|MadSci Network: Evolution|
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
Try the links in the MadSci Library for more information on Evolution.