In an eight-year experiment, twelve 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 twenty-four 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?