The apple maggot is a pest of hawthorn plants and apple trees. Before the introduction of apple trees 200 years ago, apple maggot flies laid their eggs only on hawthorns. After the introduction of apple trees, these flies started to lay eggs in apples or hawthorns. These flies tend to live and reproduce in the type of plants they were born in. Therefore, hawthorn flies tend to mate with other hawthorn flies and apple flies tend to mate with other apple flies. Some genetic differences are currently found between these two groups of flies. If these flies become different species in the future, this will be an example of what