they dont really require anything. however if you have nothing then most likely you will not get accepted.
some people do research and have little clinical experience.
others have a lot of clinical and basically no research experience.
and others have a bunch of both.
all three types get in.
now you have to ask yourself some questions...
for instance do you like research for instance...if you have absolutely no desire to do science research then i recommend you spend most of your time gaining clinical experience in any way shape or form. ie...volunteering, shadowing etc...
but ya might as well try a bit of everything out to see how you feel about it and pursue the things that interest you and not really get into the things that you dont particularly like.