TAXONOMY
Classification of Organisms
Created by: Kody Maclean
Why do we need Classification?
Classification is an important part of everyday life. Without it people would not be able to identify certain objects, behaviours or actions. We use classification to group types of cars together, group different food together and group different house hold objects together such as cleaning supplies. We classify these different objects together because it is important to be able to identify them and discuss them with another individual. These are the same reasons we need classification in Biology to be able to identify different organisms. To begin the topic of taxonomy with a class it may be beneficial to have a conversation with them about classification and ask them the question; “Why do we need Classification?”
What is Taxonomy?
Taxonomy is the practice of classifying organisms and was created by Carolus Linnaeus (1707-1778, Galbraith et al., 2001). Taxonomy uses physical characteristics to identify different species and organize them into different groups (Galbraith et al., 2001). The different groups are referred to as taxa (singular: taxon) and there are 7 different taxa. The seven taxa are; kingdom, phylum, class, order, family, genus and species (Ritter et al., 2002). Each taxa represents a new level of classification with species being the most specific and kingdom being the least specific taxa. Thus, taxonomy is a hierarchical classification system. Figure 1 demonstrates how different organisms are placed within the different taxa and how each level of classification becomes more specific.
To remember all the taxa and the order of classification many people use a mnemonic, which is a memory aid where a phrase is made from words with the same first letters of the words needed to be remembered. There are several different mnemonics that can be used and can be found at https://www.mnemonic-device.com/biology/taxonomy /domain-kingdom-phylum-class-order-family-genus-species/. A common mnemonic used is King Philip Came Over From Great Spain. Within each taxa there are different groups used to classify the different organisms.
Figure 1: A visual of the different Kingdoms and Domain of the species Ursus americanus, Retrieved from http://faculty.fortlewis.edu/dott_c/Bio125-ConsBio/ClassMeetings/Week01-Biodiversity/01_14ClassifyingLife_L.jpg
Kingdoms
Kingdoms began with only a two system group which consisted of the kingdoms Plantae and Animalia (Ritter et al., 2002). These groups represented plants and animals respectively and the kingdom taxa remained this way until single celled organism with both plant and animal characteristics were discovered (Ritter et al., 2002). These single celled organisms were placed in a third kingdom called Protista (Ritter et al., 2002). After further observations it was determined that certain microorganisms within the Protista kingdom shared a unique characteristic of lacking a true nucleus (Ritter et al., 2002). These microorganisms were the bacteria and cyanobacteria (also known as prokaryotes) and the fourth kingdom Monera was developed to classify these microorganisms (Ritter et al., 2002). Following the creation of the kingdom Monera the kingdom Fungi was created to by taxonomists as the difference between mushrooms and moulds with plants (Ritter et al., 2002). Therefore, a five kingdom classification system was created. This system was used until 1970 when microbiologist Carl Woese and his colleagues conducted research to determine that archaebacteria are distinct from other prokaryotes in the kingdom Monera (Ritter et al., 2002). Archaebacteria possess cell walls and ribosomes that are composed of different components than the other prokaryotes which have cell walls composed peptidoglycan which is a 3-dimensional structure made of carbohydrates and protein (Ritter et al., 2002). Due to this difference Woese and his colleagues proposed a six kingdom system which separates the kingdom Monera into the kingdoms Archaebacteria and Eubacteria. Therefore, we now use the six kingdom system when classifying organisms. Characteristics of the six Kingdoms are classified in figure 2, where different morphological and physiological characteristics are provided for each Kingdom.
Figure 2: The characteristics of the six different Kingdoms, Retrieved from, Ritter et al. (2002)
Figure 2: The characteristics of the six different Kingdoms, Retrieved from. Ritter et al. (2002)
Figure 3: , Examples of how different species are classified in differetn taxa. Retrieved from, McDarby (2001)
Figure 3 gives some examples of different organisms and how they are classified. Each organism is classified into a taxon at each level allowing for the classification of the organism. Providing an image such as this can assist students in understanding how the classification system works and examples of different taxa that exist such as the Phylum Chordata. Figure 3 also demonstrates how there is further classification by using supergroups and subgroups. When two or more groups in a taxa are found to be more similar than previous believed a supergroup can be created to further classify these organisms and show their relatedness (McDarby, 2001). Additionally, a group in a taxa may not be as related as once believed and can be split into subgroups (McDarby, 2001). The important aspects for students to get from this figure is that each organism is placed in a taxon at each level and all organisms in the same taxon share some characteristic.
Figure 4: A phylogenetic tree depicting how the different Kingdoms are thought to be related through evolution. Retrieved from, Ritter et al. (2002).
