Species of fruit fly larvae in the genus Rhagoletis each feed on a particular kind of fruit. Rhagoletis pomonella feeds on the small red fruit of the hawthorn tree. In 1865, farmers in the Hudson River valley found that R. pomonella flies had begun attacking their apples and then spread to apple orchards in adjacent areas of Massachusetts and Connecticut. These now separate varieties of flies, the apple and haw flies, usually don't interbreed with each other because their periods of mating coincide with the different ripening times of apples and hawthorn fruit. Each variety is becoming specialized to feed and reproduce in its own particular microhabitat and may be transitioning to separate species.If the apple and haw flies become distinct enough to be separate species, their evolution is an example of _________.
a. allopatric speciation
b. behavioral isolation
c. gametic isolation
d. sympatric speciation