Final answer:
Substitution rates of synonymous and non-synonymous mutations are used to infer the type of selection acting on a gene. The ratio of these rates indicates if there is purifying (w<1), neutral (w=1), or positive (w>1) selection. Positive selection is suggested when a gene mutation confers an advantage and leads to a w ratio greater than 1.
Step-by-step explanation:
We can use substitution rates to look for evidence that natural selection has shaped the nucleotide sequence of genes by analyzing the pattern of synonymous and non-synonymous nucleotide substitutions. In protein-coding genes, synonymous substitutions do not change the amino acid, whereas non-synonymous substitutions do. The ratio of non-synonymous to synonymous substitutions (dN/dS, where "w" represents this ratio) informs us about the type of selection acting on a gene. A w ratio less than 1 suggests purifying selection, where deleterious non-synonymous mutations are removed from the population. A w ratio equal to 1 implies neutral evolution, where there is no selective advantage or disadvantage for non-synonymous mutations. On the other hand, a w ratio greater than 1 indicates positive selection, where non-synonymous substitutions are favored and increase in frequency within the population because they confer some sort of advantage.
For example, if gene A evolves into gene B with a single nucleotide replacement and this new version of the gene provides a selective advantage, it is likely that positive selection pressures have acted on this gene. The w ratio for this gene would then likely be greater than 1. Additionally, the evolutionary history of life on Earth and the role of natural selection can be inferred by looking at these variations in nucleotide sequences over time, as well as considering the impact of gene duplication events that allow one copy to mutate and potentially adopt new functions without compromising the function of the original copy.