Question

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.

(a)   Describe one outcome that would demonstrate that a given population has evolved (think microevolution).

(b)   Identify the dependent variable measured in the experiments described above.

(c)   Predict the results obtained by the researchers when they grew the + Cit and − Cit bacteria in the medium that contained only citrate.

(d)   The researchers claim that the + Cit mutation increases the fitness of the bacteria. Provide reasoning to support the claim.

Answer 1

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