The occasional occurrence of genetic variation in individual members of populations is a prerequisite for biological evolution. Insight into the molecular mechanisms of genetic variation is gained both by following individual mutational events and by DNA sequence comparison of related genes and genomes. Spontaneous alterations of the nucleotide sequences can be attributed to many different molecular mechanisms. These can be grouped into three strategies with different qualities with regard to their contributions to biological evolution. These strategies are: (1) a local change in the nucleotide sequence, (2) the rearrangement of DNA segments within the genome and (3) the acquisition by horizontal gene transfer of a DNA segment originating in another type of organism. These processes depend both on a number of different non-genetic factors and on specific enzymes produced by so-called evolution genes. Evolution gene products act as generators of genetic variations and/or as modulators of the frequency of genetic variation. The presence of evolution genes implies a duality of the genetic information carried in the genome. The more classical genes act for the benefit of each individual and serve for the fulfilment of the individual life, while the evolution genes act for the benefit of the evolutionary development of populations, which serves for the expansion of life and insures the building up and the replenishment of a rich biodiversity under widely different living conditions. The knowledge on the mechanisms of molecular evolution has broad philosophical implications and it forms a solid basis for the evaluation of conjectural risks of genetic engineering and its biotechnological applications.