Re: why sharks never evoled?

Dear ?, 
WHen you write sharks never evolved I'm not sure what you mean.  Sharks 
have been around for millions of years and there are many forms of sharks. 
I think I'll answer your question more generally.  Some types of animal are 
often described as being 'living fossils' meaning that they appear not to 
have changed for millions of years.  Examples would be the horseshoe crab 
or the coelocanth.  This may be because the have lived in much the same 
environment for all that time and there have been no evolutionary pressures 
to select for new shapes - they just suit their environment.  However, 
bacteria and viruses are always finding new ways to attack living things 
and are evolving all the time so it is clear that the immune systems of 
the 'living fossil' creatures must have evolved to keep pace with such 
threats.  Think of how the native Americans suffered when faced with 
diseases they had never encountered before when new settlers arrived - they 
had no natural immunity to these diseases.  Even if sharks do not look as 
if they have evolved much their immune systems must have or they'd be dead. 
Remember - appearances can be deceiving!

Admin note:
Actually, sharks and their kin have gone through significant evolution
of their body shapes and sizes, and are hardly "living fossils". See:
http://www.elasmo.com/bin/menu_home.html