The following questions refer to the information and figure below.
In this eight-year experiment,12 populations of E.coli,each begun from a single cell,were grown in low-glucose conditions for 20 000 generations.Each culture was introduced to fresh growth medium every 24 hours.Occasionally,samples were removed from the populations,and their fitness in low-glucose conditions was tested against that of members sampled from the ancestral (common ancestor)E.coli population.

-Which term best describes what has occurred among the experimental populations of cells over this eight-year period?