Regarding several species that all belong to the same family

Regarding several species that all belong to the same family: species A diverged from species B 120,000 years ago and on a particular homologous gene, they have 18 nucleotide differences. Species C diverged from species D 85,000 years ago; they have 13 nucleotide differences on that homologous gene. Species E has 3 nucleotides that are different from species F for that same gene. Which of the following is a reasonable estimate of the time since the divergence of species E and F?

A) 5000 years

B) 20,000 years

C) 50,000 years

D) 100,000 years

Solution

B) 20,000 years

Justification:

For species A and B, nucleotide difference is 18 and evolution time is 120000 years. So, time per nucleotide is 120000/ 18 = ~6000 years per nucleotide.

Similarly, for C and D, time per nucleotide is 85000/13 = ~6000 years

Now, calculate for E and F i.e. 6000* 3 = ~20000 years