Final answer:
Apple flies and hawthorn flies are currently distinct populations of the same species undergoing sympatric speciation. They are differentiating due to differences in host plant preference and reproduction timing, with one population preferring hawthorn trees and the other apples.
Step-by-step explanation:
Apple flies and hawthorn flies refer to distinct populations within the same species, known as the apple maggot fly, Rhagoletis pomonella. This species is currently an example of ongoing sympatric speciation, a process where new species evolve from a single ancestral species while inhabiting the same geographic region.
Hawthorn trees, along with their associated flies, are native to the U.S. The flies are host-specific and lay their eggs in hawthorn fruits. However, with the introduction of apple trees to the U.S., which often grow near hawthorn trees, some of these flies began using apple trees as their new host. This behavior led to reproductive isolation, as the two populations evolved different breeding seasons that coincided with the fruiting times of their respective host plants.
Over time, this reproductive isolation encouraged the evolution of other genetic differences between the populations.
As a result, the apple and hawthorn flies are undergoing a steady process of becoming separate species. This speciation event is a product of a change in host preference, with one population preferring hawthorn trees and the other preferring apple trees. This event is a classic example of sympatric speciation driven by host-specific behavior of parasites.