Abstract
The evolutionary changes of viruses are primarily associated with the replication processes of viruses containing deoxyribonucleic and ribonucleic acids, which differ significantly. The genomes of most viruses containing ribonucleic acid are replicated with much less accuracy compared to the genomes of viruses containing deoxyribonucleic acid. Comparing the number of mutations in an infected cell reflects an inverse relationship between genome size and the frequency of mutations carried out by these two categories of viruses. Viruses with double-stranded deoxyribonucleic acid genomes have a low mutation rate compared to single-stranded genomes. The genome of viruses is not a stable unique structure, but rather an average, variable number of different amino acid sequences. It is in the virus population that a high mutation rate is maintained, and low variability is not beneficial for the preservation of viruses in nature. Some animal species may be intermediate hosts when new epidemic viruses appear. The introduction of non-viral nucleic acid into the viral genome can also contribute to the evolutionary changes of the virus, lead to the formation of defective genomes or to the emergence of hypervirulent strains. Viral genomes encode numerous molecules that modulate a wide range of protective immune mechanisms. The variability of viruses is also facilitated by the simultaneous integration of several proviral genomes into one cell, which activates the processes of recombination and genetic shift. An important evolutionary point may be the conversion of ribonucleic acid ribose into deoxyribose of deoxyribonucleic acid, which increases the stability of nucleic acids by more than 100 times. Horizontal gene transfer between viruses that infect different hosts is a central feature of the evolution of viruses containing ribonucleic acid. Eukaryote viruses with single-stranded deoxyribonucleic acid probably evolved from bacterial plasmids after they acquired capsid protein genes from the (+) ribonucleic acid chain of viruses. In addition to megaviruses and adenoviruses, polintons are likely precursors to bidnaviruses and virophages.