The six kingdoms can be used to create a phylogenetic tree which shows the phylogeny within the six kingdoms. Figure 4 depicts how this tree may look with the most ancestral forms present on the bottom and with the branches leading to their decedents. A theory of to how the eukaryotes, which are organisms with cells that contain a true nucleus, may have obtained their mitochondria is the theory of endosymbiosis (Di Giuseppe et al., 2003). It is believed that an earlier eukaryotic organism engulfed an aerobic Eubacteria through a similar process of phagocytosis, and these microorganisms formed a symbiotic relationship (Di Giuseppe et al., 2003). The host organism received ATP from the aerobic Eubacteria, while the aerobic Eubacteria was provided energy rich molecules and protection from the host cell (Di Giuseppe et al., 2003). Overtime it is believed that the aerobic Eubacteria became part of the host and became the mitochondria (Di Giuseppe et al., 2003). Evidence to support this theory is that the mitochondria have their own DNA and a similar double membrane as bacteria (Di Giuseppe et al., 2003). It is also believed that this same process is how cells obtained chloroplasts.
Domains
Although the six kingdom system has become popular among scientist and the scientific community there is a group of microbiologists that the kingdoms system should be replaced with a three domain classification system (Ritter et al., 2002). The three domain classification systems better reflects the evolutionary history of life and the three domains are: Archaebacteria, Eubacteria and Eukaryota (Ritter et al., 2002). These three domains better follow the evolutionary history of life that is found from DNA sequencing (Ritter et al., 2002). Figure 5 demonstrates how the six Kingdoms are related to the three Domains. The Domains act above the Kingdoms as the Domain Eukaryota encompasses all Kingdoms with eukaryotes.
Figure 5: The Three Domain classification system proposed to be used and how the six Kingdoms fit within this system.
Binomial Nomenclature
Binomial nomenclature is the system used to give each organism a unique two part scientific name (Ritter et al., 2002). The system was also created by Linnaeus and the two part system uses Latin and sometimes Greek words. Using this system creates a common language for all scientists to use despite their nationality. The first part of the scientific name is the genus of the organism. The genus is always written with a capital and can be written alone. The second part of the scientific name is the species of the organism which is also referred to as the specific epithet (Wikepidea, 2014). The species is always written in lower case and must always be written with the genus or the first letter of the genus. An example of a scientific name is Ursus maritimus or U. maritimus, which is the scientific name for the polar bear. Furthermore, the scientific name must be italicized if typed or underlined if hand written as the scientific name is always treated as if it were written in Latin (Wikepidea, 2014). The specific epithet (species) is formed in three different ways relative to the genus (Wikepidea, 2014). First the specific epithet can be formed to be a noun in apposition with the genus name meaning the genus is used to identify the specific epithet (Wikepidea, 2014). Another way is that the specific epithet can be formed to be a noun in the genitive case which connects the specific epithet to what the organism belongs too (Wikepidea, 2014). Lastly, the specific epithet can be formed to be an adjective that is based on a specific characteristics of the organism (Wikepidea, 2014).
Misconceptions
A common misconception of the species of an organism is that it is just the second part of the scientific name. For example, Homo sapiens is the scientific name for humans and many people believe that sapiens is the human’s species. However, this is incorrect as the species can not be written without the genus. Therefore, the proper term to describe the species of human’s is Homo sapiens or H. sapiens. Furthermore, some species have the same specific epithet, which is reason why the specific epithet must be accompanied by the genus. For example, a species of bird (the white throated rail) and a gazelle both have the same specific epithet of cuvieri.
Another common misconception is that common names of organisms can give a misleading ideas on the characteristics of the organism (Galbraith et al., 2001). This can lead into issues surrounding the classification of organisms by their characteristics. For example, the word “fish” is used in the naming of several animals that do not have the characteristic of a fish (Galbraith et al., 2001). Some of these animals include, starfish, crayfish, and shellfish. Although all these organism live in water like fish they have completely different characteristics from fish which can lead into issues when students are trying to understand taxonomy.
In the Activities Section see the "Proper Scientific Name" activity to address these misconceptions.
Students may also have the misconception associated with taxonomy that the classification of an organism can never change. Some students do not recognize that the classification of organisms is a theory and can be changed. Each organism is classified using different traits and characteristic. However, as new information is gathered on an organism the classification can be changed. Recently, most changes in classification is the result of new information from genetic sequencing. As a result scientists are beginning to determine that certain organisms are incorrectly classified and the organism’s classification is being changed. A way to address this misconception with students is to expose them to situations where this has occurred. The button below will take you to a news story that discusses the reclassification of a sea anemone.
Dichotomous Key
A dichotomous key is a tool developed and used by scientists to identify unknown organisms. The dichotomous key is a two part key where choices are made at each step to lead to a new choice in a key until the organism is identified (Galbraith et al., 2001). The choices in the key can either be yes/no where you look to see if the organism has or does not have a certain characteristic, or a comparison where the choice may be comparing a distinction in a characteristic. For example, a yes/no choice may be if the organism has wings or not, versus a comparison choice may be if the organism has wings bigger than its body or smaller than its body. Figure 6 below provides a visual ofr the structure of a dichotmous key and a a spider key which can be used in the creation of a dichotomous key.
Figure 6: An example of a dichotmous key (bottom of figure) that can be used to identify the different flowers. The spider key at the top of the diagram provides an example for how to organize ideas to aid in the making of the dichotomous key, Ritter et al. (2002).