Long-term experimental evolution in Escherichia coli

-We assess the degree to which adaptation to a uniform environment among independently evolving asexual populations is associated with increasing divergence of those populations. In addition, we are concerned with the pattern of adaptation itself, particularly whether the rate of increase in mean fitness tends to decline with the number of generations of selection in a constant environment. The correspondence between the rate of increase in mean fitness and the within-population genetic variance of fitness, as expected from Fisher's fundamental theorem, is also addressed. Twelve Escherichlia coli populations were founded from a single clonal ancestor and allowed to evolve for 2,000 generations. Mean fitness increased by about 37%. However, the rate of increase in mean fitness was slower in later generations. There was no statistically significant within-population genetic variance of fitness, but there was significant between-population variance. Although the estimated genetic variation in fitness within populations was not statistically significant, it was consistent in magnitude with theoretical expectations. Similarly, the variance of mean fitness between populations was consistent with a model that incorporated stochastic variation in the timing and order of substitutions at a finite number of nonepistatic loci, coupled with substitutional delays and interference between substitutions arising from clonality. These results, taken as a whole, are consistent with theoretical expectations that do not invoke divergence due to multiple fitness peaks in a Wrightian evolutionary landscape. The two most conspicuous features of biological evolution are adaptation of organisms to their environment and divergence of populations and species from each other. The contributions of natural selection, chance events, and historical constraints to adaptation and divergence are not easily separable. The divergence of populations may simply reflect their adaptation to different environments. Alternatively, genetic differences between populations may channel subsequent evolution so as to promote their further divergence even in the same environment (Wright 1932, 1982, 1988). And chance events, including both random genetic drift and the stochastic origin of mutations (Clarke et al. 1988), may give rise to differences between formerly identical populations subjected to uniform selection pressures (see, e.g., Cohan and Hoffmann 1986; Lenski 1988a). Bacterial populations have been used for some time to study evolutionary processes, such as mutation and natural selection (see, e.g., Luria and Delbruck * Present address: Center for Microbial Ecology, Michigan State University, East Lansing, Michigan 48824-1325. Am. Nat. 1991. Vol. 138, pp. 1315-1341. ? 1991 by The University of Chicago. 0003-0147/91/3806-0001$02.00. All rights reserved. This content downloaded from 130.238.188.089 on September 07, 2019 05:56:16 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). LONG-TERM EXPERIMENTAL EVOLUTION IN E. COLI 1325 TIME (Days) O? 0 75 150 225 300 0 F-I I ,