MODULE #1: Biology: The Study of Life
Alternate Forms of Taxonomy
Before we leave this discussion of taxonomy, it is important for you to know that the classification that you see in this course will not necessarily be the classification system that you see in another biology course. That's because biologists have different ideas about classification. As a result, we might classify an organism in a particular kingdom, for example, but another biology book might classify that same organism in a different kingdom. It might be hard for you to believe that there can be arguments about which kingdom a particular organism belongs in, but there are! In Module #4, for example, you will learn about the slime molds. Some biology books place these odd creatures in kingdom Protista, while others place them in kingdom Fungi. You will learn why this disagreement occurs when you study slime molds.
Not only is there disagreement about which kingdoms, phyla, classes, etc., to place organisms in, there is also disagreement on which classification system to use. The classification system that we have just taught you is based on one first developed in the 1700s by a devout Christian, Carrolus
(kair' uh lus) Linnaeus (lih nay' us). It is typically called the five-kingdom system, because it uses five kingdoms. It is the one used by the majority of biology courses on the high school and college level. However, it is important to realize that there are other classification systems that are used by some biologists and some biology courses.
For example, some biologists propose that five kingdoms really are not enough. They suggest that kingdom Monera, for example, contains organisms that are just too different from one another to justify putting them in the same kingdom. As a result, they propose splitting Monera into two kingdoms. In addition, some propose splitting kingdom Protista into two kingdoms as well, because some unicellular (single-celled) eukaryotic organisms do not have certain organelles that are present in most other unicellular eukaryotic organisms. These biologists therefore think that the lack of certain “standard” organelles is reason enough to put these organisms into a completely separate kingdom. As biologists start splitting one or more of the standard five kingdoms into several smaller kingdoms, the number of kingdoms, of course, goes up. Some biologists recommend using an eight-kingdom system. Some biologists propose classification systems that have more than twenty kingdoms. However, since the five-kingdom system is the most widely used system, we will stick with it.
Not only is the number of kingdoms in creation a point of disagreement among biologists, some biologists propose that we should scrap the five-kingdom classification system altogether and move to what is called the three-domain system. Since this system has been gaining some popularity in the field of biology, we should discuss it to some extent, even though we will not use it in this course. The three-domain system classifies all living things into one of three large domains: Archaea (ar kay' uh), Bacteria, and Eukarya (yoo' kair ee' uh).
The Eukarya domain contains all organisms with eukaryotic cells. From our five-kingdom classification system, then, the Eukarya domain would contain all members of kingdoms Protista, Fungi, Plantae, and Animalia. The organisms that our five-kingdom system puts in kingdom Monera would go into either the Archaea domain or the Bacteria domain, depending on certain characteristics. Those prokaryotic organisms that live in very extreme environments such as boiling hot springs or incredibly salty lakes belong in domain Archaea, while those prokaryotic organisms that live in more “normal” environments would belong in domain Bacteria.
Once you have decided the domain in which an organism should be placed, you then assign it a kingdom, phylum, class, etc. Most users of the three-domain system have only one kingdom in Archaea and only one kingdom in Bacteria, but they have many kingdoms in Eukarya. Some users of the three-domain system have kingdoms Protista, Fungi, Plantae, and Animalia in the Eukarya domain, but most split domain Eukarya into many, many different kingdoms. Let's summarize the three-domain system with a figure so that you can you understand it a bit better.
Now you might ask yourself what the reasoning behind the three-domain system is. After all, it is similar to the five-kingdom system in that it still uses kingdom, phylum, class, etc. However, it simply adds a grouping called “domain,” and depending on those who use it, the system might have several more than five kingdoms. Well, the main rationale behind the three-domain system is that those who use it believe in the hypothesis of evolution, which we will discuss in detail in an upcoming module. In this hypothesis, all life on earth descended from one (or a few) “simple” life form (or forms) that lived on earth billions of years ago and was (or were) formed through abiogenesis. As a result, all organisms are “related” to one another in some way, and the three-domain system tries to separate organisms based on those relationships. The Archaea are supposed to be most closely-related to the original life form or forms that were the result of abiogenesis, while the Bacteria are more distantly related, and the Eukarya are even more distantly related.
As you will learn when we study the hypothesis of evolution in depth, there is precious little evidence for such an idea and quite a bit of evidence against it. As a result, it does not make sense to us to base a classification system on such a tenuous hypothesis. Instead, it makes more sense to base our classification system on the observable similarities among organisms. This is the essence of what Carrolus Linnaeus developed in the 1700s, and it has served biology well since that time.
Since we have touched on a classification system that has been inspired by the hypothesis of evolution, we should at least mention a classification system that has been proposed by those who believe that the earth and the life on it were specially created out of nothing by God. This classification system, usually called baraminology (bear' uh min ol' uh jee), attempts to determine the kinds of creatures that God specifically created on earth. Indeed, the word “baraminology” comes from two Hebrew words used in Genesis: bara, which means “create,” and min, which means “kind.” Thus, baraminology is the study of created kinds.
Those who work with baraminology think that God created specific kinds of creatures and that He created them with the ability to adapt to their changing environment. As time went on, then, these created kinds did change within strict limits that we will discuss later on in the course. This led to a greater diversity of life on the planet than what existed right after creation. As a result, baraminologists think that all organisms we see on the planet today came from one of the many kinds of creatures that God created during the creation period discussed in the first chapter of Genesis. Baraminologists, then, try to define groupings called “baramins.” Any organisms that exist within a baramin came from the same originally-created organism. For example, some baraminologists place domesticated dogs, wild dogs, and wolves into the same baramin because they believe that God created a basic kind of creature called a “dog,” and the various forms of dogs and wolves that we see today are simply the result of that basic kind of creature adapting to a changing environment.
Although we think that there is a lot of evidence in favor of this new classification scheme, we still do not think that it should be used in this course. It is still relatively new and not fully developed. We doubt that it will be fully developed for many, many years to come. As a result, we think that the five-kingdom system still provides the best overall means by which to classify the organisms of God's creation, and we will limit ourselves to that system. Nevertheless, we will mention the other systems (the three-domain system and baraminology) from time to time, so it is important that you understand the basics of each.