Answer:
The separation of the single population (allopatric speciation) led to speciation.
Step-by-step explanation:
"Speciation" refers to the evolutionary process of organisms in order to become a "distinct" species. There are several factors affecting speciation and these are: Genetic Drift, Natural Selection, Mutation and Geographic Isolation.
The situation above is an example of the factor called "Geographic Isolation." This type of isolation is reproductive in nature. This happens when organisms are separated or divided by a barrier. In the case above, the fruit flies were divided into different cages. One cage was for fruit flies who fed on maltose-based foods, while the other cage was for fruit flies who fed on starch-based foods.
More specifically, this case may also be referred to as "allopatric speciation." It happens when a single population is divided. Thus, over time the separated species acquire genetic differences. These differences prevent them from breeding with each other once they are reunited. This is the reason why the maltose-based flies prefer to mate only maltose-based flies as well.