Genetics with the Basics of Selection - M.P. Myhun - 2008

CHAPTER VI. Population Genetics

6.3. Genetic Heterogeneity of Natural Populations

Natural populations of organisms are predominantly genetically heterogeneous, consisting of individuals with different genotypes. This genetic diversity of populations is primarily explained by The phenomenon of multiple allelism and the continuous emergence of new (mostly recessive) Gene alleles through mutation, which serve as the primary source of genotypic Variability in organisms. Mutational variability makes a substantial contribution to the genetic Structure of both Mendelian and non-Mendelian populations.

If a gene in a population exists in only two alleles—dominant (A) and recessive (a)—the individuals of a Mendelian population are divided into three classes according to their allelic composition: (AA) homozygous dominants, (Aa) heterozygotes, and (aa) homozygous recessives. In reality, a gene in a population may be represented by a much larger number of allelic variants, meaning the number of genotypic classes can exceed three.

The ratio of homozygotes to heterozygotes in a population depends on many factors, primarily the mode of reproduction (sexual or asexual, cross-pollination or self-pollination), the population size, the type of variability, and The Nature of isolation.

The degree of heterozygosity is always higher in a panmictic population, whereas a population of self-pollinating plants mainly consists of individuals belonging to pure (i.e., homozygous) lines. This was demonstrated in 1903 by the Danish physiologist W. L. Johannsen in his work "On Inheritance in Populations and Pure Lines". Studying the inheritance of seed weight and size traits in self-pollinating plants (such as peas and beans), he selected heavy and light seeds from each plant separately over the course of 6-7 generations—Selection within pure lines. Selection within pure lines proved ineffective, as light and heavy seeds continued to reappear because variability within a pure line is primarily non-heritable and modificational. Populations of autogamous plants and animals consist of pure yet genetically distinct lines that do not interbreed or exchange Genetic information with one another.

Changes in the genetic structure of such populations occur mainly through the Mutational Process and the selection of hereditarily distinct lines and clones that possess certain adaptive advantages under given conditions.

Consequently, any autogamous Organism can serve as the founder of a new race, subspecies, and species, as well as a cultivar or breed. Homozygosity is never absolute; in populations of self-pollinating plants (such as wheat, tomatoes, and flax), cross-pollination occasionally occurs, and Mutations also arise that disrupt the homogeneity of pure lines.

Vegetative Reproduction in agamous organisms (those that do not form Gametes, such as Fungi and Algae) leads to The formation of clones. A clone is a group of descendants originating from a single ancestor. Therefore, a population of organisms that reproduce exclusively asexually consists of individual clones. The genetic structure of each clone and its degree of homo- or heterozygosity are determined by the genotype of the parental form. Within a single clone, genetic heterogeneity among organisms is possible due to the appearance of mutant forms.

In allogamous organisms, a population is formed through the free interbreeding of diverse sexes with different genotypes, that is, on The basis of panmixia. Thus, The Diversity of genotypes in a panmictic population is the result of both mutational and combinatorial variability.

In such a population, the proportion of individuals with a particular genotype in each generation is determined by the frequency of Formation of the corresponding zygotes and, consequently, by the ratio of various classes of gametes in their total pool. This means that the traits and properties of individuals are preserved and distributed within the population According to the laws governing the dissemination of allelic and non-allelic genes.

Allelic genes that arise in populations As a result of mutations are overwhelmingly recessive and manifest phenotypically only at a sufficiently high concentration of heterozygotes, within which mutant genes are preserved and propagated. For this reason, natural Mendelian populations are saturated with various mutations that most frequently remain in a heterozygous state. The probability of these mutant genes becoming homozygous decreases as the population size increases.

S. S. Chetverikov was the first to draw attention to these features of panmictic populations while studying the genetic heterogeneity of natural populations. Researchers (D. Jones, E. East, R. Fisher, S. Wright, M. P. Dubinin, D. D. Romanov, S. M. Gershenzon) developed the balance model of Mendelian population structure. Its essence is that populations do not contain standard "wild-type" genes. Most gene loci, and perhaps all of them in the Chromosomes of individuals, are occupied by genes belonging to series of multiple alleles. Evolutionary shifts in a population occur not through the selection of a single gene, but through the selection of many genes whose alleles exist in a specific ratio (balance) relative to one another.

Prior to this, W. Johannsen’s classical model prevailed, which viewed natural populations as collections of homozygotes for dominant (wild-type) alleles.

It later became clear that dominant homozygotes quite often yield in viability to other genotypes, especially heterozygotes, whose widespread occurrence in populations gives rise to the phenomenon of heterosis.

The genetic heterozygosity of Mendelian populations is not accidental. It is the result of entirely predictable genetic processes governed by specific rules and laws. The high heterozygosity of these populations helps preserve multiple allelism and enhances their adaptive and, consequently, evolutionary potential.