|MadSci Network: Evolution|
It does not have to be physical isolation.
It could be spatial or temporal. Scarcity of food supply or breeding habitats etc. could lead to differential selection pressure. Some members might go east to feed others go west. Some may feed in the day others at night. Some high in trees, others on ground. They may divide up the limited resources by time or location. This is how we think that bird migration and different breeding times may have evolved. I am not sure of the title of the book but I think the author is Collenveaux.
Try the links in the MadSci Library for more information on Evolution.