Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 7. Natural Selection

7.1. The Struggle for Existence

Charles Darwin formulated The Theory of natural Selection based on The phenomenon of the struggle for existence, the necessity of which stems from the limitless drive of organisms to reproduce. It arises as a consequence of the geometric progression of reproduction, meaning that life resources are not sufficient for everyone.

Natural heterogeneity serves as the foundation for the struggle for existence.

Even before Darwin, scientists pointed out the significant discrepancy between the potential fecundity and the actual reproduction of various animal and plant species. Moreover, in the drive of organisms to reproduce, disperse, and conquer natural space, The rate of reproduction per se does not play a decisive role. The manifestation of their vital energy as agents of nature is not limited solely to the capacity for reproduction. Rather, conversely, high fecundity compensates for a substantial percentage of elimination of individuals, thus indicating a low adaptability of a given species and significant activity of its enemies.

Clear Examples of a sharp increase in Organism Abundance include Water "blooms," The Development of plankton representatives, or mass outbreaks of forest and agricultural pests (species with an eruptive type of population waves). Normally, the potential capacity for reproduction is not realized due to METABOLISM/18.html">The Influence of limiting environmental factors, which can be regarded as the pressure of life. A reduction in barriers, even a slight easing of elimination, will, according to Darwin, promote an almost instantaneous growth of species abundance to any scale. In the infinity of the geological duration of life, according to V.I. Vernadsky's theory, the geochemical effect of both morphological forms—the elephant and the bacterium—will ultimately be identical in its essence. An organism that reproduces most slowly is just as enormous a force in The Biosphere as an organism that reproduces most rapidly in terms of environmental modification.

The production of a greater number of organisms than can possibly survive, coupled with the heterogeneity of the latter, precipitates the struggle for existence. It should be understood more broadly than a mere fight: it represents the dependence of one creature upon others, including with respect to its capacity to realize its reproductive potential.

If the population ceiling of a certain species is established primarily by the limitation of vital resources, then organism reproduction encounters environmental counteraction. Due to a shortage of nutritional Materials, organisms fall into a state of physiological depression and cease to reproduce. The intensification of active competition leads, as a consequence, to the elimination of many individuals, a decrease in fecundity, and promotes maximum economy in the utilization of respective resources by organisms.

Since resource limitation, in the absence of parental care, impacts the young first and foremost, their numbers may increase significantly. An increase in egg size, the number of offspring, or the overall size of animals can be considered the simplest way to provide progeny with everything necessary for survival. Such a trend of changes is characteristic primarily of carnivorous organisms, whose development may be constrained precisely by a shortage of food resources.

If, however, species abundance is limited mainly by systematic destruction by other organisms that feed upon it, the environmental counteraction is weaker. Reproduction encounters no obstacles; on the contrary, it is encouraged by the availability of an abundant food base. Therefore, all advantages lie with the most prolific organisms. The predominance of passive forms of competition and direct elimination under these conditions fosters increased fecundity, earlier sexual maturity, and even progressive miniaturization as the simplest means of life preservation. All of this is inherent to the least protected organisms, which occupy predominantly lower rungs of the phylogenetic ladder. However, similar consequences can be traced today in natural populations actively exploited by humans (commercial fish, game animals, etc.).

The struggle for existence is a result of The activity of the organisms themselves.

In addition to environmental activity that counteracts the reproduction and dispersal of organisms, Various Forms of the organisms' own activity acquire even greater significance in the struggle for existence. The struggle for existence implies a certain activity of organisms in their relations with the environment from which they derive the resources necessary for their vital activity. An appropriate level of activity is one of the mandatory leading characteristics of living matter. Moreover, the activity of the environment itself is a consequence of the direct action of the organisms inhabiting it.

The forms and degrees of organism activity (individual, social, intra- and interspecific behavior, etc.) may vary. Nevertheless, the directional thrust of this activity is common to all organisms—the preservation of life. Initially, it consists in capturing the necessary matter and energy, and subsequently in protecting the acquired resources from other organisms.

According to Yu.M. Plyusnin's social archetype of behavior (a theoretical prototype for the entire spectrum of a specific category of phenomena), the Social behavior of any organisms is based on just four vital needs. Individually necessary are The Need for self-preservation through the appropriation of certain resources (territorial behavior) and reproduction in future generations through mating behavior. Social needs relate to the individual's capture and retention of its place within the community, manifested through hierarchical relationships, and the preservation of the community, which is impossible without friendly relations among kin (alliances). The struggle for existence relies heavily on these vital needs of organisms.

Oddly enough, even in plants, this type of environmental relationship can explain The formation of the struggle for existence. Firstly, individual vital needs for resources for self-maintenance and reproduction in plants have the same essence as in animals; secondly, in plant communities, individual specimens also strive to occupy their place, which is realized through tiering; thirdly, plants also endeavor to preserve their community, which consists in the greater resilience of certain stands compared to individual specimens.

