Combinative variability and its evolutionary significance

Combinative variability is the main cause of the intraspecies diversity of all living organisms. But this kind of gene modification only leads to the formation of a new combination of already present characteristics. And never the combinative variability and its mechanisms cause the appearance of any fundamentally different gene combination. The emergence of entirely new properties due to various gene variations is possible only in the case of intraspecific mutational changes.

Combinative variability is determined by the nature of the reproductive process. For this type of gene modification, the emergence of new genotypes is based on newly formed gene combinations. Combinative variability manifests itself already in the phase of formation of gametes (sex cells). Moreover, in each such cell only one chromosome from each homologous pair is represented. It is characteristic that the chromosomes enter the germ cell in a random way, as a result of which the gametes in one organism can vary quite widely in the set of genes. At the same time, there are no chemical transformations in the direct carrier of hereditary information.

Thus, the combinational variability is due to a variety of recombinations of already existing genes in the chromosome set. This type of gene modification is also not associated with changes in gene and chromosomal structures. Only processes occurring during the reduction of cell division (meiosis) and fertilization can be sources of combinative variability.

The elementary (smallest) unit of various recombinations of hereditary material, which causes the formation of new gene combinations, is called a recon. Each such reicon corresponds to two nucleotides (a nucleic acid building material) in double-stranded DNA molecules and one nucleotide when it comes to the single-stranded structure of the nucleic acid of the viruses. The rekon is not divided by crossing-over (exchange process between pair homologous chromosomes during conjugation) and in all cases is transmitted whole.

Combinative variability in eukaryotic cells is produced in three ways:

1. Gene recombination in the process of crossing-over, resulting in the formation of chromosomes that have new combinations of alleles.
2. Independent random discrepancy of chromosomes during anaphase of the first stage of meiosis division, as a result of which all gametes acquire their own genetic characteristics.
3. Accidental encounters of germ cells during fertilization.

Thus, through these three mechanisms of combinative variability, each zygote cell, formed by the fusion of gametes, acquires a completely unique set of genetic information. It is such hereditary modifications that explain the enormous intraspecific diversity. Genetic recombination is extremely important for the evolution of any biological species, because it creates an incalculable variety of genotypes. This gives any population heterogeneity. The appearance of organisms endowed with their own individual features predetermines the high efficiency of natural selection, allowing it to leave only the most successful combination of hereditary traits. Thanks to the inclusion of new organisms in the reproductive process, the genetic makeup is continuously improved.