Natural selection is the central concept in our formulation of evolutionary theory. Thus, it is important to study it and to understand its causes and effects by careful thought, observation, and experimentation. The effects-how selection changes gene frequencies have been extensively studied by population geneticists. The causes-how genetic variation within an environment creates selective differences have been less well studied because of many difficulties-such as understanding the development of the phenotype and defining the important components of the environment (19). This review describes experiments with microorganisms that provide insight into the causes of natural selection and consequently the evolutionary process, insights that are difficult or impossible to obtain if evolutionary biology concentrates solely on multicellular eukaryotic organisms. A statement of the importance of studying microorganisms to increase understanding of the evolutionary process is required because of the near total exclusion of microbiology from the neo-Darwinian synthesis (61, 101). This exclusion was not intentional but occurred in part because bacterial species and their phylogenetic relationships were nearly impossible to define until recently. Consequently, microbiology has remained the least evolution-oriented of the