D. Some organisms, which were previously characterized together, were determined to be genetically very different.
Explanation:
Domains are the highest taxons within the biological classification of living beings. Like other taxons, domains are used to group living beings in a scientifically based classification.
They were added to the classification system after the other taxons were already established. The presence of domains was necessary to group individuals in relation to evolutionary factors. This was necessary because some organisms, previously characterized together, were determined to be genetically very different.
