The modern theory of evolution is based on three main mechanisms of evolution. They include natural selection, gene drift, and horizontal gene transfer.
Evolution through natural selection is a process in which mutations are fixed increasing the fitness of organisms. Organisms with different genetic traits have different survival rates and the ability to reproduce. Such conditions create competition between organisms in survival and reproduction and are necessary for evolution through natural selection. Thus, organisms with hereditary traits that give them a competitive advantage are more likely to pass them to their offspring compared to organisms with hereditary traits that do not have such an advantage.
Gene drift is a change in the frequency of gene alleles from generation to generation, due to random processes during the survival and reproduction of organisms. If the selection effect is relatively weak or absent, then the frequency of the alleles tends to accidentally drift up or down. The drift stops when the allele is fixed in the population, disappearing or completely replacing other alleles of this gene in the population. As a result, some alleles can be eliminated due to random processes occurring in the population.
Horizontal gene transfer is the transfer of genetic material from one organism to another organism. The most common horizontal gene transfer is observed in bacteria. In particular, the process contributes to the spread of antibiotic resistance, due to the fact that after the emergence of antibiotic resistance genes in one bacterium, they can be quickly transmitted to other species.