Final answer:
Positive selection at the MHC locus was determined by observing a greater rate of non-synonymous mutations compared to synonymous mutations, signaling that these mutations confer an advantage that is selected for.
Step-by-step explanation:
The MHC locus shows evidence for positive selection by having a rate of non-synonymous mutations that is greater than the rate of synonymous mutations. When the ratio (often represented as ω) of non-synonymous to synonymous substitutions in a protein-coding gene is greater than 1 (ω > 1), it indicates that non-synonymous substitutions are favored by natural selection. This is because such mutations change the amino acid composition of the protein and can give rise to advantageous traits that are then selected for. In contrast, a ratio less than 1 suggests purifying selection, where deleterious non-synonymous mutations are being removed from the population, and a ratio equal to 1 indicates neutral evolution, where the mutations do not affect the organism's fitness.