Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 8. Adaptation as a Biological Phenomenon

8.4. Pathways of Adaptation Formation

The formation of adaptations is a consequence of the microevolutionary process, meaning it requires the presence of elementary evolutionary material and the action of elementary evolutionary factors.

The Emergence of a selectively valuable genotype (an Organism with adaptive traits) is considered an elementary adaptation event.

However, this cannot yet be considered a finished adaptation—it becomes such only after a shift in the reaction norm of the entire population, and subsequently, of the species. This change occurs through the gradual dying out of organisms with certain traits (which serves as a sort of evolutionary price paid for Selection) and the more active reproduction of those with others.

Currently, three main pathways for the formation of adaptations are distinguished: preadaptive, combinatorial, and postadaptive.

The preadaptive pathway of formation implies that an already established adaptation acquires a new adaptive significance under certain conditions. The corresponding structures form even before their usage becomes fully specialized. This can be traced in The Development of the fontanelle on the heads of newborn humans. While in other mammals the presence of fontanelles is associated with the specifics of Brain development, in humans they acquire an additional function—facilitating childbirth due to the mobility of the bones. Due to the large size of the mammalian brain compared to reptiles, these forms faced the need to restructure the braincase to make it more capacious. The original braincase forms only the floor, while the roof and walls are formed by dermal elements where the fontanelles develop. After the brain reaches a more or less definitive size (subsequent growth slows down compared to the growth of other body parts), the fontanelles close up, and the braincase becomes a solid bony capsule (discussed in more detail below).

Another example of the preadaptive pathway of organismal adaptation is found in fish Lungs. As it turned out, these were evolutionarily older structures than the swim bladder, though in most fish they never acquired adaptive significance. Lungfishes, which inhabit oxygen-deficient Water bodies, are the only example of modern ray-finned fish where lungs have an adaptive function. In the coelacanth, which inhabits great depths, the lungs have remained in a preadaptive state.

Combinatorial pathway: adaptations are formed by combining multiple Mutations followed by the fixation of the most successful variant. An example includes the complex adaptations discussed earlier.

It can be argued that the majority of adaptations are created precisely through this combinatorial pathway. Here, the creative role of natural selection is most clearly manifested: the most successful combination achieves maximum prevalence in the population.

The postadaptive pathway of adaptation formation is observed primarily in cases of the reduction of certain structures or Organs due to changing environmental conditions. Examples include the reduction of eyes in moles and certain other mammals following their transition to a subterranean lifestyle (under such conditions, fully developed eyes would only be a hindrance), the reduction of legs in snakes that adopted a new mode of locomotion, and so on.

A.N. Severtsov proposed an original Classification of the pathways of adaptation formation (which, in our view, has not lost its relevance to this day):

1) hereditary changes in Organization — through this mechanism, significant adaptive changes in organ Structure and organismal Functions occur slowly in response to gradual environmental transformations;

2) non-hereditary functional changes in structure: in this way, organisms are able to adapt to minor environmental changes that occur rapidly.

Examples of the second type of adaptations, which have a purely adaptive significance, include wound healing and regeneration processes. In addition, There is a wide range of functional (non-hereditary) organ modifications: the organism responds to external environmental changes with minor yet rapid structural shifts of an adaptive nature. For instance, the seasonal variation in the length and density of fur in a specific individual mammal within the limits of its reaction norm, depending on Temperature conditions.

Apart from the two aforementioned mechanisms, which are common to both animals and plants, A.N. Severtsov proposes distinguishing two more, characteristic exclusively of animals and associated with adaptations through behavioral changes without structural alterations. In highly developed animals, psychological activity reaches a level that human observers might describe as "intelligent action." Unlike instincts, these actions are not inherited; only a certain level of psychological organization is passed down.

The Significance of this phenomenon lies in the fact that it significantly enhances animal adaptability to rapid environmental changes, responding with behavioral shifts rather than structural modifications. For example, depending on weather conditions within their range, some migratory birds in Europe may either roam or become sedentary. Behavioral changes in animals can also be observed during hunting: having sensed danger associated with the actions of a hunter, animals often alter their behavior and no longer respond to lures or other tricks.

Unfortunately, all this brings us back to the eternal struggle of ideas between Darwinism and Lamarckism. In particular, according to Darwin's view, non-hereditary changes are insignificant for the evolutionary process. Indeed, the evolutionary mechanisms proposed by Lamarck turned out to be largely artificial and far-fetched. However, the complete departure of Darwin, and later of the Modern Synthesis, from the organism as the primary unit of life cannot be considered correct either. Our main task is to find the golden mean. If we Touch upon the problems of Human Evolution, it turns out that such a non-hereditary trait as learning played a leading role in the entire process of anthropogenesis (discussed further below). Learning is also a mandatory prerequisite for the development of behavior in the vast majority of birds and mammals. Evolutionary changes occur throughout ontogeny. If certain non-hereditary changes promoted the active and successful reproduction of an individual, we cannot deny their adaptive significance.

In its final version, the classification of pathways of adaptive development proposed by A.N. Severtsov looks as follows:

1. Hereditary adaptations to very slow environmental changes.

1.1. Hereditary changes in organismal structure.

1.2. Hereditary changes in animal behavior without structural alterations (Reflexes, instincts).

2. Non-hereditary adaptations to relatively rapid environmental changes.

2.1. Functional changes in organism structure.

2.2. Behavioral changes in "intelligent-type" animals.

These types of adaptive development are largely independent of one another; that is, some evolutionary lineages are dominated by certain pathways, while others rely on different ones. For example, Arthropods—one of the most progressive animal groups—predominantly adapt their behavior to environmental changes through reflex and instinct. However, due to their small size, rapid generation turnover, and high fecundity, their overall rate of adaptation remains quite rapid. Conversely, in vertebrates, where individual specimens can play a significant role in preserving the species as a whole (especially among larger representatives), behavioral adaptation predominantly occurs through the accumulation and subsequent application of individual experience.

One of the Prerequisites for the development of non-heritable adaptations is the multifunctionality of organs. In other words, an animal or plant may begin utilizing a certain organ differently than its ancestral forms did. Naturally, the suitability of such an organ for performing new functions will initially be limited, but in the presence of appropriate mutations and with the support of natural selection, corresponding hereditary Changes in the organism's structure may gradually develop.

Food for Thought

The question regarding the purposiveness of biological adaptations and the mechanisms of their formation has long been a stumbling block in evolutionary theories. The close match between many organism structures and their environmental conditions was viewed as nothing less than Divine Providence. Only by grasping the relative nature of adaptations did science realize that any adaptation is the outcome of the evolutionary process. Yet, debates soon arose over the immediate causes of such evolutionary changes. This entire chapter is dedicated to demonstrating the fundamental importance of the organism and its baseline activity in achieving the effectiveness of any adaptation. At the same time, the author of these lines does not consider himself a proponent of Lamarckism. Perhaps the problems of adaptogenesis call for novel Perspectives and approaches to be resolved?