house finches (a bird species) were found only in western north america until 1939, when a few birds were released on the east coast in new york city. these birds established a new breeding population in eastern north america and gradually expanded their range to the west. the western population also expanded its range to the east, and the two populations have recently come in contact. if the two forms were unable to interbreed when their expanding ranges met, it would be an example of .