Convergent evolution.
<P>
   Fast  swimmers  such  as  tuna (a fish), penguin (a bird), and seal (a
   mammal) all have stramlined bodies.
<P>
   They  evolved  independently  but  made similar adaptations in form to
   perform the same function of moving through water.
<P>
   Similar  adaptations among unrelated organisms to similar environments
   is called convergent evolution.