What change can occur to a small population as a result of genetic drift?
There is real possibility that the small population will interbred.
This could produce a number of individuals that have a mutation that would make them unable to deal with life in that environment.
Or the same change would allow them to do exceedingly well.
This mutation would, over a very long time, produce a different species.
Speciation that results when a population is separated by a physical barrier is allopatric speciation.
Sympatric speciation is speciation that occurs without physical separation of members of the population.
Some examples of sympatric changes occurred long ago when people live in small villages. There was very little travel between villages and people did look very similar.
One example is seen in areas were groups of people have red hair, blue eyes, and freckled skin. Of course, they are still the same species but they were different from others who live in far away villages.