Evolution of microbial diversity during prolonged starvation

Models of evolutionary processes postulate that new alleles appear in popul ations through random spontaneous mutation. Alleles that confer a competiti ve advantage in particular environments are selected and populations can be taken over by individuals expressing these advantageous mutations. We have studied the evolutionary process by using Escherichia coli cultures incuba ted for prolonged periods of time in stationary phase. The populations of s urviving cells were shown to be highly dynamic, even after many months of i ncubation. Evolution proceeded along different paths even when the initial conditions were identical. As cultures aged, the takeovers by fitter mutant s were incomplete, resulting in the coexistence of multiple mutant forms an d increased microbial diversity, Thus, the study of bacterial populations i n stationary phase provides a model system for understanding the evolution of diversity in natural populations.