In  late  1986,  a  Florida  dentist  tested positive for HIV. Several
   months  later,  he  was  diagnosed with AIDS. He continued to practice
   general  dentistry  for  two  more years, until one of his patients, a
   young  woman with no known risk factors, discovered that she, too, was
   infected  with HIV. When the dentist publicly urged his other patients
   to have themselves tested, several more were found to be HIV positive.
   Did  this  dentist  transmit  HIV to his patients, or did the patients
   become infected by some other means?

   At  first  glance,  this  situation  appears  to  be a long way from a
   discussion  of  evolution;  however,  the movement of a virus from one
   individual  to  another  is  similar  to  the founding of a new island
   population  by  a  small  number  of  migrants.  Gene flow between the
   ancestral  population  and  the new population is nonexistent, and the
   populations  are  free  to  diverge,  as  a  result  of genetic drift,
   adaptation to different environments, or both.

   If  the  dentist  passed  his  HIV infection to his patients, then the