Researchers grew populations of identical Escherichia coli (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 concentration 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 them 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. Describe one outcome that would demonstrate that a given population has evolved.