Question

question 2(multiple choice worth 2 points)
(05.02a nc)
a molecule that can be used as a molecular clock has a neutral mutation rate of one mutation per 10 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 six mutations?
6 million years
30 million years
10 million years
60 million years

Explanation:

Step 1: Understand the mutation rate

The molecular clock has a neutral mutation rate of 1 mutation per 10 million years. This means that for each mutation, 10 million years pass.

Step 2: Calculate total time for 6 mutations

If there are 6 mutations, we multiply the time per mutation by the number of mutations. So we calculate \( 6\times10 \) million years.

\( 6\times10 = 60 \)? Wait, no, wait. Wait, actually, when two species diverge, the mutations accumulate in both lineages. Wait, no, the problem says "the molecules found in these two species differ by a total of six mutations". Wait, maybe I misread. Wait, the mutation rate is 1 mutation per 10 million years. So if the total number of mutations between them is 6, then the time since common ancestor is \( 6\times10 \) million years? Wait, no, maybe each lineage has mutations. Wait, no, the problem says "a neutral mutation rate of one mutation per 10 million years". So if the two species differ by 6 mutations, then the time since common ancestor is \( 6\times10 \) million years? Wait, no, let's think again.

Wait, the molecular clock: the number of mutations between two species is related to the time since they shared a common ancestor. If the mutation rate is \( r = 1 \) mutation per 10 million years, and the total number of mutations between the two species is \( m = 6 \), then the time \( t \) is \( t = m\times10 \) million years? Wait, no, maybe it's that each mutation takes 10 million years, so 6 mutations would take \( 6\times10 = 60 \) million years? Wait, but let's check the options. The options are 6 million, 30 million, 10 million, 60 million.

Wait, maybe I made a mistake. Wait, when two species diverge from a common ancestor, the mutations in each lineage add up. So if the mutation rate is 1 mutation per 10 million years per lineage, then the total number of mutations between the two species is the sum of mutations in each lineage. But the problem says "the molecules found in these two species differ by a total of six mutations". So maybe the total number of mutations is 6, so the time since common ancestor is \( 6\times10 \) million years? Wait, no, 1 mutation per 10 million years. So 6 mutations would be \( 6\times10 = 60 \) million years? Wait, but let's see:

Wait, the formula is: time since common ancestor = (number of mutations) × (time per mutation). So number of mutations is 6, time per mutation is 10 million years. So \( 6\times10 = 60 \) million years. So the answer should be 60 million years? Wait, but let's check again.

Wait, maybe the mutation rate is 1 mutation per 10 million years, so for each mutation, 10 million years. So 6 mutations would be 6×10 = 60 million years. So the correct answer is 60 million years? Wait, but let's check the options. The options are 6, 30, 10, 60. So 60 million years is an option. Wait, but maybe I messed up. Wait, maybe the mutation rate is 1 mutation per 10 million years, so the time for 6 mutations is 6×10 = 60 million years. So the answer is 60 million years.

Wait, but let's re-express. The mutation rate is 1 mutation per 10 million years. So if the two species have 6 mutations different, then the time since common ancestor is 6×10 = 60 million years. So the correct option is 60 million years.

Answer:

D. 60 million years