Abstract
Adaptation to an environment is enabled by the accumulation of beneficial mutations. How do adaptive trajectories and pleiotropic effects of adaptation change in response to "subtle" changes in the environment? Since there exists no molecular framework to quantify "subtle" environmental change, designing experiments to answer this question has been challenging. In this work, we address this question by studying the effects of evolution in environments which differ solely in the way sugars are presented to a bacterial population. Specifically, we focus on glucose and galactose, which can be supplied to an E. coli population as a mixture of glucose and galactose, lactose, or melibiose. We evolve six replicate populations of E coli for 300 generations in these three chemically correlated or "synonymous" environments, and show that the adaptive responses of these populations are not similar. When tested for pleiotropic effects of fitness in a range of non-synonymous environments, our results show that despite uncorrelated adaptive changes, the nature of pleiotropic effects is largely predictable based on the fitness of the ancestor in the non-home environments. Overall, our results highlight how subtle changes in the environment can alter adaptation, but despite sequence-level variations, pleiotropy is qualitatively predictable.