Bacteriophage ?X-174 was forced to reproduce at high temperatures on the host Salmonella typhimurium as a substitute of its normal host Escherichia coli to form two separate lines of viruses and marked as TX and ID, set aside separately even though both are exposed to the similar conditions. Both the viruses developed the capacity to reproduce in the new host at high temperatures. Among the two lines, one line has the capacity to reproduce on Escherichia coli still existed, and the other line loses its ability. The bacteriophage possess 11 genes and there are 15 DNA changes in TX strain situated in 6 unusual genes, whereas in ID strain there are 14 changes situated in four different genes. In seven cases, the modifications to the two strains are same, involving a large deletion, but still the similar alterations developed in each line in a dissimilar order.
The variation in the order of mutation fixed among the two dissimilar lines denotes that the order is not expected, although a number of the fixed mutations are identical in the two lines, the mutations are not fixed in the similar order. It is expected that the mutations in a third virus line possibly fixed in an unusual order, but include at least some of the identical mutations, for instance those that are the same between TA and ID and looks like that the mutation in a third line would occupy the A and F proteins, because the fixed mutations generally occur in those two.