A scientist notes that some individual fish survive better in low-oxygen environments than others of the same species. She decides to compare the genes that code for hemoglobin, the protein that transports oxygen in the blood. The table below shows the DNA sequences for hemoglobin isolated from individual fish of this species. It also shows each individual's phenotype. How did these differences in DNA sequence arise, and should the scientist characterize the phenotypes as helpful, harmful, or neutral?