Lesson 14.1: Form and Function

Lesson Objectives

Define taxonomy, and understand why scientists classify organisms.

Describe Linnaean taxonomy and binomial nomenclature.

 

Introduction

Billions of years of evolution on Earth have resulted in a huge variety of different types of organisms. For more than two thousand years, humans have been trying to organize this great diversity of life. The classification system introduced by the Swedish botanist Carolus Linnaeus in the early 1700s has been the most widely used classification for almost 300 years.

 

 

Taxonomy

Scientific classification is a method by which biologists organize living things into groups. It is also called taxonomy. Groups of organisms in taxonomy are called taxa (singular, taxon). You may already be familiar with commonly used taxa, such as the kingdom and species. A kingdom is a major grouping of organisms, such as plants or animals. A species includes only organisms of the same type, such as humans (Homo sapiens) or lions (Panthera leo). The modern biological definition of a species is a group of organisms that are similar enough to mate and produce fertile offspring together. In a classification system, kingdoms, species, and other taxa are typically arranged in a hierarchy of higher and lower levels. Higher levels include taxa such as kingdoms, which are more inclusive. Lower levels include taxa such as species, which are less inclusive.

 

 

This type of hierarchical classification can be demonstrated by classifying familiar objects. For example, a classification of cars is shown in Figure 14.1. The highest level of the classification system includes all cars. The next highest level groups cars on the basis of size. Then, within each of the size categories, cars are grouped according to first one and then another trait. Higher taxa (for example, compact cars) include many different cars. Lower taxa (for example, compact cars that are blue and have two doors and cloth seats) contain far fewer cars. The cars in lower taxa are also much more similar to one another.

 

Figure 14.1: Cars can be classified, or grouped, on the basis of various traits. In this classification, the most inclusive groups are the size categories, such as all compact cars or all mid-size cars. The most exclusive groups in this classification share several additional traits, including color, number of doors, and type of seats. Note that just one group for each trait is further divided as an example. (19)

 

 

Why do biologists classify organisms? The major reason is to make sense of the incredible diversity of life on Earth. Scientists have identified millions of different species of organisms. Among animals, the most diverse group of organisms is the insects. More than one million different species of insects have already been described. An estimated nine million insect species have yet to be identified. A tiny fraction of insect species is shown in the beetle collection in Figure 14.2.

 

 

Figure 14.2: Only a few of the more than one million known species of insects are represented in this beetle collection. Beetles are a major subgroup of insects. They make up about 40 percent of all insect species and about 25 percent of all known species of organisms. (4)

 

As diverse as insects are, there may be even more species of bacteria, another major group of organisms. Clearly, there is a need to organize the tremendous diversity of life. Classification allows scientists to organize and better understand the basic similarities and differences among organisms. This knowledge is necessary to understand the present diversity and the past evolutionary history of life on Earth.

 

Early Classification Systems

One of the first known systems for classifying organisms was developed by Aristotle. Aristotle was a Greek philosopher who lived more than 2,000 years ago. He created a classification system called the “Great Chain of Being” (See Figure 14.3). Aristotle arranged organisms in levels based on how complex, or “advanced,” he believed them to be. There were a total of eleven different levels in his system. At the lower levels, he placed organisms that he believed were less complex, such as plants. At higher levels, he placed organisms that he believed were more complex. Aristotle considered humans to be the most complex organisms in the natural world. Therefore, he placed them near the top of his great chain, just below angels and other supernatural beings.

Aristotle also introduced two very important concepts that are still used in taxonomy today:

 

 

Figure 14.3: The Great Chain of Being was Aristotle’s way of classifying organisms. The basis of Aristotle’s classification was the presumed complexity of organisms. On that basis, Aristotle placed plants near the bottom of the classification and humans near the top. (16)

 

genus and species. Aristotle used these two concepts in ways that are similar to, but not as precise as, their current meanings. He used the term species to refer to a particular type of organism. He thought each species was unique and unchanging. He used the term genus (plural, genera) to refer to a more general grouping of organisms that share certain traits. For example, he grouped together in the same genera animal species with similar reproductive structures.

As early naturalists learned more about the diversity of organisms, they developed different systems for classifying them. All these early classification systems, like Aristotle’s, were based on obvious physical traits of form or function. For example, in one classification system, animals were grouped together on the basis of similarities in movement. In this system, bats and birds were grouped together as flying animals, and fishes and whales were grouped together as swimming animals.

 

Linnaean System of Classification

The most influential early classification system was developed by Carolus Linnaeus. In fact, all modern classification systems have their roots in Linnaeus’ system. Linnaeus was a Swedish botanist who lived during the 1700s. He is known as the ”father of taxonomy.” Linnaeus tried to describe and classify the entire known natural world. In 1735, he published his classification system in a work called Systema Naturae (”System of Nature”).

