Subject: How did the bat's wing (pentadactyl limb) evolve?

I know that the pentadactyl limb is a homology, because the human arm, 
whale flipper, dog leg, etc. all evolved from that limb of a common 
ancestor.  I'm trying to think in terms of structure vs. function: the 
limb has evolved to provide the bat with flight,  but I'm still confused 
as to how the limb changed (at random) over time, while still allowing the 
new population/species to be successful and reproduce while in competition 
with the other ground animals, before their wings enabled them to fly?
(thus allowiing them to exploit a new niche).  I am also confused about 
the whale's flipper.  I understand the end structure and function of the 
limb for different mammals, and the common ancestor, but not about the 
intermediate phase.  Can you help?

Re: How did the bat's wing (pentadactyl limb) evolve?