Dynamics of adaptation in fluctuating environments - an experimental evolution study with Escherichia coli

Abstract

Resource conditions in nature can fluctuate markedly and how organisms adapt to survive in these conditions is of great interest in the fields of evolutionary biology and ecology. Experimental evolution using microbes has been shown to be effective in answering general evolutionary questions. Using this technique, I studied the bacterium Escherichia coli adapting to fluctuating environments to investigate the evolution of growth traits and the dynamics of adaptation. My results have provided general insight into bacterial adaptation which may allow for better prediction of growth trait evolution in a range of conditions. (1) I have shown that evolution in both predictable and unpredictable environments resulted in the evolution of a reduced lag phase, an increased growth rate and a higher maximum population size. My results suggest that bacteria do not adapt to conditions by anticipating the timing of the resource renewal. (2) I found that a trade-off exists for evolved populations between a reduced lag phase and a higher mortality rate in all environments, and propose this as an explanation as to why some bacteria retain a lag phase. (3) I show that the dynamics of adaptation do not differ between populations adapted to conditions which involved varying periods of time in stationary phase between transfers. There seem to be different mutations for different traits, with mutations to the lag reducing first, followed by growth rate, and finally population size. These findings highlight the dynamics of growth trait evolution in environments in which a complex interplay exists between reproducing and growing faster than competitors, and being able to survive in starvation conditions.