Suppose you have a single population that is split in two by a change in climate, so the middle of the geographical region is uninhabitable by that species? This happens, e.g., with mountain-dwelling species in the West. When the climate was colder and wetter, they were distributed all over the region. When the climate dried and heated up, they were forced upwards into isolated mountain ranges, and since have drifted apart. The Great Plains has had the same effect on many species of birds. There are the various relic species of pupfish in the lakes of SW Nevada and SE California; speciated after one big lake started to dry up. What happened, in terms of my hand-waving mathemetical analogy, is that they inhabited a fairly flat region of evolutionary space, which develped a saddle or even a ridge thanks to external changes.