Sometimes it seems as though evolution witnesses a continuous increase in The Significance of the individual, with organismal activity directed primarily toward self-preservation. In reality, almost all organismal activity is concentrated on preserving the continuity of life and securing it in descendants. Therefore, reproduction reveals the most primitive activity of organisms, although an increase in fecundity in itself does not solve all problems of species preservation. Increasing individual lifespan and reproductive capacity is a more effective method of the struggle for existence (better-adapted organisms leave more offspring), but in this case, plasticity may be lost and subsequent evolutionary opportunities for the organism constrained.

In higher animals, forms of parental care are refined, a family (in the broad sense) emerges, and in some species, even a society. High organismal activity also ensures high paces of life, which has positively influenced the evolution of corresponding forms.

In the struggle for existence and reproduction, a vast number of organisms perish due to adverse physical impacts, interactions with other organisms, shortages of food or other resources, and so forth. Since it is predominantly the weaker ones that die or fail to reproduce, it can be considered that the struggle for existence drives the survival of the fittest, which constitutes the operation of natural selection.

According to C. Darwin, the struggle for existence can be considered a universal phenomenon because its action never ceases. Naturally, it does not necessarily culminate in natural selection; in the event of mass mortality of organisms due to various catastrophic abiotic causes, the latter may not occur at all. Such phenomena never pass without consequences for a population—ranging up to complete extinction, which befalls smaller organisms first and foremost, such as insects.

Forms of the struggle for existence. The most important forms of the struggle for existence were outlined by Darwin, yet no universally accepted Classification of them exists to this day. A significant portion of modern evolutionists does not mention it at all, reducing evolutionary mechanisms to the molecular-genetic level. In our conviction, those scientists are right who consider the realization of any evolutionary events impossible without the struggle for existence.

Being transparent in meaning, the term "struggle for existence" should be perceived quite metaphorically—as the activity of an organism manifested in self-preservation and the reproduction of its kind. However, as A. Schopenhauer asserts, the cause of such activity can be a "blind will to life," through which an organism attempts to do everything possible for its self-preservation. Yet the latter always encounters a similar will on the part of other organisms in the pursuit of their vital needs.

It was proposed to distinguish Direct and Indirect forms of elimination and competition within the struggle for existence. C. Darwin singled out intraspecific and interspecific struggles for existence, to which the so-called constitutional struggle (where organisms better resist abiotic factors due to their structural features) was subsequently added. We note that constitutional features may also play an important role in Other forms of struggle. Regarding the division into direct and indirect forms, elimination and competition, drawing a clear boundary between them is rather difficult, moreover, the presence of one form does not exclude others. The main point is that, regardless of the name, the struggle for existence consists in sorting out the uncompetitive individuals. Therefore, we shall adhere to classical views regarding the Classification of the struggle for existence.

The intraspecific struggle for existence occurs within a single species among the individuals that comprise it. Its primary cause is the intrinsic activity of individuals in realizing the will to life through the satisfaction of vital needs. Even the simple consumption of resources already leads to the struggle for existence, because a nonspecific transformation of the habitat takes place. The vital activity of highly developed organisms with complex behavior is accompanied by diverse, often quite unexpected manifestations.

Dividing this type of relationship into individual, interfamily, or intergroup struggle is, in our view, rather conventional, since any relationships both within a population and in the biogeocoenosis as a whole are tied to their agents—individual organisms. Therefore, all forms of the struggle for existence are manifested solely through individuals. For instance, in primitive eusocial insects (such as bumblebees and certain species of wasps), alongside interfamily competition, there is—at least periodically—an individual struggle among particular females for the status of queen.

This form of the struggle for existence is the most severe, because the organisms involved are closely related biological systems and, consequently, share similar environmental requirements and resource demands. Organismal competition can escalate into cannibalism, lead to the death of one of the contenders—for example, As a result of mating duels—or even turn into outright elimination. Thus, dividing the individual struggle for existence into competition and elimination is likewise artificial.

The interspecific struggle for existence involves organisms of different species connected by food chains. Three MAIN TYPES OF interspecific relationships can be distinguished.

The first type is competition, wherein the fecundity, survival, and development of individuals of one species directly depend on how intensively another species utilizes a specific resource. It arises only when the resource in question is available in insufficient quantities. Competition can take various forms depending on The Nature of the resources contested. For example, competitive interactions between two bedstraw species (Galium hercinicum and G. pumilum) in Great Britain result in the former species growing predominantly on acidic soils under natural conditions, whereas the latter thrives on alkaline ones. Cultivation experiments have shown that both species perform well on various soils when grown in isolation. However, when grown together on acidic soils, only G. hercinicum grew normally, while on alkaline soils, only G. pumilum did so; in other words, the outcome of competition in this example depends on the environmental conditions in which it occurs.

