what is one reason why many mutations may be deleterious?

One widely cited reason is that most random genetic changes disrupt protein function or regulatory elements, reducing an organism’s fitness. In other words, many mutations are deleterious because they tend to impair biochemical processes or gene regulation, and organisms carrying those mutations are less likely to survive or reproduce. This creates a natural tendency for deleterious variants to be selected against over generations, though their persistence can occur in small populations or due to other factors like genetic drift, linkage, or environments where the mutation’s effect is context-dependent.