Tajima's D is a statistical test used in population genetics to assess the neutrality of mutations in a given DNA sequence. It compares the number of segregating sites (variations) in a sample to the average number of nucleotide differences. A positive Tajima's D value suggests an excess of low-frequency variants, indicating possible balancing selection or population expansion, while a negative value indicates an excess of high-frequency variants, which may suggest a recent population decline or selective sweep. This test is particularly useful for understanding the evolutionary history of populations and can help identify regions of the genome that may be under selection. By analyzing DNA sequences from different individuals, researchers can gain insights into the genetic diversity and evolutionary pressures acting on a species.