Maybe it's the sympatric speciation. What is that?
Speciation without the usual geographical isolation (which would make it "allopatric" speciation). One part of the population starts making a living in a new way, experimenting with eating something different. For a while, the populations stay completely intermingled, but after a time divergent selection pressures make them look and act differently. Interbreeding stops. Eventually it becomes impossible.
This particular scenario is not thought to account for that much of the diversity we see.