EVOLUTION OF COMPETITIVE ABILITY IN ESCHERICHIA COLI

The study reported here was undertaken to determine whether the amount of adaptation of a population to a novel environment is more nearly related to the number of generations spent in the new environment or to the absolute time spent there. The issue is relevant to the interpretation of an empirical finding concerning the rate at which amino-acid substitutions become incorporated into proteins (or nucleotide substitutions become incorporated into the genome) over evolutionary time-namely, that the rate of such substitutions seems to be a function of absolute time and independent of the generation interval of the organisms involved (Sarich and Wilson, 1966, 1967; Nei, 1975; Fitch, 1976). Some authors (e.g., Nei, 1975; Kimura, 1979, and earlier) interpret the existence of this "molecular clock" as implying that most substitutions incorporated into the genome are selectively neutral or nearly neutral. On the other hand, as Fitch (1976) has pointed out, so little is known about actual mechanisms of selection that selectively driven allele substitutions might also have certain clocklike properties. Unfortunately, the issues are not amenable to direct experimental test in higher eucaryotes because their generation interval is not easily manipulated and because the time scale involved is too long. We have studied the question in bacterial chemostats using the procaryote Escherichia coli. A chemostat is a bacterial culture vessel having an input aperture for the addition of nutrient medium and an overflow aperture for the drainage of excess exhausted medium, cells, and cellular debris (Monod, 1950; Novick and Szilard, 1950b). The rate of division of cells within the chemostat can be varied by a factor of ten by appropriate adjustment of the rate of inflow, but an equilibrium will eventually be reached in which the number of new cells created by division is exactly balanced by the number of cells washed out in the overflow (Kubitschek, 1970; Kossen, 1979). Thus, chemostats provide an environment in which the rate of cell division can be manipulated but population size is held constant. For E. coli, whose natural habitat is the mammalian intestine, a chemostat represents a novel environment. Escherichia coli populations in nature typically grow very quickly when food passes through the intestinal cavity but otherwise may not grow at all (Koch, 1971). In nature, E. coli respires largely anaerobically in an environment rich in competitors (Shedlofsky and Freter, 1974; Savage, 1977). In the chemostat, cells grow at a constant moderate rate in the presence of excess oxygen and in the absence of competing species. Populations of E. coli maintained in chemostats become progressively more adapted to their environment. Although the absolute amount of adaptation of a population is difficult if not impossible to measure, it is possible to assess the amount of adaptation of one population relative to that of another population. Such an assessment for two bacterial strains involves inoculation of a chemostat with cells from both strains, and the rate of change in the relative frequencies of the two strains provides a measure of their relative "fitness"; in the present context, relative fitness as operationally defined above may be considered as equivalent to the relative degree of adaptation. The experiments reported here were designed to study the increase in fitness of E. coli pop-