Spontaneous mutation, selection, and isolation are key elements in biological evolution. Molecular genetic approaches reveal a multitude of different mechanisms by which spontaneous mutants arise. Many of these mechanisms depend on enzymes, which often do not act fully at random on the DNA, although a large number of sites of action can be observed. Of particular interest in this respect are DNA rearrangement processes, e.g., by transposition and by site-specific recombination systems. The development of gene functions has thus to be seen as the result of both DNA rearrangement processes and sequence alterations brought about by nucleotide substitutions and small local deletions, insertions, and duplications. Prokaryotic microorganisms are particularly appropriate for studying the effects of spontaneous mutation and thus microbial evolution, as they have haploid genomes, so that genetic alterations become rapidly apparent phenotypically. In addition, bacteria and their viruses and plasmids have relatively small genomes and short generation times, which also facilitate research on evolutionary processes. Besides the strategy of development of gene functions in the vertical transmission of genomes from generation to generation, the acquisition of short DNA segments from other organisms appears to be an important strategy in microbial evolution. In this process of horizontal evolution natural vector DNA molecules are often involved. Because of acquisition barriers, the acquisition strategy works best for relatively small DNA segments, hence at the level of domains, single genes, or at most operons. Among the many enzymes and functional systems involved in vertical and horizontal microbial evolution, some may serve primarily for essential life functions in each individual and only secondarily contribute to evolution. Others, however, might serve primarily for evolution and thus exert their biological functions at the level of populations rather than at that of a single organism.