 

 

Linnaean taxonomy divides all of nature into three kingdoms: animal, vegetable (or plant), and mineral. (The mineral kingdom does not include living organisms, so it is not discussed further here.) Both plant and animal kingdoms are subdivided into smaller and smaller categories of organisms. An updated version of Linnaean taxonomy is shown in Figure 14.4.

Linnaean Classification System (Revised)

The classification in Figure 14.4 includes a few more taxa than Linnaeus identified. However, it follows the same general plan as Linnaeus’ original taxonomy. The taxa are below:

 

Kingdom—This is the highest taxon in Linnaean taxonomy, representing major divi- sions of organisms. Kingdoms of organisms include the plant and animal kingdoms.

Phylum (plural, phyla)—This taxon is a division of a kingdom. Phyla in the animal kingdom include chordates (animals with an internal skeleton) and arthropods (animals with an external skeleton).

Class—This taxon is a division of a phylum. Classes in the chordate phylum include mammals and birds.

Order—This taxon is a division of a class. Orders in the mammal class include rodents and primates.

Family—This taxon is a division of an order. Families in the primate order include hominids (apes and humans) and hylobatids (gibbons).

 

Figure 14.4: This is an updated version of Linnaeus’ original classification system. In this classification system, organisms are classified into a hierarchy of taxa. First, all organisms are divided into kingdoms. Further subdivisions place organisms in smaller, more exclusive taxa, all the way down to the level of the species. (7)

 

Genus—This taxon is a division of a family. Genera in the hominid family include

Homo (humans) and Pan (chimpanzees).

Species—This taxon is below the genus and the lowest taxon in Linnaeus’ system. Species in the Pan genus include Pan troglodytes (common chimpanzees) and Pan paniscus (pygmy chimpanzees).

 

To remember the order of the taxa in Linnaean taxonomy, it may help to learn a mneumonic, a sentence to help remember a list, in which the words begin with the same letters as the taxa: k, p, c, o, f, g, and s. One sentence you could use is: King Philip came over for green sugar. Can you think of others?

 

Table 14.1 shows the classification of the human species. The table also lists some of the physical traits that are the basis of the classification. For example, humans are members of the animal kingdom. Animals are organisms capable of independent movement. Within the animal kingdom, humans belong to the mammal class. Mammals are animals that have fur or hair and milk glands. At each lower taxon, additional physical traits further narrow the group to which humans belong. The final grouping, the species sapiens (as in Homo sapiens), includes only organisms that have all of the traits listed in the table.

Table 14.1:

 

Taxon Name Traits1

Kingdom Animal Organisms capable of mov- ing on their own.

Phylum Chordate Animals with a notochord (flexible rod that supports the body).

Class Mammal Chordates with fur or hair and milk glands.

Order Primate Mammals with collar bones, grasping hands with fingers.

Family Hominid Primates with three- dimensional vision, rela- tively flat face.

Genus Homo Hominids with upright pos-

ture, large brain.

Species sapiens Members of the genus Homo with a high forehead, thin skull bones.

 

1 Only one or two traits per taxon are listed in the table as examples. Additional traits may be needed to properly classify species. (Source : http://en.wikipedia.org/wiki/ Linnaean_taxonomy)

Although Linnaeus grouped organisms according to their physical similarities, he made no claims about relationships between similar species. Linnaeus lived a century before Charles Darwin, so the theory of evolution had not yet been developed. Darwin explained how evolution, or changes in species over time, can explain the diversity of organisms (see the Evolutionary Theory chapter). In contrast, Linnaeus (like Aristotle before him) thought of each species as an unchanging ”ideal type.” Individual organisms that differed from the species’ ideal type were considered deviant and imperfect.

 

Binomial Nomenclature

The single greatest contribution that Linnaeus made to science is his method of naming species. This method, called binomial nomenclature, gives each species a unique, two- word name (also called a scientific or Latin name). Just like we have a first and last name, organisms have a distinguishable two word name as well. The two words in the name are the genus name and the species name. For example, the human species is uniquely identified by its genus and species names as Homo sapiens. No other species has this name.

 

 

Both words in a scientific name are Latin words or words that have been given Latin endings. The genus name is always written first and starts with an upper-case letter. The species name is always written second and starts with a lower-case letter. Both names are written in italics.

As another example, consider the group of organisms called Panthera. This is a genus in the cat family. It consists of all large cats that are able to roar. Within the genus Panthera, there are four different species that differ from one another in several ways. One obvious way they differ is in the markings on their fur as shown in Figure 14.5, Panthera leo (lion species) has solid-colored fur, Panthera tigris (tiger species) has striped fur, and the other two Panthera species have fur with different types of spots. As this example shows, the genus name Panthera narrows a given cat’s classification to big cats that roar. Adding the species name limits it to a single species of cat within this genus.

