized representatives of ancestral groups. These very forms possess a higher potential for fundamentally new adaptations, enabling them to embark on the path of arogenesis. Thus, mammals originate from the least specialized reptile representatives, closely approximating cotylosaurs. Any specialized forms lack the necessary potentials for the subsequent formation of new evolutionary directions due to A large number of constraints.

The problem of neoteny, mentioned earlier, is closely related to the rules of E. Cope and Ch. Depéret. In many specialized organisms (especially sessile and parasitic forms), larvae remain unspecialized and, serving for dispersal, function as active stages. Therefore, it is believed that through the transition to neoteny, organisms are able to lift the constraints caused by narrow specialization and continue along the path of progressive development. While not disputing the prevalence of such transformations in nature, we note that they do not negate the reality of the aforementioned rules.

H.F. Osborn's rule of adaptive radiation (1902): the phylogenesis of any group is accompanied by its branching into separate lineages that diverge in different adaptive directions (distinct ecological niches) from the ancestral state. This rule is fully consistent with Charles Darwin's principle of divergence and is supported by an extraordinary number of Examples (Fig. 13.10). The basis for the adaptive radiation of groups is the reduction of competition during the formation of multiple species occupying different ecological niches (it should be recalled once again that interspecific Struggle for Existence is less acute than intraspecific).

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Fig. 13.10. Adaptive radiation using placental mammals as an example (after A. Romer, T. Parsons, 1992)

I.I. Schmalhausen's rule of the alternation of main evolutionary trends (1939): arogenic evolution alternates with periods of allogenic evolution across all groups. The necessity of such alternation is explained by the fact that entering a new adaptive zone (arogenesis) is a rather complex process accompanied by the extinction of almost all intermediate forms. Consequently, it occurs only when the capacities of the previously mastered adaptive zone are exhausted and further allogenesis becomes difficult. Accessing a new (yet unoccupied) adaptive zone is always accompanied by speciation followed by the occupation of the entire zone.

I.I. Schmalhausen's rule of the intensification of biological system integration (1961): As a result of evolution, biological systems become more integrated, accompanied by the complexification of the mechanisms ensuring this integration (ecosystem Homeostasis and its maintenance mechanisms). This rule demonstrates the impossibility of reducing biological systems of any level of organization to a mere aggregate of their components. The entire course of the evolutionary process confirms this rule both at the ontogenetic level (autonomization and canalization of ontogeny) and at the population-species level (complex STRUCTURE OF THE species) and ecosystem level (interconnection and interdependence of all representatives of respective ecosystems).

The list of evolutionary rules could be continued, but even the ones cited have certain exceptions and limitations. The specifics of biology as a scientific discipline lie in the fact that theoretical generalizations are largely empirical in nature and rely heavily on comprehension and belief rather than on proof and constant verification in nature. Almost every biological phenomenon is unique (which is why we cannot recreate past evolutionary events or predict what will happen in the future) and therefore may require its own specific rule, which science will likewise be unable to test.