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. The cells in the 12 cell lines grown in low-glucose conditions showed the effects of which of the following processes?
1.8 1.6 1.0 5,000 10,000 5,00 20,000 Generation
a. natural selection and mutation
b. conjugation and transformation
c. natural selection and gene flow
d. gene flow and genetic drift