Self-study
Support 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. █ ████████████ ████████ ██ ███████ ██ █ ██████████ ██ ███ █████ █████ ███ ██ █████████ ███ ████ ████████ ██ █ █████████████ ██████████ ██ ███ █████████ ██████ ███ ████ ████ █████ ███ ████ ███████ ████████ ████ ████ █████ █████ ██ ███████ █████ ███ █████████ █████ ████ ███ ████████████ ██ ██ █████ ██ ███████ █████ ████
Summarize Argument
The author concludes that the hepadnavirus present in the chromosomes of a zebra finch and dark-eyeed junco is at least 25 million years old. This is because after a hepadnavirus inserts itself into a chromosome of an animal, it’s based on to all of that animal’s descendants. In addition, the hepadnavirus is found in the same location in the corresponding chromosome of the finch and the junco. And, these two species diverged around 25 million years ago.
Notable Assumptions
The author assumes that having a hepadnavirus in the same corresponding location in the chromosome tells us something about when the hepadnavirus was inserted. The author also assumes the hepadnavirus didn’t insert itself into each bird’s chromosome more recently than 25 million years ago.
22.
Which one of the following, ██ █████ ████ ███████████ ███ █████████
a
Viruses can affect ███ █████████ ██ ██ ████████ ███ ███ ███████ █████████ ███ ██████████ ██ █████ █████████ ████ ███ ████████
This suggests viruses might have influenced the divergence of the two species. But this doesn’t suggest anything about the age of the hepadnavirus. Perhaps other viruses caused the species to diverge, and the hepadnavirus was inserted in each chromosome after divergence.
b
The chromosomes of ███ █████ █████ ███ ███ █████████ █████ ███████ █████████ ██ ██ █████ █████ ████ ███ █████████████
This tells us there’s no other virus in the chromosomes of the birds. But this doesn’t reveal anything about the age of the hepadnavirus.
c
When a virus ███████ ██████ ████ ██ ██████████ ███████████ ███ █████████ ██████ ██ █ ██████ █████
This strengthens by casting doubt on the possibility that the virus inserted itself into each chromosome after the divergence 25 million years ago.
d
Many bird species █████ ████ ███ █████ █████ ███ ███ █████████ █████ ███████ █████████ ██ ███ █████████████
The presence of the virus in other birds has no clear impact on how long the virus has been around.
e
The presence of █ ████████████ ██ ██ ██████ ███████ ████ ███ ██████ ███ ██████████ ██ ████ ██████████ █████████
Whether the virus affects chances of a species’ survival has no clear impact on how long the virus has been around.