Abstract
The evolutionary responses of organisms to changes in environmental factors such as temperature and pH have been of longstanding interest, because they are naturally variable and can often reach stressful or even lethal levels. Laboratory evolutionary experiments permit the quantitative study of adaptation to both constant and variable environments and to both benign and stressful environments. In this study, we use the bacterium Escherichia coli to study evolutionary responses over 2,000 generations to changes in environmental pH. Populations of E. coli (in six-fold replication) were exposed to one of six different conditions: constant benign ancestral pH of 7.2, constant stressful acidic pH of 5.4, constant stressful alkaline pH of 8.0, constant benign novel pH of 6.6, daily alternation between pH of 5.4 and 8.0, or a random daily environment of pH 5.4, 6.6, 7.2, or 8.0. We are currently examining several questions concerning the resulting evolutionary adaptation. First, are there fitness tradeoffs: do improvements in fitness in a new pH environment result in a fitness loss at other pH levels? Second, is adaptation to stressful environments more rapid and extensive than to benign environments? Results to date suggest highly significant adaptive evolution to acid stress and more modest fitness gains to alkaline stress, accompanied by significant tradeoffs in fitness to other environments for both alkaline and acid adaptations.