The  Isthmus  of  Panama, which created a land bridge connecting North
   and  South America and simultaneously separated the Caribbean Sea from
   the  Pacific Ocean, formed roughly 3 million years ago. Nancy Knowlton
   and  colleagues  took  advantage  of  a  natural  experiment  that the
   formation of the Isthmus of Panama had performed on several species of
   snapping  shrimps  (Figure  26-9).  After  identifying seven Caribbean
   species  of  snapping  shrimp,  they  matched  each one with a Pacific
   species to form a pair. Members of each pair were more similar to each
   other in structure and appearance than either was to any other species
   in  its  own  ocean.  Analysis  of  allozyme  allele  frequencies  and
   mitochondrial  DNA  sequences  confirmed that the members of each pair
   were one another's closest genetic relatives.

   Figure 26-9. A snapping shrimp (genus Alpheus).

   The Knowlton team's interpretation of these data is that, prior to the
   formation  of  the  isthmus,  the ancestors of each pair were a single
   species.  When  the  isthmus  closed, the seven ancestral species were