Classification schemes are in a state of flux because of the availability of large volumes of data generated by molecular sequencing of DNA and RNA. As might be expected, disagreements among biologists are common. For example, not all biologists believe widely different‐appearing and behaving organisms should be grouped together just because they have similar DNA base pair sequences. But, the cladists do (and are willing to debate the doubters).
In the middle of the eighteenth century, Linnaeus' ideas transformed biological classification. In the latter half of the nineteenth century, Darwin revolutionized biology with an irrefutable theory of evolution. At the end of the twentieth century, molecular sequencing is changing the phylogeny of the entire tree of life. Appropriately enough, a major adjustment has already been made at the roots of the tree: There appear to be three main lines of development from the primitive milieu. The hodge‐podge of prokaryotes (unicellular, non‐nucleated organisms) clearly belong to two separate groups: the Bacteria and the Archaea (Archaebacteria). The nucleated organisms (eukaryotes)— plants, animals, and so forth— fit in a separate lineage, the Eukarya (Figure . The Linnaean hierarchy is modified and a new name added for these three “super kingdoms,” the Domain.