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Question 19

Differences between the nucleotide base sequences of mitochondrial DNA can be used to estimate the length of time since two species diverged from a common ancestor. The greater the number of differences, the longer the time that has elapsed since the two organisms shared a common ancestor.

The graph below represents the line of best fit for the differences in mitochondrial DNA between pairs of species in the bear family (Ursidae) plotted against the time since the two species diverged from a common ancestor.

A graph showing percentage of mitochondrial DNA base differences against time since divergence in millions of years for Ursidae. The x-axis is labeled 'Time since divergence (millions of years)' and ranges from 0 to 20 with major grid lines at intervals of 5. There are 10 subdivisions between each major interval, so each small grid square represents 0.5 million years. The y-axis is labeled 'Percentage of mitochondrial DNA bases that are different' and ranges from 0 to 8% with major grid lines at intervals of 1.0%. There are 5 subdivisions between each 1.0% interval, so each small grid square represents 0.2%. A straight line of best fit starts at the origin (0, 0) and goes up to (20, 7.2). A point representing the divergence of brown bears and polar bears is plotted at a percentage difference of 1.8%.

a.

Calculate the rate of mitochondrial DNA change using the line of best fit shown in the graph. Give your answer to three significant figures.

[2]
b.

The mutation rate in Ursidae can vary by as much as 15% between lineages. Use the graph to calculate the estimated time since brown bears and polar bears diverged (where the percentage difference is 1.8%), and the range over which this estimate may vary.

[3]

Analysis Questions

  1. A Level
  2. /Biology
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