Final answer:
Changes in genotype frequency within a population over time represent evolutionary change, thus making option A (True) the correct answer to the student's question. It is the basis of population genetics and is influenced by various evolutionary mechanisms like natural selection, genetic drift, and more.
Step-by-step explanation:
Evolution is defined in the context of population genetics as a change in the frequency of an allele in a population over time. For instance, the allele frequency in the ABO blood type system such as the frequency of IA, is the total number of that specific allele compared to all copies of the ABO gene in the population. If the genotype frequency changes over time, this does indeed mean evolution has occurred.
Option A, stating that 'Evolution has not occurred, because the frequency of the heterozygotes is different 5 years later compared with the original population.' is incorrect because a change in the frequency of heterozygotes over time does represent an evolutionary change. Option B suggests that evolution has also not occurred if the frequency of heterozygotes remains the same, which also would be an incorrect statement. In contrast, statements indicating that a change in frequency means evolution has occurred, like in Option D, are correct.
It's crucial to distinguish between different mechanisms of evolution, such as natural selection, genetic drift, gene flow, and mutations. These processes can influence allele frequencies in different ways. For example, natural selection tends to increase the frequency of alleles that are advantageous, while genetic drift involves random changes in allele frequencies and can occur in any direction.