Final answer:
Nonsynonymous DNA polymorphisms are genetic variants that alter phenotypes by changing the amino acid sequence and thus the function of proteins. These include missense and nonsense mutations and can affect the organism's fitness.
Step-by-step explanation:
Genetic variants that affect phenotypes through changes in protein function are called nonsynonymous DNA polymorphisms. These variants lead to changes in the amino acid sequence of proteins, potentially altering protein structure and function. Types of nonsynonymous mutations include missense mutations where a different amino acid is incorporated, and nonsense mutations that create a premature stop codon resulting in a truncated, usually nonfunctional, protein. On occasion, such changes can be beneficial, granting a selective advantage under specific environmental conditions.