In this essay we will focus on biological classification. I will approach it first from an historical perspective, discuss the rationale for classifying things, both biological and physical, and we will learn some of the details of biological classification.
If we stop and think for a moment, classifying things is a normal and natural thing to do. But why do we do it? I think it is based upon a simple need to organize. We do it every day in many facets of life. We organize our thoughts; we jot down notes to help us remember. We organize our desks, our closets, our clothes, drawers, and, yes, even our daily activities. How do we organize, that is, on what basis? I would suggest that it is based on similarities and differences of some quality or attribute such as shape, color, size, useful purpose, etc. I used to tell my students that “suppose you went to a grocery store to buy milk, bread, bananas, and detergent. Imagine how difficult it would be if the store wasn’t organized with similar items together.” Then we would do a little exercise in naming some things we organize or classify, things like clothes, music, cars, and students to name a few. For example, we may organize clothes by summer vs. winter, music as classical, rock, country etc. and cars by manufacturer (i.e. Ford, General Motors, Chrysler, etc.) Biological classification can be traced back as far as Aristotle. The modern system can be attributed to the Swedish scientist Carolus Linnaeus (1707-1778) who sought to systematically organize all the plants native to his country. He developed a system we now know as binomial nomenclature. In other words, he arranged plants into hierarchical levels or categories and then placed the plants or taxa into these levels according to similar characteristics based mainly on appearance (morphology). To give an example using geography, consider the following.
Category Taxa
Country United States
State Illinois
County Cook
City Chicago
As you can[LB1] see the taxa are simply examples of the categories. Also note that as you proceed down the columns you are being led to more specific areas while more and more areas are being excluded. The same is true of biological taxonomy. Consider the following:
Kingdom Plantae Kingdom Animalia
Division Anthrophyta Phylum Chordata
Class Dicotlydones Class Mammalia
Order Sapindales Order Primates
Family Aceraceae Family Homnidae
Genus Acer Genus Homo
Species (acer) rubrum Species (Homo) sapiens
Red maple human
As you proceeded upward, each categorical level (and therefore, taxa) contains members that are less like each other. That is, they have fewer characteristics in common. Members of class mammalia are more like each other than they are like frogs and turtles (class amphibia). There are evolutionary implications here too. As you proceed upward, you are progressing backward in time. The various divisions (plants) of which there are many, should represent various plants before many new groups split off.
A key question here involves the origin of a similarity or difference. Does a similarity reflect inheritance from a common ancestor, (divergent evolution) or does it reflect adaptation to similar environments that do not share a common ancestor (convergent evolution)? Consider the converse statement: Does a difference reflect separate phylogenetic histories or the adaptations of closely related organisms to different environments (divergent evolution)?
In Aristotle’s time all living organisms were considered to be plants or animals. However, once the microscope was invented in the early 1600’s, and a whole new world was discovered, it became apparent that not all forms of life would fit comfortably in either category. In the 1880’s Ernst Haeckel, a German scientist proposed a third kingdom, Protista that included unicellular microscopic organism
Up until that time organisms were classified according to morphology (structural or anatomical) similarities and differences as well as motility and kingdom Protista included photosynthetic (plant-like) and heterotrophic (animal like) organisms. For other reasons there were huge differences between members of this kingdom. In 1969 R.H. Whittaker proposed the five kingdom system which added kingdom Fungi and kingdom Monera comprised of bacteria only. Monerans were put into a separate kingdom because they lack a membrane bound nucleus and have a few other differences too. They are also called prokaryotic since they lack a nucleus whereas all other kingdoms are called eukaryotes (true nucleus)
Kingdom Monera
(bacteria)
Kingdom Protista
(Spirogyra spp.(algae))
Physarum (slime mold)
Kingdom Plantae
Narcissus spp.
(King Alfred daffodil)
Kingdom Animalia
Canus lupus familiaris
With the advent of electron microscopy, many organisms were reclassified. For example, during my undergraduate years I recall studying a group called blue-green algae. Later they were renamed cyanobacteria. Thus, they were moved into a completely different kingdom. However, most classification systems were, by and large, based on structural similarities and differences, but now are based on the cellular and molecular level.
Within the last few years it has become possible to classify organisms by molecular data that is based on protein, DNA, and RNA comparisons. Remember that proteins are involved in cellular respiration (see essays VII and VIII ) in all aerobic organisms. For example, the difference in a molecule called Cytochrome c between chickens and ducks is only 3 but the difference between chickens and humans is 13. This indicates that, as we already knew, chickens and ducks are more closely related than chickens and humans. Likewise, base pair sequences in DNA and RNA can be used to establish relatedness. Base pair sequences refer to the pairing of the nucleotides along the length of the DNA molecule where adenine and thymine (A-T) pair together and guanine and cytosine (G-C) pair together. The more closely the DNA of two organisms resemble each other, the closer they are related. Conversely, the more their DNA sequences differ, the less they are related. Furthermore, as suggested above, since the normal progression in protein synthesis goes from DNA to mRNA to rRNA to amino acid sequencing that ultimately results in protein synthesis (called “central dogma”), this means that closely related organisms should have fewer differences in proteins such as Cytochrome c. In some viruses (retroviruses) the reverse occurs in which RNA codes for DNA. Based on the above newer methods, many organisms that once we thought to be closely related have been reclassified. In fact, whole new systems have arisen. In 1990 C.R. Woese et. al. used rRNA sequences to generate a “universal phylogenetic tree” predicated on a three domain classification system. They are:
Domain Archaea, Domain Bacteria, Domain Eukarya (eukaryotes, cells with a true nucleus)
In the five kingdom system all prokaryotes are grouped into kingdom Monera whereas in Woese’s system all prokaryotes are grouped into two domains (Archaea and Eubacteria, (true bacteria)). The term Monera is obsolete. Davis, S. (2012) The following diagram represents the above classification system. (Offner 2013) A really good diagram of the DNA molecule follows
Credit to: factfile.org
For an excellent account of the discovery of the correct structure of the DNA molecule I would suggest reading the book The Double Helix by James Watson. It not only was written in a way that laymen can easily understand but describes how the world situation in the 1950’s played an important role in who won the race that surely would result in a Nobel Prize. For example, this was during the McCarthy era and the great American biochemist, Linus Pauling who was also working on the DNA problem was detained in Europe and not allowed to return to the United States for some time and lost precious time. It also exemplifies the inequality of the role of women in science (Rosalind Franklin) who provided the critical X-ray diffraction photograph but never received proper recognition for her contribution.
Other classification systems have been proposed. Obviously, modern taxonomy is in a fluid state. If this seems kind of crazy, remember that all levels except perhaps species of any system are simply constructs of the human mind to organize.
Offner, S. (2013) Making the Connection – Genetics and Evolution The American Biology Teacher, 614
Davis, S. (2012) Applying the Scientific Method & Phylogenetics to Understand the Transition from Kingdoms to Domains: Does One Plus One Equal Five, Six, or Three?
The American Biology Teacher, 332
Watson, J. (1968) The Double Helix Touchstone New York
