Evolutionary Theory and Experiments With Microorganisms $ MJ Wiser and RE Lenski, Michigan State University, East Lansing, MI, United States r 2017 Elsevier Inc. All rights reserved. Abbreviations GASP Growth Advantage in Stationary Phase MOI Multiplicity of infection Glossary Adaptation A feature of an organism that enhances its reproductive success and that evolved by natural selection. Epistasis An interaction among genes, such that the effect of a mutation depends on the genetic background in which it occurs. Evolution Change in the genetic properties of populations and species over generations, which requires the origin of variation (by mutation or mixis) as well as the subsequent spread or extinction of variants (by natural selection and genetic drift). Fitness Average reproductive success of a genotype in a particular environment, usually expressed relative to another, standard genotype. Genetic drift Changes in gene frequency caused by the random sampling of genes during transmission across generations (rather than by natural selection). Mixis Production of a new genotype by recombination of genes from two sources. Natural selection Changes in gene frequency caused by specific detrimental or beneficial effects of those genes. Population Group of individuals belonging to the same species and living in close proximity, so that individuals may potentially recombine their genes, compete for limiting resources, or otherwise interact. Defining Statement Evolution in action can be studied by experiments in the laboratory using bacteria and other microorganisms with suitably rapid generation. These experiments have confirmed the main principles of modern evolutionary theory, while also providing new insights into the genetics, physiology, and ecology of microorganisms. Review of Evolutionary Theory Evolutionary theory seeks to explain observable patterns of biological diversity in terms of a few fundamental evolutionary processes. These processes are presumed not only to have operated in the past, but also to continue to operate today. Thus, they can be studied experimentally in the laboratory. Before discussing a broad range of experiments that have used microorganisms to examine evolutionary processes, the major elements of evolutionary theory will be reviewed. Evolutionary Patterns The three most conspicuous products of organic evolution are (1) the wealth of genetic variation that exists within almost every species; (2) the divergence of populations and species from one another and from their common ancestors; and (3) the manifest adaptation, or fit, of organisms to the environments in which they live. Genetic variation The existence of extensive genetic variation within species has been demonstrated by a variety of means. Variation in certain traits, such as seed shape in pea plants and blood type in humans, can be shown to have a genetic basis by careful examination of pedigrees. For many other traits, such as milk production in cows or body weight in humans, quantitative genetic analyses are required to partition the phenotypic variation that is due to genetic versus environmental influences. Biochemical and molecular techniques have also revealed extensive variation in DNA sequences and the proteins they encode. ☆ Change History: August 2016. M.J. Wiser and R.E. Lenski updated Sections “Glossary,”“Genetic and Physiological Bases of Fitness,”“Evolution of New Metabolic Functions,” and “Evolution of New Genetic Systems.” Reference Module in Life Sciences doi:10.1016/B978-0-12-809633-8.13042-0 1