Classification Intro
Humans often organize sets of objects by classifying them. For example, musical instruments are placed in several groups, brass, percussion,
woodwind, and string. Motor vehicles can be categorized as cars, trucks, and vans. Each broad group is then subdivided into several more specific groups. Woodwinds consist of clarinets, saxophones, and oboes. Cars consist of sedans, coupes, and convertibles. As new instruments or motor vehicles are developed, they can be placed in the proper category in the classification system. A classification system brings order and logic to a set of related objects.
Recall that there are nearly two million known species of organisms. Biologists have long been interested in classifying them. Today, organisms are classified in categories called taxa (singular, taxon). The science of classifying organisms is called taxonomy (tak SAHN uh mee). A classification system for organisms brings order to the great diversity of life forms. It also serves as a basis for identification of newly discovered organisms.
THEORY OF CLASSIFICATION
Rather than dealing individually with millions of different organisms, biologists place them in major groups. Each group has a certain set of features. When a new organism is discovered, its characteristics are studied, and it is then added to the proper group. If its features are unique, it may lead to the formation of an entirely new group. New
organisms are being discovered continually. For example, recent exploration of marine caves has led to discovery of more than twenty different species of crustaceans (the group that includes shrimp, lobsters, and crabs).
Both these crab species share many characteristics, and are classified very close together.
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12: 1 The Need for Classification
What are some of the reasons for classifying organisms? One reason is the need for order and organization. You know it would be difficult to find a certain book in a disorganized library. The same is true for organisms. It would be difficult to find
information about a certain organism if organisms were not in some order. Because they are put in a certain order, information about similar organisms is also easier to find.
Another reason to classify organisms is that a logical means for naming organisms is needed. Common names are inadequate for use in a uniform classification system. The word frog, for instance, suggests a certain mental image to you, but it is inaccurate as a scientific label. What kind of frog is it? Is it a grass frog, a tree frog, or a bullfrog?
Consider also the common word worm. To you, this name probably suggests an animal that is slimy and soft; but biologists are familiar with many worms such as roundworms, flatworms, and segmented worms. Also, there are organisms such as ringworms, meal -worms, and acorn worms that are not worms. A ringworm is a fungus, a mealworm is an insect in the larval stage, and an acorn worm is a simple relative of the vertebrates.
Also, common names vary country to country and language to language. They even vary region to region. Consider that puma, cougar, and mountain lion refer to the same organism.
Mountain lion? Puma? Cougar?
Horseshoe crab. Really a crab??
12:2 Binomial Nomenclature
Linnaeus introduced a two-term naming system, binomial nomenclature (bi NOH mee ul • NOH mun klay chur), for classifying organisms. Each organism is given a two-word Latin name. The first word, a noun, is the genus OEE nus; plural genera, JEN ur uh) to which an organism belongs. Its first letter is capitalized. The second word, an adjective, represents the species (the specific name). Its first letter is not capitalized. Notice that the name of a species consists of both the genus and the specific name.
Scientific names list the genus and species of organ -isms. Both animals are considered frogs. However, (a) the bullfrog, Rana catesbeiana, and (b) the tree frog, Hyla versicolor, are classified in different genera. What is the genus of each?
Genus and species are different taxa. Genus is a broader category than species. Thus, a single genus may contain many different species. For example, many cats belong to the genus Felis (FEE Ius), but there are many species of cats. A wildcat is Felis sylvestris (sihl YES trus), an ocelot is Felis pardalis (PAR duh Ius), a cougar is Felis concolor, and a house cat is Felis domeslicus (doh IVIES tih kus). Oak trees belong to the genus Quercus (KWUR kus). A red oak is Quercus rubra and a white oak is Quercus alba. Note that in print these names are usually italicized. In handwriting the names should be underlined. Sometimes the genus name is abbreviated by using only the first letter. Felis sylvestris is sometimes written as F. sylvestris.
a) African wildcat
b) Domestic cat
c) Mountain lion
Felissylvestris Felisdomesticus
12:3 Bases for Classification
Understanding of evolution provides the modern basis for classification. Biologists group organisms based on their evolutionary relationships. Lines of evidence that support the concept of evolution are used today as guides to classification. Often, several types of evidence are used together. Organisms that have a common ancestor are closely related, and they are grouped together. The various species of cats are thought to have evolved from a common ancestor. They are all members of the same genus, Felis. Organisms not descended from a common ancestor are placed in different groupings. For example, snakes and worms are in the same kingdom, but their classification differs from that point on. A major line of evidence for classification is the presence of
homologous structures (Section 10:3). Although Linnaeus did not realize it, by using structure as his basis, he was grouping organisms based on evolutionary relationships. That is why much of his classification remains valid today.
Looking at these skeletons can you pick out similarities? Skeletal structure is one of the homologies that can help classify organisms.
