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Journal Article

Selection for Tryptophan Auxotrophs of Escherichia coli in Glucose-Limited Chemostats as a Test of the Energy Conservation Hypothesis of Evolution

Daniel Dykhuizen
Evolution
Vol. 32, No. 1 (Mar., 1978), pp. 125-150
DOI: 10.2307/2407415
Stable URL: http://www.jstor.org/stable/2407415
Page Count: 26
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Selection for Tryptophan Auxotrophs of Escherichia coli in Glucose-Limited Chemostats as a Test of the Energy Conservation Hypothesis of Evolution
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Abstract

Evolutionary arguments are often based upon the assumption that there is selection for the organisms which most efficiently use a scarce resource. It is assumed that most often energy will be the limiting resource, so I will speak in terms of energy limitation. The most obvious formalization, but not the only one possible, is that the amount of selective advantage an organism obtains depends upon how efficient the organism is in its use of energy. Energy can be saved by eliminating unneeded processes and structures. Thus evolution to auxotrophy can be explained by an efficiency argument. Zamenhof and Eichhorn (1967), after showing that several auxotrophic mutants of Bacillus subtilis are selected over wildtype B. subtilis in energy-limited chemostats, used an efficiency argument to explain their results. This argument, formally stated as the energy conservation hypothesis, says that the rate of selection is determined by the amount of energy saved by eliminating unneeded processes. I tested this hypothesis using tryptophan mutants of Escherichia coli competing against isogenic wild-type E. coli in a glucose-limited chemostat with excess tryptophan. The selection for the mutant was found to be three orders of magnitude larger than the theoretical percentage of the energy budget which could be saved. No demographic differences could be found to explain this large selection. Predicted relative differences in selection rates between different mutants in the same environment or the same mutant in different environments were not found. Therefore, I concluded that the selection for tryptophan auxotrophs was not caused by differences in energy efficiency between the auxotroph and wild-type. This implies that selection for energy efficiency is less important in evolution than traditionally assumed.

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