THINK: A scientist finds what she thinks is a new species of…

THINK: A scientist finds what she thinks is a new species of rodent on a small Pacific island. However, some similar-looking rodents inhabit some nearby islands. She mates the new rodent with the nearby rodents and gets viable (living) but sterile offspring. Why?  [Hint:  2 categories of allopatric speciation are: Dispersal = which occurs when a few members of a species move to a new geographical area Vicariance = which occurs when a natural situation arises to physically divide organisms]