Correct option is B
Purifying selection (also known as negative selection) is a type of natural selection that removes deleterious alleles from a population. It acts to preserve functional integrity by eliminating harmful mutations. Under purifying selection, the evolutionary patterns of synonymous and non-synonymous mutations are significantly different.
Analysis of the Given Statements:
Statement A (Correct): "Most non-synonymous mutations are selected against."
- This is true under purifying selection because non-synonymous mutations alter amino acid sequences and can negatively impact protein function.
- Since many of these mutations are harmful, natural selection removes them, leading to their lower fixation in the population.
Statement B (Correct): "Synonymous mutations can accumulate."
- This is also true because synonymous mutations do not change the amino acid sequence of proteins.
- Since they are selectively neutral, they are more likely to accumulate over time, primarily through genetic drift.
Statement C (Incorrect): "The ratio of non-synonymous to synonymous substitutions is high."
- This is false under purifying selection.
- The dN/dS ratio (ratio of non-synonymous (dN) to synonymous (dS) substitutions) is typically low (<1) under purifying selection because non-synonymous mutations are removed more frequently than synonymous ones.
Statement D (Incorrect): "Non-synonymous sites accumulate mutations at higher rates."
- This is false under purifying selection.
- While non-synonymous mutations occur, they are not fixed in the population because they are selectively removed.
- Instead, synonymous sites tend to accumulate mutations at a higher rate since they are not exposed to strong selective pressures.
Thus, the correct answer is Option 2 (C and D) because these two statements do not align with purifying selection.
