the processes of genetic drift, mutation, and
natural selection cause populations to diverge from one another
Different mutations are likely to
arise by chance in two allopatric populations, further augmenting their differences.
Finally, no two geographically isolated populations will experience exactly the
same selective conditions, and any differences in selective conditions can cause
the populations to diverge by natural selection. In the long run, these processes
may lead to multiple forms of reproductive isolation between these populations.
This is because gene flow between geographically isolated populations may be
permanently eradicated when the members of one population lose the ability to
breed successfully with members of the other population due to differences in
geographic range, genetics, behavior, or reproductive physiology.