Researchers grew populations of identical E. coli bacteria in a growth medium that contained a low concentration of glucose and a high concentration of citrate, a substance that is not typically consumed by E. coli. For thousands of generations, the bacteria used only glucose as an energy source and grew relatively slowly and to a low density because of the low concentrations of glucose. After about 30,000 generations, one population emerged that began to rapidly grow to a much higher density. The researchers hypothesized that the bacteria evolved the ability to use citrate as an energy source and referred to the as Cit+. To test the hypothesis, the researchers grew separate populations of the Cit+ bacteria and bacteria from the original population (Cit-) in a growth medium that contained only citrate. (a) Describe one outcome that would demonstrate that a given population has evolved.
(b) Identify the dependent variable measured in the experiments.
(c) Predict the results obtained by the researchers when they grew the Cit+ and Cit- bacteria in medium that contained only citrate.
(d) Provide a reason to support the idea that the researchers claim that the Cit+ mutation increases the fitness of the bacteria.