CHEMOSTATS USED FOR STUDYING NATURAL-SELECTION AND ADAPTIVE EVOLUTION

Chemostats are open systems in which nutrients are continually added at a constant rate and spent medium plus cells removed at the same rate, such that a constant volume is maintained. Thus, they are designed to provide a constant, homogeneous environment in which the cells grow at a constant rate. Chemostats are used in two ways to study evolution. The first approach attempts to characterize natural selection in terms of how selection coefficients change as either the genetics or the environment is changed. Two strains are mixed and the initial selective difference between the strains is measured over the first 50 generations. The second approach follows the course of adaptive evolution of a population in a particular environment. A single initial strain is monitored over hundreds to thousands of generations to determine the evolutionary changes. The advantage of the first approach is that the genetic differences are known and the effects of these differences in various environments can be explored. The advantage of the second approach is that the actual evolutionary trajectory of fitness changes is followed. © 1993