Why is Linnaeus’ method of naming organisms so important? Before Linnaeus introduced his method, naming practices were not standardized. Some names were used to refer to more than one species. Conversely, the same species often had more than one name. In addition, a name could be very long, consisting of a string of descriptive words. For example, at one time, common wild roses were named Rosa sylvestris alba cum rubore folio glabro. Names such as this were obviously cumbersome to use and hard to remember.

For all these reasons, there was seldom a simple, fixed name by which a species could always be identified. This led to a great deal of confusion and misunderstanding, especially as more and more species were discovered. Linnaeus changed all that by giving each species a unique

 

 

Figure 14.5: Species in the Genus Panthera. All four species in the Panthera genus are similar, but each is a unique type of organism, clearly identified by its combined genus and species name. (6)

 

and unchanging two-word name. Linnaeus’s method of naming organisms was soon widely accepted and is still used today.

 

Changes in the Linnaean System

Linnaean taxonomy has been revised considerably since it was introduced in 1735. One reason revisions have been needed is that many new organisms have been discovered since Linnaeus’ time. Another reason is that scientists started classifying organisms on the basis of evolutionary relationships rather than solely on the basis of similarities in physical traits.

Scientists have had to add several new taxa to the original Linnaean taxonomy in order to accommodate new knowledge of organisms and their evolutionary relationships. Examples of added taxa include the subphylum, superfamily, and domain.

 

A subphylum is a division of a phylum that is higher than the class. An example of a subphylum is Vertebrates (animals with a backbone). It is a subphylum of the Chordate phylum (animals with a notochord).

A superfamily is a taxon that groups together related families but is lower than the order. An example of a superfamily is Hominoids (apes). This superfamily consists of the Hominid family (gorillas, chimps, and humans) and the Hylobatid family (gibbons).Figure 14.6 shows species from both of these families of the Hominoid superfamily.

A domain is a taxon higher than the kingdom. An example of a domain is Eukarya, which includes both plant and animal kingdoms. You can read more about domains in Lesson 14.3.

 

 

Figure 14.6: The Hominoid superfamily includes the Hominid and Hylobatid families. Mem- bers of the Hominid family are chimpanzees (Pan troglodytes, left), gorillas, orangutans, and humans. Members of the Hylobatid are all gibbons (Nomaskus concolor, right). (5)

 

Lesson Summary

Taxonomy is the scientific classification of organisms. Scientists classify organisms in order to make sense of the tremendous diversity of life on Earth.

Linnaean taxonomy groups organisms in a hierarchy of taxa, based on similarities in physical traits. Linnaeus’ binomial nomenclature gives each species a unique two-word name.

 

Review Questions

 

Further Reading / Supplemental Links

1. Define taxonomy.

2. What contributions did Carolus Linnaeus make to taxonomy?

3. List the order of taxa in Linnaean taxonomy, from most to least inclusive.

4. What is binomial nomenclature?

5. Create a hierarchical taxonomy to classify writing implements, such as pens and pencils. Use a diagram to show your taxonomy.

6. Assume that a new organism has been discovered. It has a notochord, fur, forward- facing eyes, and grasping hands with fingers. In which taxa should the new organism be placed? Justify your answer.

   Explain why biologists need to classify organisms.

7. Why was Linnaeus’ naming system such an important contribution to biology?

Wilfrid Blunt, Linnaeus: The Compleat Naturalist. Princeton University Press, 2002.

Paul Lawrence Farber, Finding Order in Nature. Johns Hopkins University Press, 2000.

Judith Winston, Describing Species. Columbia University Press, 1999.

 

Vocabulary

binomial nomenclature Linnaeus’ method of naming species using a unique two-word name made up of the genus and species names.

 

class Taxon that is a division of a phylum. family Taxon that is a division of an order. genus Taxon that is a division of a family.

kingdom Major grouping of organisms, such as plants or animals.

 

Linnaeus Swedish botanist who lived during the 1700s and is known as the “father of taxonomy.”

 

order Taxon that is a division of a class.

 

phylum Taxon that is a division of a kingdom.

 

species Group of organisms that are similar enough to mate and produce offspring together.

 

taxa Categories of organisms in a taxonomy.

 

taxonomy Method of organizing living things into groups.

 

 

Points to Consider

Linnaeus grouped together organisms on the basis of similarities in physical traits.

 

Can you think of other similarities that could be used to group organisms?

What other types of traits might related organisms share?

What about similarities in molecules, such as DNA, among related organisms?