As human knowledge grows ever more vast, it becomes more and more difficult for the nonspecialist to keep tabs on such basic things as the classification of life. Remember back in junior high, when you “learned” that there were three basic categories of “thing”: animal, vegetable, mineral? (Actually, you probably learned to make this distinction by playing Twenty Questions, old school style–that is, out loud, with other humans, instead of on a computer.) Well, unfortunately, this distinction is now out of date.
Dealing with purely biological organisms, there are a bewildering array of classificatory schemes. At the species level, the modified Linnean system, with its binomial nomenclature (genus and species), still serves us fairly well, even if we can’t all agree on just what it is that constitutes a species.*
Whatever species concept you adhere to, though, you still need to back up the tree to figure out how all these species relate to each other. For that, you need to go back to the fundamental differences between species, that between Kingdoms. While there are several competing “big picture” divisions, I adhere to the simple one described in Scientific Style and Format: the Council of Science Editors Manual for Authors, Editors, and Publishers. In this compendious reference work, section 22.2.2. describes the big picture as follows (and, as befits an editor, I paraphrase, rather than reproduce wholesale, because I alone possess the editorial acumen to make things Really Understandable):
There are 5 kingdoms, organized into 2 superkingdoms:
- Superkingdom Prokarya, with the unique kingdom Bacteria (with 2 subkingdoms, Archaea and Eubacteria).
- Superkingdom Eukarya, with 4 kingdoms: Protoctista, Animalia, Plantae, and Fungi.
*There are two main contenders for defining a species:
- The “biological species concept”, in which a species is defined as a group of organisms capable of interbreeding and producing fertile offspring. That sounds pretty good, but what do we mean by “capable”? Biologically capable? What about organisms that would be capable of such breeding, but they are separated by unbridgeable geographic divides? Are they two separate species now? Or do we have to wait until natural selection enhances certain characters and discontinues others in the two separate populations, until they are both biologically and geographically incapable of breeding and producing fertile offspring? Ernst Mayr, who pretty much invented the concept, incorporates this problem into his definition, solving the problem head-on: species are “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.”
- The “phylogenetic species concept”, in which species are groups of organisms whose most recent common ancestor is their own species. This is a circular definition, because I haven’t figured out how to make it noncircular. One good website defines the phylogenetic species as “the smallest set of organisms that share an ancestor and can be distinguished from other such sets.”
[UPDATE: What about viruses? Where are they in the kingdoms of life? Well, they aren’t. Viruses, according to the CSE, are not living things. They “are neither prokaryotes nor eukaryotes; they are nucleic acid molecules that…are not cellular, do not grow, have no observable activity except replication, and function only within living cells” (366). So there, H1N1.]