Use the following to answer questions :
A gene family has been accumulating nucleotide substitutions at a rate of 0.25 substitutions per million years.On average,there have been 80 substitutions between the common ancestor of this gene family and each of the various living forms of this gene.
-How many substitutions are likely to have evolved between any pair of living forms of genes from this gene family?
A) 20
B) 40
C) 80
D) 160
E) 320

Question

Quizplus · Accepted Answer

If the gene family has been accumulating nucleotide substitutions at a rate of 0.25 substitutions per million years, and on average there have been 80 substitutions between the common ancestor of the gene family and each of the various living forms, then we can calculate the time since divergence as follows:

80 substitutions / 0.25 substitutions per million years = 320 million years

Therefore, any pair of living forms of this gene family diverged from a common ancestor about 320 million years ago, and would have accumulated substitutions at the same rate of 0.25 substitutions per million years since then.

So, the number of substitutions that have evolved between any pair of living forms can be estimated by multiplying the time since divergence by the rate of substitution:

320 million years x 0.25 substitutions per million years = 80 substitutions

However, since we are asking for the number of substitutions between a pair of living forms, we must remember that each lineage would have accumulated substitutions independently, so we need to double the estimate:

80 substitutions x 2 = 160 substitutions.

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