A molecule that can be used as a molecular clock has a neutral mutation rate of one mutation per 5 million years. How many years ago did two species share a common ancestor if the molecules found in these two species differ by a total of eight mutations?
1.5 million years
10 million years
20 million years
40 million years
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The question is saying that a molecule starts out the same in two species that have just split. It says the molecule mutates (changes in some small way) once every 5 million years.
So, after 5 million years, you expect to see one change in the molecule. After 10 million years, you expect to see 2 changes. So, how long do you have to wait to see 8 changes?
The answer is 20 million years. I took this test 3 times and every time the answer was 20 million years! :)