Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 4. The Mutational Process as an Elementary Evolutionary Factor

4.2. The Mutational Process

The Mutational Process is generally considered to be the primary supplier of elementary evolutionary material. Its essence lies in the spontaneous emergence of Mutations and their subsequent combination during Crossing-over, which drives hereditary changes in populations (some scientists, notably A.S. Severtsov, argue that Variability as a whole should be viewed as this factor, rather than the mutational process alone).

We have previously discussed the profound biological importance of variability in nature generally. Here, it must be emphasized that the mutational process is specifically what introduces fundamentally new material into a population. Gene recombination, gene drift, and other processes that shape The Diversity of existing phenotypes are secondary factors that rely on pre-existing material for their action.

The mutational process is characterized by specific features and exerts a distinct influence on populations of All living organisms. A certain proportion of individuals in a population carry various mutations, and The Emergence of new ones causes a shift in the frequencies of one allele relative to others.

Mutation rates can vary significantly and depend both on environmental influences and the specific genotype of a given species. Specific life forms—including population size and dynamics driven by feeding habits, reproductive traits, lifespan, and the presence of predators or parasites—determine the mutation rate in each particular case. Although mutations themselves arise only under METABOLISM/18.html">The Influence of changing environmental conditions, the genotype may prove more or less resilient to various factors, resulting in differing mutability among individual genes.

When considered per single gene, the pressure of the mutational process is usually insignificant given the total number of genes in an Organism; nevertheless, this elementary evolutionary factor (along with genetic recombination) significantly impacts populations as a whole. For instance, the mutation rate of individual genes in Drosophila does not exceed 0.001-0.004%, whereas the overall mutation rate reaches up to 25% per generation.

The emergence of mutations can be seen as the result of specific external and internal factors acting on the organism. They arise randomly, and even in the case of induced mutations, we cannot fully direct this process. Utilizing even the most specific mutagens does not allow for targeted mutations due to the wide spectrum of organismal responses. Only Introduction/32.html">Genetic Engineering makes it possible to resolve this problem (at least partially).

In a given species, characteristic mutations may recur, and closely related species often develop similar mutations (such as LEAF SHAPE IN plants or animal pigmentation). Certain mutations appear in a given organism under specific environmental conditions with a certain frequency. Changes in conditions lead to corresponding shifts in the intensity of the mutational process, though The ratio of frequencies of different mutation types may remain nearly constant.

The rate of spontaneous mutation can range from 1 · 10^-5 to 1 · 10^-7 per gene per generation. It is believed that a higher mutation intensity would not only fail to accelerate the evolutionary process, but could actually retard it.

The mutational process is reversible, but reverse mutations typically occur at least an order of magnitude less frequently. This process may proceed more easily in one direction because a prior mutation facilitates the occurrence of a subsequent one in the same direction (allowing one to speak of directed mutation). However, such a phenomenon is extremely rare, and even rarer is its potential positive evolutionary significance (primarily in the reduction of certain Organs).

Neutral or indifferent deviations are genotype variations that manifest as phenotypic changes within the norm of reaction. The majority of recessive mutations in a heterozygous state do not manifest at the phenotypic level at all.

The vast majority of arising mutations are detrimental compared to the initial state of any organism. This is explained by the fact that the genotype, as an integrated whole, has been shaped over a long period through the Selection of the most "successful" adaptations, meaning its alterations are predominantly negative. The transition of new genetic changes into a heterozygous state neutralizes these negative consequences (buffering of mutations), and in some cases, heterozygotes even acquire greater viability.

The consequences of the mutational process also include the so-called "genetic load"—that portion of hereditary variability responsible for the appearance of less viable individuals who are subject to selective elimination during natural selection. The sources of genetic load can include mutational, segregational, and substitutional (replacement or transitional) loads.

Mutational load is caused by the recurrent appearance of mutant alleles in a population. Since natural selection acts against these alleles, their frequency is low, and they are maintained in the population through mutational pressure. Meanwhile, recessive mutations in the heterozygous state are completely suppressed or exert only a minor deleterious effect, thereby persisting in the population.

Segregational load arises As a result of heterozygous parents segregating less adapted homozygous offspring. It is assumed that a significant proportion of mutations exert a positive effect in the heterozygous state (overdominance effect) and are continuously maintained by selection across generations. Substitutional load arises during Changes in the Adaptive Value of individuals and persists in the population until one allele replaces another.

Regardless of The Nature of this genetic load, homozygotes experience negative consequences. At the same time, this detrimental effect should be viewed as a relative phenomenon, because genetic load can serve as a genotypic reserve for evolution by maintaining genetic diversity and, consequently, the evolutionary plasticity of the population. This reserve can provide the basis for genetic systems that give rise to new adaptive traits in populations. One of the most famous Examples of such a negative mutation in humans is Sickle-Cell Anemia. Individuals suffering from this disease (where red Blood Cells assume a sickle shape under oxygen deprivation) typically perish before reaching maturity. In the parents of affected individuals (who are heterozygous for this mutation), red blood cells undergo only partial changes, meaning they do not suffer from the severe form of anemia. At the same time, heterozygotes are resistant to such a severe disease as malaria, which is why this mutation has become relatively widespread in certain equatorial regions.