Plants Among the Diversity of Organisms

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).

Major groups and current ways of grouping of organisms

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.

Controversial as this change has been, shifts among groupings of the Eukarya are even more controversial, not because the data are suspect, but because biologists differ on how best to organize the new information with the old. Organisms in the five-kingdom approach of the recent past are now distributed among four kingdoms of the Domain Eukarya and the two domains of prokaryotes, Domain Bacteria and Domain Archaea.

This change among groupings brings up a problem for botanists. What do you do if the organisms you study are evicted from the plant kingdom? Are you still a plant scientist if you no longer study plants? Many of the ousted groups are included in plant biology textbooks with the justification that, because the groups share many of the features of plants, it's appropriate for botanists to study them.

Classifications are based on current knowledge, which is constantly changing, so rearrangements are bound to occur along with differences of opinion about what belongs where. Rarely do all parties agree. Some of the old group names survive the advent of new classification schemes and are useful ways to discuss informally some groups.