Question

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.

Answer 1

Researchers tested the hypothesis that Cit+ E. coli populations evolved the ability to metabolize citrate by measuring growth in a citrate-only medium. Growth of Cit+ confirmed evolution, and the ability to use citrate likely increased bacterial fitness by providing an additional energy source when glucose was limited.

Step-by-step explanation:

The question addresses the concept of evolution in bacterial populations, specifically focusing on E. coli's ability to utilize different energy sources. A possible outcome demonstrating evolution would be the Cit+ populations’ ability to grow on citrate-only medium, indicating a heritable change allowing citrate metabolism.

The dependent variable measured in these experiments would be the growth rate or population density of the E. coli in the citrate-only medium.

If the Cit+ bacteria grew in the citrate-only medium but the Cit- did not, this would support the hypothesis that Cit+ bacteria had evolved the ability to metabolize citrate. The Cit+ mutation would be considered to increase fitness if these bacteria could use the more abundant citrate in their environment, potentially allowing for higher growth rates and survival when glucose is scarce.