CK-12 Biology I - Honors

by CK-12 Foundation

  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 below.

  Linnaean Classification System (Revised)

  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.

  The classification in Figure above 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 divisions 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).

  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 below 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.

  Classification of the Human Species Taxon Name Traits1

  Kingdom Animal Organisms capable of moving 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, relatively flat face.

  Genus Homo Hominids with upright posture, 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 below, 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.

  Figure 14.5

  . 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.

  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 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 below 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. Members of the Hominid family are chimpanzees (, left), gorillas, orangutans, and humans. Members of the Hylobatid are all gibbons (, right).

  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

  Define taxonomy.

  What contributions did Carolus Linnaeus make to taxonomy?

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

  What is binomial nomenclature?

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

  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.

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

  Further Reading / Supplemental Links

  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?

  Lesson 14.2: Phylogenetic Classification

  Lesson Objectives

  Understand the concept of phylogenetic classification.

  Outline how cladistics generates cladograms and identifies clades.

  Compare phylogenetic and Linnaean classification systems.

  Explain how nucleic acid base sequences are used in phylogenetic classification.

  Introduction

  In the century after Linnaeus published his system of classification, ideas about classifying organisms began to change. In 1859, Darwin published his major work on evolution, On the Origin of Species by Natural Selection. After that, there was more and more interest in classifying organisms, incorporating the evolutionary history, including the genetic relationships, of the organisms.

  Phylogeny

  The evolutionary history of a group of genetically related organisms is called a phylogeny. It includes ancestor species and descendant species. A phylogeny is usually represented by a tree diagram called a phylogenetic tree. An early example of a phylogenetic tree is Darwin’s “Tree of Life” (see Figure below). In this diagram, Darwin was trying to show how he thought evolution had occurred. The tree shows how species evolved through time, from the bottom of the tree to the top. As species evolved, they formed new branches on the tree of life. Some of these species eventually branched into additional descendant species. Others died out, or went extinct, without leaving any descendants.

  Figure 14.7

  . This branching diagram represents the evolutionary histories of different species. It is the only diagram that originally appeared in Darwins famous 1859 book,

  Modern biologists still use phylogenetic trees to represent evolutionary histories. A simple phylogenetic tree is shown in Figure below. The tips of the branches represent genetically related species. The branching points represent common ancestors. A common ancestor is the last ancestor species that two descendant species shared before they took different evolutionary paths. In the tree in Figure below, species 1 and 2 shared a more recent common ancestor with each other than with species 3. Therefore, species 1 and 2 are more closely related to one another than to species 3.

  Figure 14.8

  . This phylogenetic tree shows how hypothetical species 1, 2, and 3 are related to one another through common ancestors.

  Ancestor species are like your own ancestors. Your most recent common ancestor with any siblings you may have is a shared parent. Your most recent common ancestor with a first cousin is a shared grandparent. Your most recent common ancestor with a second cousin is a shared great-grandparent. In general, the more distant the relationship between you and relatives in your own generation, the farther in the past you shared a common ancestor. The same holds true for related species. The more distant the relationship between two related species, the farther back in time they shared a common ancestor.

  The most common method of incorporating information into phylogenetic trees is called cladistics. Cladistics depicts hypotheses about how organisms are related, based on traits of ancestor and descendent species. Cladistics was developed in the 1950s by a scientist named Willi Hennig. Over the next several decades, it became very popular, and is still widely used today.

  The term cladistics comes from the word clade. A clade is a group of organisms that includes an ancestor species and all of its descendants. A diagram showing evolutionary relationships within one or more clades is called a cladogram. A clade is a relative concept. How you define a clade depends on which species you are interested in classifying. Small clades can includes as few as two species and their common ancestor. The larger clades can include many more species and their common ancestors.

  As another example, consider the cladogram of insect phylogeny shown in Figure below. According to this cladogram, beetles first branched off from their common ancestor with other insects. Then, the group that includes wasps, bees, and ants branched off. Finally, flies branched off from their common ancestor with butterflies and moths. All insects can be considered a clade because they have a common ancestor. Butterflies, moths, and flies can also be considered a clade for the same reason. Can you identify other clades in Figure below? For example, can you find the clade of all nonbeetle insects?

  Figure 14.9

  Cladogram of Insect Phylogeny. Based on this cladogram, flies shared a more recent common ancestor with butterflies and moths than either group shared with other insects. What other evolutionary relationships does the cladogram reveal?

  Generating Cladograms

  How do sc
ientists construct cladograms like the one in Figure below? The starting point is a set of data on traits of a group of related species. The traits could be physical traits, genetic traits, or both (see Evidence for Evolutionary Relationships below). The next step is deciding which traits were inherited from the common ancestor and which traits evolved only in a descendant species after splitting off from the common ancestor. Traits inherited from a common ancestor are called ancestral traits. Traits that evolved since two groups shared a common ancestor are called derived traits and both types of traits are illustrated in Figure below.

  Ancestral and Derived Traits in Cladistic Analysis

  Figure 14.10

  In this cladogram, the ancestor species has traits A, B, and C, so these are ancestral traits. During the process of evolution, trait A evolves to trait a and trait B evolves to trait b. These new traits (a and b) are derived traits. Organisms can be classified into separate groups (species #1 or species #2) on the basis of these derived traits.

  Consider birds as an example. A derived trait in birds is feathers. The trait is present only in birds and was not inherited from a common ancestor of birds and other organisms. An example of an ancestral trait in birds is the presence of eyes. Eyes are present not only in birds but also in many other groups of animals that share a common ancestor with birds. Therefore, the presence of feathers can identify an organism as a bird, but the presence of eyes cannot. In cladistics, the sharing of derived traits is the most important evidence for evolutionary relationships. Organisms with the same derived traits (such as feathers) are grouped in the same clade.