After a hepadnavirus inserts itself into a chromosome of an animal, fragments of the virus are passed on to all of that animal' s descendants  A hepadnavirus fragment is present in a chromosome of the zebra finch and in precisely the same location in a corresponding chromosome of the dark-eyed junco
The fact that these two bird species diverged from each other about 25 million years ago ttherefore means that the hepadnavirus is at least 25 million years old
Viruses can affect the evolution of an organism and can thereby influence the likelihood of their diverging into two species.