In Dianne Dodd's Experiments with Drosophila Pseudoobscura,different Populations Were Established

Essay

In Dianne Dodd's experiments with Drosophila pseudoobscura,different populations were established that were raised on diets of either maltose or starch.When flies from the different populations were allowed to mate,some flies mated with flies fed on a different food source,but there was a preference for mating with flies raised on the same food source.What does this phenomenon illustrate about the timescale of speciation?