You compare homologous nucleotide sequences among several pairs of species with known divergence times. A pair of species that diverged 1 million years ago has two nucleotide differences, a pair that diverged 2 million years ago has four nucleotide differences, and a pair that diverged 3 million years ago has six nucleotide differences. You have sequence data for another pair of species for which the divergence time is unknown. There are five nucleotide differences between them. Based on your clock, when did their line of ancestry diverge?

Question

Quizplus · Accepted Answer

Based on the data provided, we can use a linear regression of nucleotide differences against divergence time to estimate the time of divergence for the unknown pair of species. Using the pairs of species with known divergence times and nucleotide differences, we can calculate a molecular clock rate of 2 nucleotide differences per million years (slope of the regression line). We can then use this rate to estimate the divergence time for the unknown pair of species: 
(5 nucleotide differences) / (2 nucleotide differences per million years) = 2.5 million years ago. 
Therefore, the best choice is C.

You Compare Homologous Nucleotide Sequences Among Several Pairs of Species