Question

If a gene is evolving ____________, we expect the he ratio of the rate of nonsynonymous to synonymous mutations (dN/dS) to be equal to one. If the dN/dS ratio is greater than one, it suggests that the gene is evolving ____________.

1) neutrally; purifying selection
2) under positive selection; neutrally
3) neutrally; under positive selection
4) under purifying selection; neutrally
5) under purifying selection; under positive selection

Answer 1

The correct answer is (3) neutrally; under positive selection. A dN/dS ratio equal to one implies neutral evolution, whereas a ratio greater than one suggests the gene is evolving under positive selection, indicating adaptive evolution.

Step-by-step explanation:

If a gene is evolving neutrally, we expect the ratio of the rate of nonsynonymous to synonymous mutations (dN/dS) to be equal to one. If the dN/dS ratio is greater than one, it suggests that the gene is evolving under positive selection. Therefore, the correct answer is 3) neutrally; under positive selection.

In the context of molecular evolution, synonymous mutations are those that do not result in a change to the amino acid sequence of a protein, whereas nonsynonymous mutations do change the amino acid sequence and can affect the protein's function. The dN/dS ratio is used as a gauge of evolutionary pressure: a dN/dS ratio of less than one suggests that purifying selection is occurring, meaning that nonsynonymous mutations are being selected against because they are deleterious.

A dN/dS ratio of exactly one implies neutral evolution, where mutations do not affect the organism's fitness. Lastly, a dN/dS ratio greater than one indicates adaptive evolution, where advantageous nonsynonymous mutations are being favored by natural selection.