Направленность эволюции - Северцов А.С.
ISSN 5—211—00917—7
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equivalent to the maintaining of the selection favouring the existing trends of . adaptation. These selective vectors are differently directed to counterbalance each other. This leads to the stabilization of the population which looses the capacity to be subject to any directional change. In spite of the environmental fluctuations, this provides a basis for the stabilizing selection, especialy for its canalising sub form.
Fluctuations in the ecosystem result in the alteration of the relative strength of different selective vectors. In the restricted environmental conditions the strengthening of a given vector automatically leads to the strengthening of some other selective vector in the opposite direction. Provided that we are considering a character with a wide reaction norm, this stimulates the selection favouring its farther expansion. Provided that we are considering characters possesing the strick genetic determination, this lead to selection, favoring heterosigous genotypes (Dobzhansky 1970), i. e. the increasing of the genetic variability.
The directional selection arises from the disbalance of the actualy existing selective vectors or as a result of the appearence of the new one caused by the change of the environmental conditions. The adaptation of the pests to pesticides is an example. Thus, the counterbalance of the selective requirements which arises at the ecosystemal level of life organization leads to the evolutionary stability of populations. At the same time, this makes them evolutionary flexible, this-flexibility being itself an adaptation to the fluctuating environmental conditions.
The consideration of the directionality of evolution at the level of macroevo-lution, when "we are taking into account the change on the scale of the object (from population to individual), as well as the change on the scale of the time-(the «geological» time), alters the notion of the agencies which determine the-(Iirection of phylogenesis. The individual variability is out of account. The individual organization, instead of offering a mechanism for the rapid directional adaptations arising from the phenotypic flexibility, imposes constraints on the number of potentially possible ways of evolution. The flexibility of characters is determined not by the expansion of their reaction norm, but rather by the ex-etnt of their polyfunctionality. The ways of the phylogenetic change of characters in relation to their function are extremly variable. Coordination provide a mechanism which delimits the responce to the selection favouring any of the functions of the given functional system. This is a reason why the possibility of changes in the direction of phylogenesis is restricted by the coordinations within the organism (or by the degree of heterobatmia giving the mesure of the lack of coordination).
In the sophisticated complexion of coordinations the secondary changes oc-curing in the several coordinated units may become the primary one with respect to some other chains of coordinations thus leading to alterations directed frome the center to the periphery of these chains. This is the reason why the coordinations lead not only to the limitation of the number oof potentialy possible ways of phylogenesis. When changing the organization, they switch a reaction of the organism to the environmental changes thus leading to the increase of their evolutionary plasticity. Another way to avoid the lost of the evolutionary flexibility concerned with the increase in the degree of coordination between the parts of an organism is offered by means of recombination and (or) transposition of the functional blocks (Ugolev 1985). The evolution on the level of recombination of functional blocks seemed to play an important role in the progress of the organization. Finally, the coordinations themselves may be ruptured or rebuilded in the course of the progressive evolution resulting in the release of some components of the coordination chains along with the fixation of other components thus leading to the change of their potencies to the further rearrangements.
Only in the relatively rare cases the coordinations do impose constraints which prevent a given evolutionary trend, in spite of the existence of the corresponding selective vector. This happens when the rupture in the coordination chains leads to the lost of fitness. But in the more usual situation, the ^preser-vance of the existing direction of the evolution in spite of environmental fluctuations and decrease of evolutionary flexibility is due to specialization which, at the level of the organism, is expressed in the drop of polyfunctionality of the
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organs, which are immediately connected with the adaptation to the environment. This tendency to conserve the existing trend should be considered as the most prominent manifestation of the directional nature of the evolution.
The constraints imposed on the direction of phylogenesis by the organization which is inherent to the individuals of a given taxon are to be realized in the concrete environmental conditions, i. e. inside the adaptive zone of this taxon. The limits of the adaptive zone usually become more rigid in the process of the phylogenesis. This is due to the splitting of the ancestral adaptive zone into the daughter ones in the process of the divergence of this taxon or due to the formation of the coadaptive complexes. In both cases this leads -to the strengthening of the competition at the boundaries between the daughter subzones in the process of their coherent evolution. Another reason for the process of their coherent evolution. Another reason for the fixation of bondaries between adaptive zones is the loss of heterobatmia in the course of the phylogenesis of the taxon. The interdependence between the organization of taxa inhabiting the neighboring zones produces the parallelisms which are very attractive for the investigators of phylodenesis.
