There is no "choosing" in animals and evolution.
There are however different types of of reproductive isolating mechanisms observed in fruit flies, such as gametic isolation (female eggs cannot be fertilized), ethological isolation (lack of attraction and mating), hybrid inviability (offspring are not viable) and hybrid sterility (offspring are sterile), which all lead to the same thing, e.g.re speciation.
In any case interbreeding capabilities have nothing to do with species differentiation. Many species of animal interbreed.
I agree many species of animals interbreed. However, there appear to be boundaries that as Dr. Ekkehard Lönnig stated, "cannot be transgressed." A fruit fly remains forever a fruit fly. Lönnig's conclusion? Mutations cannot transform an original species into an entirely new one. This conclusion agrees with all the experiences and results of mutation research of the 20th century taken together as well as with the laws of probability.