Succinctly describe the process of evolution by natural selection and what it means for an organism to be "adapted."

How does mutation fit in to the process of natural selection?

A tuberculosis patient starts a treatment of antibiotics.  At first the numbers of bacteria in his lungs decrease.  However, after continued treatment, the numbers again rise and, unfortunately, the bacteria isolated from the patient turn out to be antibiotic-resistant. 

Part A) Assuming that this patient is not in contact with other carriers of tuberculosis, where did this antibiotic resistant strain come from?

Part B) Relative to the period of drug treatment, when was the first resistant bacterium likely present in this patient?

Part C) Your microbiologist friend tells you that antibiotic resistant strains of bacteria are often less ‘fit’ in the absence of the antibiotic (that is, resistant strains grow slower than sensitive strains without the antibiotic around).  Suppose the tuberculosis patient stopped his antibiotic treatment and several months later the bacteria isolated from this patient are again found to be antibiotic resistant.  Given your friend’s comments, how would you explain this data?