There is a case of a species of arctic birds, (arctic frigate birds as I recall), that separated, one group migrating to the east, the other group migrating to the west. Many decades—and generations—later, the two groups met more or less on the other side of the earth (still in the arctic, but approximately 180° of longitude from where they parted company), and there had been sufficient changes (mutations) in the DNA of each group that they could mate, but could produce no offspring. They had become, by definition, different species.

Nature experiments all the time. It doesn't take much. It's pretty random, but the mutations that increase an individual's (it always happens in an individual, not in a whole group) chances of surviving to reproduce and pass on the newly modified gene may eventually lead to the emergence of a new species. That's the way it works.

Don Firth