Environmental fluctuations do not select for increased variation or population-based resistance in Escherichia coli
Shraddha Madhav Karve, Kanishka Tiwary, S Selveshwari, Sutirth Dey
In nature, organisms often face unpredictably fluctuating environments. However, little is understood about the mechanisms that allow organisms to cope with such unpredictability. To address this issue, we used replicate populations of Escherichia coli selected under complex, randomly changing environments. We assayed growth at the level of single cells under four different novel stresses that had no known correlation with the selection environments. Under such conditions, the individuals of the selected populations had significantly lower lag and greater yield compared to the controls. More importantly, there were no outliers in terms of growth, thus ruling out the evolution of population-based resistance. We also assayed the standing phenotypic variation of the selected populations, in terms of their growth on 94 different substrates. Contrary to extant theoretical predictions, there was no increase in the standing variation of the selected populations, nor was there any significant divergence from the ancestors. This suggested that the greater fitness in novel environments is brought about by selection at the level of the individuals, which restricts the suite of traits that can potentially evolve through this mechanism. Given that day-to-day climatic variability of the world is rising, these results have potential public health implications. Our results also underline the need for a very different kind of theoretical approach to study the effects of fluctuating environments.