Questions: Positive (Directional) Selection on Beneficial Mutations

Question 1

A genomic study of MHC antigen-binding regions across primate species finds synonymous substitutions at 0.015 per site and non-synonymous substitutions at 0.042 per site. What is the most likely interpretation?

• A. These regions are under strong purifying selection to conserve protein function
• B. These regions are evolving neutrally with no selective pressure on amino acid sequence
• C. These regions are under positive selection, with amino acid changes being actively favored
• D. The elevated non-synonymous rate reflects a higher baseline mutation rate in MHC genes

Answer: (C)

dN/dS = 0.042/0.015 = 2.8, well above 1. The dN/dS ratio tests whether amino acid changes accumulate faster than the neutral baseline (represented by synonymous changes). When dN/dS > 1, amino acid changes are being *favored* — the molecular signature of positive selection. This makes biological sense for antigen-binding regions: new amino acid variants that recognize novel pathogens provide strong fitness advantages. Option A (purifying selection) would yield dN/dS < 1. Option D conflates mutation rate with substitution rate — synonymous and non-synonymous mutations arise at similar rates, but selection filters them differently.

Question 2

A domesticated crop has a genomic region flanking a disease-resistance gene showing unusually low genetic diversity compared to the rest of the genome. This pattern is most consistent with:

• A. A neutral bottleneck affecting only that chromosomal region
• B. Balancing selection maintaining multiple alleles at that locus
• C. A selective sweep where a beneficial allele fixed rapidly and dragged nearby neutral variants to fixation
• D. Purifying selection removing deleterious mutations from that region over many generations

Answer: (C)

A selective sweep occurs when a beneficial allele rises rapidly toward fixation. Because recombination doesn't have time to break up associations between the favored allele and nearby neutral variants, flanking variation gets dragged to fixation alongside it (genetic hitchhiking), producing a long haplotype block with unusually low diversity. Option A (neutral bottleneck) would affect the entire genome proportionally, not one region. Option B (balancing selection) maintains multiple alleles and tends to *increase* diversity at a locus, the opposite of what's observed. Option D (purifying selection) removes harmful alleles but doesn't cause dramatic regional diversity reductions.

Question 3

A dN/dS ratio less than 1 indicates that a gene is under positive selection because non-synonymous mutations are being preferentially retained over synonymous ones.

• T. True
• F. False

Answer: False

A dN/dS ratio less than 1 is the signature of *purifying* (negative) selection, not positive selection. It means non-synonymous mutations accumulate *more slowly* than the neutral baseline — harmful amino acid changes are being removed before they can fix. Positive selection is indicated by dN/dS *greater than* 1, where amino acid changes accumulate faster than the neutral rate because they are being favored. This is counterintuitive: most students assume 'selection' means retaining changes, but purifying selection works by removing them.

Question 4

Synonymous substitutions are used as a neutral baseline in dN/dS analysis because they change the amino acid sequence without affecting the DNA sequence, making them invisible to selection.

• T. True
• F. False

Answer: False

This reverses the relationship. Synonymous substitutions change the *DNA* sequence without changing the *amino acid* — not the other way around. Because the amino acid is unchanged, the protein function is unaffected, so these mutations are largely invisible to natural selection and accumulate at the background mutation rate. Non-synonymous substitutions change the amino acid and are therefore visible to selection. Synonymous = silent at protein level; non-synonymous = amino acid change.

Question 5

Why does a dN/dS ratio greater than 1 specifically indicate positive selection, rather than simply indicating that a gene has a higher overall mutation rate?

Model answer: The dN/dS ratio controls for mutation rate by using synonymous substitutions as an internal baseline. Synonymous mutations arise at approximately the same rate as non-synonymous mutations within the same gene — they experience the same underlying mutation process — but accumulate at the neutral rate because they are invisible to selection. By comparing non-synonymous to synonymous rates *within the same gene*, the ratio isolates the effect of selection on amino acid changes, net of mutation rate differences. A higher overall mutation rate would elevate both dN and dS proportionally, leaving the ratio near 1. When dN/dS > 1, it means something beyond mutation rate is driving amino acid change — specifically, positive selection.

This is the elegance of the dN/dS approach: it uses the gene as its own control. The synonymous substitution rate is the yardstick by which the non-synonymous rate is judged. A ratio above 1 cannot be explained by a higher mutation rate, because the denominator (dS) would also increase. It can only be explained by selection that preferentially retains amino acid-changing mutations.