Genetics and Fundamentals of Selection - M.P. Myhun - 2008

CHAPTER IV. Variability and its Genetic Foundations

4.1. Hereditary (Combinative, Mutational) and Non-hereditary (Modificational) Variability

When examining a population of similar organisms, their immense diversity caused by trait Variability becomes immediately apparent. The capacity of living nature to change is a fundamental prerequisite and driving force of the evolutionary process. Variability serves as a source of novel genotype and phenotype variants for natural and artificial Selection, reflecting the complex interaction between organisms and their environment.

Variability refers to the ability of living organisms to acquire new traits and properties.

The phenotypic variability of organisms comprises hereditary and non-hereditary forms.

Non-hereditary, or modificational, variability is not associated with genotype alterations and is not transmitted to descendants. Instead, it represents the Organism's capacity to respond to environmental conditions within the limits of its reaction norm. The underlying mechanism of modification is genomodulation—namely, the alteration of Gene function.

Each genotype possesses a specific reaction norm to various environmental factors. Consider humans in the sun: they tan, but to different extents. Europeans have lived in tropical regions for over 300 years and have deeply tanned (brown) Skin, yet their children are born white. Well-known Examples include submerged and aerial leaves in aquatic plants. These modifications are adaptive in nature and have repeatedly driven Evolutionary Processes. However, certain environmental factors can exceed the protective capacity of an organism—particularly when a damaging agent acts during Critical Periods of individual development; such occurrences are termed morphoses. Phenotypically, morphoses may resemble Mutations, but since they are not inherited, they are referred to as genocopies. Non-hereditary changes also occur during ontogenetic development, known as ontogenetic variability. In essence, this is the realization of the organism's genetic developmental program, i.e., the manifestation of a given genotype's reaction norm over time. The non-heritable nature of modifications was not understood immediately, but definitive experimental evidence has proven this fact. Today, the impossibility of inheriting acquired ontogenetic changes is supported by molecular biology—Genetic information flows from Nucleic Acids to Proteins, never in the reverse direction.

Nevertheless, one should avoid the misconception that the cellular genetic apparatus is entirely unrelated to modifications, as modificational changes occur strictly within the Structure/21.html">Limits of the reaction norm, which is ultimately determined by the genotype.

Hereditary variability is subdivided into combinative and mutational.

Hereditary variability is driven by changes in organismal traits dictated by the genotype—specifically, the structures of hereditary material (genes, Chromosomes)—which are stably preserved across generations.

Combinative variability is caused by two phenomena:

1) chromosome reassortment during Meiosis and the random combination of Gametes;

2) genetic recombination during Crossing-over. In this type of variability, The structure of genes and chromosomes remains unchanged, but gene combinations and their patterns of interaction within The Genome are altered.

Mutational variability results from The Emergence of new allele variants and structural rearrangements within the cellular genetic apparatus. Biological evolution is made possible because the direct material carrier of heredity—genetic nucleic acid—can change from generation to generation. Such changes were studied by S.I. Korzhinsky (1899) and H. de Vries (1901), who introduced the term "mutation" and referred to The process of their occurrence as "mutagenesis." Investigating the mutation process, scientists concluded that mutational trait changes in organisms arise suddenly and discontinuously, combine with all other GENES OF THE genotype, and are stably inherited across generations. Therefore, the processes of mutation and gene recombination form the foundation of hereditary variability.

A mutation is a sudden, heritable change in a trait caused not by gene recombination, but by an alteration in their structure.

Mutations can affect any traits and properties of an organism indiscriminately, leading to biochemical, physiological, and morphological changes.

The accumulation and combination of mutations within populations of organism species serve as a leading factor in evolution. An analysis of trait variability patterns demonstrates that all three forms of phenotypic variability (mutational, combinative, and modificational) collectively drive species evolution.