Competition also exists among different species of barnacles inhabiting rocky shores in southwestern Europe. Balanus balanoides outcompeted or at least suppressed the development of Chthamalus stellatus within the sublittoral zone, whereas the latter prevailed in the littoral zone due to their lower susceptibility to desiccation.

Competition between different species may involve specific microhabitats, primarily refuges essential for successful reproduction. This is clearly observed in cavity-nesting birds (such as flycatchers, tits, and sparrows) or burrowing mammals (such as foxes, raccoon dogs, and badgers). Food competition in nature is relatively rare, but it is invariably intense and even brutal when species inhabit temporary biotopes, which is characteristic mainly of small animals. Such biotopes include decaying organic matter (such as carcasses or manure) and its associated fauna. Because relationships in these settings are extremely tense, organisms develop special adaptations. For example, adult insects possess well-developed Sensory Organs that enable them to locate such food sources first, while larvae can produce chemical substances that inhibit The Emergence of competing forms such as nematodes and Fungi.

Interspecific competition can manifest in two ways—Interference and exploitation—which are largely interconnected. Exploitation is driven by the indirect interaction of individuals of different species: the utilization of a resource by one species reduces the viability of individuals of another. Interference is the Direct impact of one species upon another, up to physical displacement. Interspecific competition is frequently asymmetrical, meaning one species barely affects the Development of the other while remaining dependent on it (a phenomenon known as amensalism).

In some cases, interspecific competition can be more intense than intraspecific competition, fostering mutual antagonism. In animals, this phenomenon may involve overt aggressiveness and even mutual predation, whereas in plants, it manifests as allelopathy (the release of chemical substances that inhibit the development of other species). Competition can lead to the extinction of one of the species (the one with lower ecological plasticity) or disrupt ecological equilibrium even as a result of minor environmental changes.

The second type can be considered predation. Naturally, a predator always exerts a negative impact on the specific individual it consumes. Nevertheless, for the prey population, this effect can be positive, since the eliminated individuals are typically weaker or inferior, and the survivors can engage compensatory mechanisms.

Numerous observations of the dynamics of various species in food chains demonstrate that predators predominantly kill individuals lacking an individual territory, as well as wounded, sick, young, or very old animals. Consequently, The impact of predation on a population is considerably milder than one might expect: the individuals that perish are primarily those destined to die anyway, while the resources they failed to utilize remain available to other organisms.

Survivors compensate for the impact of predators primarily by reducing intraspecific competition among themselves. This is clearly evident in prey populations experiencing high density, which renders the intraspecific struggle for existence particularly intense. At the and time, one must bear in mind that the potential for such compensation is almost always limited (humans frequently overexploit certain populations, which can ultimately drive them to extinction). Easing the pressure of one type of predator (or enemy) on a prey population can lead to an intensification of similar pressures from other organisms.

Quite often, the evolutionary traits of predators and their prey become mutually coordinated. In particular, the impact of predation on the phylogenesis of prey can drive the development of toxic or other defensive chemical substances, cryptic coloration, and the like. The reciprocal effect involves the development of enhanced sensory organs in predators, specialized Methods for capturing prey, and so forth. In other words, a certain specialization of predators occurs, rendering them incapable of consuming arbitrary prey (thus establishing evolutionary constraints). It is known that some interspecific predator-prey systems coevolve, mutually stimulating each other's development (although direct proof of this in nature is currently lacking).

A much deeper interrelation resulting from coevolution occurs in host-parasite interactions, which constitute the third type of interspecific struggle for existence. Undoubtedly, the evolution of diseases proceeded in parallel with the development of living organisms: descending from free-living ancestors, parasites evolved alongside their hosts. The scale of harm inflicted by parasitic organisms on their hosts depends directly on the population density of the former. However, confirming the death of a given host due to specific parasites is rather difficult, requiring post-mortem pathological examinations, and because parasites impair host health without causing immediate mortality (the problems of parasitism are detailed in the works of E. N. Pavlovsky, V. N. Beklemishev, V. A. Dogiel, and others).

Thus, the interspecific struggle for existence is relatively mild and aims at developing predominantly minor adaptations that facilitate the coexistence of several species. This is because interspecific relationships are far less pronounced than intraspecific interactions and are consequently less tense. They tend to escalate primarily during direct physical contact between individuals of different species, which may result in organismal mortality.

The struggle against unfavorable environmental conditions is classically termed the "constitutional struggle for existence," which is not entirely accurate. According to ecological data, abiotic factors exert a significant influence on a given organism population when they occur in deficit or excess. For organisms, this manifests as unfavorable environmental conditions and is temporary and periodic in nature. The impact of this form of the struggle for existence on populations may involve the direct elimination of individual organisms or operate by intensifying the intra- and interspecific struggle for existence.