Fundamentals of Evolution - Korph O.P. - 2006

Part I. MICROEVOLUTION

Chapter 7. Natural Selection

7.2. Essence and Biological Significance of Natural Selection

Having proposed THE PRINCIPLE OF natural Selection based on the results of artificial selection, Charles Darwin predicted similar mechanisms in natural environments. The most crucial differences in the outcomes of natural and artificial selection are that the former preserves traits vital to a given Organism, whereas the latter may fix properties that are actually harmful to the organism yet useful to humans. Consequently, the mechanisms of these phenomena differ somewhat: artificial selection picks individuals best suited to human requirements, whereas natural selection eliminates unfit individuals or excludes them from reproduction. For this reason, artificial selection cannot drive The formation of new species, as it primarily affects isolated traits that are inconsequential to the species' biology.

Natural selection can be characterized as the differential survival and reproduction of individual organisms (genotypes) that differ in certain traits from those that fail to survive or realize their reproductive potential. While it was previously believed that selection operates exclusively through elimination (I.I. Schmalhausen), modern scientists have concluded that the primary driver is the exclusion of certain forms from reproduction (which is biologically equivalent to elimination). The mere survival of organisms, or even their attainment of an "advanced" age, has no evolutionary consequences in itself; far more important is each individual's contribution to the Gene pool of the respective population, meaning the realization of its reproductive potential.

One of the fundamental features of natural selection is that it always selects phenotypes—meaning it does not act at the level of individual genes or even genotypes as a whole, but solely against the Background of realized Genetic information. Therefore, individuals with even the finest genetic traits who lack the opportunity to express them may prove uncompetitive under specific conditions and thus perish in the Struggle for Existence.

Virtually all traits and properties of organisms fall under the action of natural selection, yet not as isolated loci or features, but strictly as components of a single, integrated organism.

Moreover, properties in themselves, detached from their bearers, carry no biological meaning whatsoever. Therefore, reductionist theories of gene-centric evolution should be approached with caution. We cannot deny the Evolutionary Significance of the presence or absence of certain traits upon which selection acts most effectively, yet we must understand the absolute relativity of such "sites of selection application." Unevenness is a primary characteristic of the evolutionary process, which applies equally to the action of natural selection on specific organismal traits.

The intensity of natural selection can vary greatly and largely depends on the severity of the forms of the struggle for existence. Occasionally, the general direction of selection may reverse, returning organisms to ancestral traits. A key feature in this process is the shaping of species-favorable traits that may positively benefit multiple forms (coevolution). Selection does not permit the fixation of traits detrimental to the species as a whole: even if characteristics harmful to an individual organism emerge, they are invariably useful to the species collectively.

One of the Functions of natural selection, recognized unconditionally by nearly all evolutionists, is maintaining a certain level of population adaptation (the stabilizing effect) for survival under specific conditions. This applies to both constant and changing environments. Since evolution cannot proceed in the direction of declining fitness, The rate of population change keeps pace with environmental changes (otherwise, the population will go extinct).

Natural selection is the sole elementary evolutionary factor that directs the overall course of evolution, which constitutes its fundamental evolutionary essence.

The action of all other elementary evolutionary factors is random and undirected, manifesting primarily as unpredictable shifts in the genotypic composition of a population. Natural selection controls this rather chaotic process and imparts a specific direction to it through the differential reproduction of individual organisms.

A prime example of the directional action of natural selection was industrial melanism, which developed in several butterfly species due to environmental pollution. The outcome of natural selection, much like evolution as a whole, is the ADAPTATION OF ORGANISMS to prevailing conditions (this will be discussed in greater detail in the chapter on adaptations).

According to some evolutionists, a distinction should be made between the directional action of selection and the general evolutionary trend. Thus, under specific conditions (such as intense elimination), selection may favor smaller body sizes, earlier sexual maturation, and increased fecundity. A completely different matter is K- and r-strategies, sometimes referred to as K- and r-selection.

K-selection aims to increase the survival probability of individual organisms (where K represents the upper equilibrium population size, i.e., its maximum carrying capacity). Populations of these organisms inhabit either constant or predictable environments with minimal fluctuations in parameters (the tropical rain forest can be considered the most stable terrestrial ecosystem). Conversely, r-selection is a strategy utilized by organisms in unstable environmental conditions, geared toward maximizing reproduction and rapidly increasing population size.

These strategies often contradict one another. The vast majority of organisms occupy an intermediate position, and these forms can be distinguished only conditionally.

In general, we can speak of K- and r-strategists only in relative terms. For instance, mice are r-strategists compared to elephants, but K-strategists relative to the vast majority of insects. K- and r-strategies are significant less for the directional action of selection than for shaping specific life cycles and successional processes.

To this day, the central problem of evolutionary theory remains the status of natural selection: is it the creator of evolutionary changes, or does it merely function as an editor? In the view of S.S. Shvarts, debates regarding The Essence of selection (differential survival versus differential mortality) cannot be considered fundamental, since transformations in a population's genetic makeup are only possible when certain organisms leave more offspring than others. In some cases (purely mechanically), we can compare selection to a sieve that merely filters out the best-suited variants from the available organisms endowed with hereditary Variability. That is, in this instance, the factor is viewed as nothing more than an editor that introduces nothing novel to pre-existing variants. Sometimes the action of natural selection is compared to an artist-sculptor who creates a work of art merely by removing excess material. The latter case (no less mechanistically) acknowledges the "creative" significance of even simple "elimination of the superfluous."

Moving away from extremes, we must acknowledge that the organism itself is the primary object of natural selection. But What is the action of natural selection as a whole? It is nothing other than the intrinsic vital activity of organisms colliding with the identical activity of other organisms or with the "aggression" of the environment (the pressure of life); thus, selection is not an independent mechanism of anything in itself, but arises solely from the interaction of biologically active organisms. The successful realization of this activity promotes the survival and reproduction of individual organisms. This process is continuous and persists throughout the lifetime of each specific organism, which is why the creative nature of natural selection depends entirely on the organisms themselves.

The directionality of natural selection can be viewed as a specific pressure of life under conditions of a concrete environmental state. The activity of organisms realized in these settings is adjusted accordingly to align as closely as possible with the demands of life. The exact same organism under different conditions falls under METABOLISM/18.html">The Influence of differently directed factors, resulting in a different adjustment of its activity. The gradual accumulation of minor positive deviations across generations ultimately drives the formation of a new trait, which we may classify as the creative action of natural selection directed toward the gradual adaptation of individuals.

Thus, natural selection, as the generalized pressure of life exerted on specific individuals, is currently considered the only directed elementary factor of evolution that calibrates the action of other factors and shapes organismal adaptation to concrete conditions. At the same time, by itself, devoid of other evolutionary mechanisms, it does not function: the realization of an elementary evolutionary phenomenon is possible only within a population—somewhat isolated from similar populations—possessing a necessary number of genotypically and phenotypically diverse individuals engaged in the struggle for existence.

Natural selection should not be envisioned as a special "creative principle." According to I.I. Schmalhausen, at any given moment, natural selection is merely a reflection of the relationship established between the organism and the environment. This very relationship, constantly shifting during the course of evolution, manifests in Various Forms of the struggle for existence and determines the character and direction of natural selection and evolution as a whole.