I'm always fascinated by the gene's ability to do what is necessary.... like if we (humans)were forced back into the sea, would we naturally just start getting webbed fingers and toes( ala Waterworld), without mutation? Just by the genes activating the "waterborn" subroutines, instead of the airborn ones

A gene cannot alter its own behaviour. Rather, we humans (at least most of us!) just don't possess the gene which produces webbed feet. Or, they are inactive. So say we are forced back into the sea, many of us are going to die and thus our genes. The gene responsible for webbed feet will slowly rise in numbers in the gene population thus giving us webbed feet! What I'm hoping to say is that some percentage of humans would be possessing the inactive gene for webbed feet and their offspring will be the ones who survive.