The Hawthorne and apple maggot flies appear to have become reproductively isolated from each other largely because e.)they have adapted to different hosts.
Initially, these two species are the same species and their host is the hawthorn tree. Speciation occurs when European settlers introduced apple trees in North America. Some flies of species born in the Hawthorne tree accept it as a host and continue to feed on this plant and mate with others flies on the Hawthorn tree. Flies that are born on apple trees accept it as a host and continue to feed on this tree and mate with other flies in this tree. Slowly after some time, these flies differentiated into two species.
This type of speciation which occurs without physical separation Or geographical separation is called sympatric speciation.
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