Final answer:
Populations change over time due to evolution, which is the alteration in the genetic composition of a population over generations, primarily driven by mutations and natural selection. Individuals do not evolve; rather, the population as a whole does through differential survival and reproduction. The correct term for the provided description is Evolution (A).
Step-by-step explanation:
Evolution is the process through which all organisms undergo changes in their DNA over time, resulting in population changes from generation to generation. A population's genetic makeup changes over generations as a result of individuals with specific alleles reproducing differently from one another. The diversity within populations is largely driven by mutations, which are changes in an organism's DNA. Natural selection may cause some mutations to be harmful and swiftly eliminated, while other mutations may be advantageous and spread widely.
However, it's crucial to realize that individuals do not evolve when talking about the malleability of DNA and how it affects species. Evolution is the term used to describe the shift in the average value of a characteristic in a population over time, not the changes in a person's genes as they age. A change in a trait, such as the average size of a finch's bill, can be measured when a sufficient number of individuals possessing specific advantageous traits live and procreate.
Therefore, the correct answer to the student's question is A. Evolution. It is the most accurate term that describes the process through which the genetic makeup of a population changes over time.