What is the neutral theory of molecular evolution and its key implication?

• A. Most variation is due to positive selection at all sites
• B. Polymorphism is driven by migration
• C. Most genetic variation is due to random drift of neutral mutations rather than selection; explains high levels of polymorphism at synonymous sites among others
• D. Polymorphism is negligible

Answer: (C)

Most molecular variation comes from random genetic drift acting on mutations that do not affect fitness. In the neutral theory, many DNA changes are effectively neutral, so natural selection doesn’t systematically shape most variation. A primary consequence is that high levels of polymorphism are expected, especially at synonymous sites where a nucleotide change does not alter the amino acid sequence and is usually neutral. This provides a simple baseline: many sequence differences persist or fix due to drift rather than because they’re beneficial. It also lets us test for selection by comparing rates of changes at neutral (often synonymous) sites to those affecting proteins; when nonsynonymous changes accumulate faster than synonymous ones, it suggests positive selection, whereas slower rates imply purifying selection.