Answer
One example would be if a difference in mating behavior appeared between the two populations, preventing mating between them because males and females from different populations would then not mate (behavioral isolation). Another example would be if a mutation spread through one population causing its member to switch from diurnal to nocturnal activity (temporal isolation): no mating would occur between populations since males and females of opposite populations would not be awake at the same time.
Work Step by Step
There are many possible answers-- scan through pp.502-503 and p.510 for possible mechanisms, chose two, and use them for your answer.
