Final answer:
A change in the base subunit sequence during DNA replication can lead to variation within an organism, as these changes, known as mutations, are essential for the process of natural selection and evolution.
Step-by-step explanation:
A change in the base subunit sequence during DNA replication can lead to various outcomes, one of which is the introduction of variation within an organism. This happens because errors in copying the DNA sequence can occur, resulting in a mutation. Such a mutation can change the function or structure of a protein, leading to phenotypic variation. Mutations are not solely negative; they are essential for evolution and are the bedrock for genetic diversity. Without mutation, evolution through natural selection could not occur because there would be no genetic variation from which advantageous traits could be selected.
Mutations can be of different types, including spontaneous mutations due to errors during DNA replication or induced mutations caused by external agents such as ultraviolet rays, chemicals, or radiation. Whether a mutation is beneficial, harmful, or neutral depends on how it affects the organism's ability to survive and reproduce.