Mutations,the changes in the structure of genetic material respecting the rules of writing of genetic information, are the only source of variability and evolutionary innovation at the level of the species; in their absence, biological evolution would sooner or later stop.If more fundamental changes occur in the environment, organisms that would not be capable of undergoing mutation processes and thus of adapting to changes in the environment, would die out.However, at the level of individual populations, gene flow and genetic recombination are the main source of evolutionary innovation.In fact, even in species without regular sexual reproduction, for example bacteria, in which recombination must occur with the participation of relatively ineffective processes of transformation and transfection, the recombination is responsible for the formation of new alleles 10x more frequently than mutations. Mutations can be differentiated according to a number of criteria.According to their physical nature, they can be classified as point mutations, mutations at the level of DNA sections (chromosomes) and at the level of the entire genome.Mutations can be encountered in the nuclear DNA and in the organelle DNA.“Mutations” that occurred during RNA transcription and that are thus not at all connected to the DNA exhibit the character of vanishing mutations, i.e. the mutations whose manifestation becomes weaker over time.In single-cell organisms with a short generation time and long mRNA lifetime, these mutations can peter out over many generations.On the other hand, mutation can occur in multicellular organisms only during ontogenesis or in the adult organism, so that the cells containing the given mutation can be present only in some tissues.If these somatic mutations do not reach the germinal organs and tissues, they are of no evolutionary importance.
According to their effect on the biological fitness, mutations can be differentiated as selectionally positive mutations (useful or also advantageous – increasing the biological fitness of their bearers), selectionally negative (detrimental or also disadvantageous – reducing the biological fitness of their bearers) and selectionally neutral (with no effect on the biological fitness of their bearers).In studying genetic drift, it was found that it is necessary to also differentiate the extremely numerous category of slightly negative mutations (see V.6).This category includes those mutations that, while they have a negative selection coefficient, this is simultaneously so low that their fate in the studied population tends to be determined by genetic drift (see Chap. V) or genetic draft (see IX.5.2) rather than by selection.