Modern genetics has become a very important tool of taxonomy.
For example, organisms that have similar numbers and kinds of chromosomes may be closely related. Recall that DNA (genes) of different species can be compared. How is this done? Double stranded DNA from each of two different organisms is separated into single strands. The DNA of one of the organisms is made radioactive. The DNA of the two organisms is then mixed. Due to base-pairing, the single strands bond with one another to form double-stranded molecules. Bonding will only occur where the genes (sequences of bases) are similar. Thus, the more bonding, the more closely related the organisms. This technique has shown that closely related organisms have many genes in common.
Recently, mitochondrial DNA has been used to study evolutionary relationships. DNA of mitochondria from different species can be analyzed and compared. This DNA is known to mutate at a certain rate. Using these known mutation rates and comparing the DNA, biologists can approximate the time when two species began to diverge from a common ancestor.
Research comparing the mitochondrial DNA of the quagga, zebra, and horse may be useful in understanding their evolution.
Species having mitochondrial DNA with very few differences in base sequence are closely related. Such studies can be a basis for classifying organisms. Recent studies have even analyzed mitochondrial DNA of the quagga, a zebra extinct since 1883! (The DNA was extracted from some preserved muscle tissue.) Two segments of quagga DNA have been analyzed and compared to DNA of other zebras. This research may provide new insight into the evolution and classification of various zebra species and the relationship between zebras and horses.
REVIEWING IDEAS
1. What is taxonomy? What are taxa?
2. How did Aristotle classify organisms?
3. What is binomial nomenclature?
SYSTEM OF CLASSIFICATION
Using the evidence discussed in Section 12:3, a system of classifying living things has been devised. Each organism is given a two-word name (binomial nomenclature) that is its genus and species. This name is used because it is the most specific. Each species is a distinct form of life.
a b
(a) The paper birch, Betula papyri!era, and (b) the yellow birch, Betula aileghaniens;s, are members of the same genus.
12:4 Classification groups
Genus and species are not the only classification groups. Before organisms are placed in these quite specific categories they are first grouped more broadly. Placement in each taxon is based on similar features. In the complete classification of an organism the groups are kingdom, phylum (F] lum), class, order, family, genus, and species.
Each group from kingdom to species becomes more specific as each step narrows the number of organisms of the previous group. A kingdom is the broadest of all taxa. Recall that there are five kingdoms in today's classification scheme (Section 2:8). All the organisms in each kingdom share the same basic features. Each kingdom is subdivided into phyla. Phyla in turn are subdivided into classes, classes into orders, orders into families, families into genera, and genera into species. Each species is one certain type of organism.
12:5 Some Examples
Consider the complete classification of a common house cat. Because it is an animal, it is placed in Kingdom Animalia (an uh MAY lee uh). A cat is classified as an animal based on the very broad features of all animals. A cat eats rather than makes its own food. A cat also moves around and has a nervous system.
The Brahman cattle and cattle egrets shown here are in separate classes of the same phylum, Chordata. The birds are in the class Aves, and the cattle are in the class
Mammalia. The grasses are classified in a separate kingdom, the plants.
CLASSIFICATIONS OF SOME ANIMALS
Division House Cat Dog Human Grasshopper
KINGDOM Animalia Animalia Animalia Animalia
PHYLUM Chordata Chordata Chordata Arthropoda
SUBPHYLUM Vertebrata Vertebrata Vertebrata
CLASS Mammalia Mammalia Mammalia Insecta
ORDER Carnivora Carnivora Primates Orthoptera
FAMILY Felidae Canidae Hominidae Locustidae
GENUS Fefis Canis Homo Schistocerca
SPECIES Felis domesticus Canis familiaris Homo sapiens Sahistocerca americana
Because of certain other features, a cat is then put into Phylum Chordata (kor DAHT uh). An important subphylum of Chordata is Vertebrata (vurt uh BRAHT uh), animals with backbones. The complete classification of a common house cat appears in the tabie above. Each group represents more specific features than the previous group. Notice that in writing the species name, the genus is repeated.
Compare the classification of a cat to that of a dog. Because cats and dogs share the same broad features, they are in the same kingdom, phylum, subphylum, class, and order. At the family level; the classification differs, because a dog's features are distinct from those of a cat. Each of the remaining groups is, of course, also different from the others.
what level do the classifications differ?
Finally, compare these three animals to the common American grasshopper. The only common group among the grasshopper, cat, dog, and human is kingdom - all of these organisms are animals. What are some of the features of the grasshopper that cause biologists to classify it in a phylum different from that of the cat, dog, and human?
12:6 The Kingdom Problem
You know that most biologists today use a five-kingdom system of classification. For many years, though, all organisms were classified as either plants or animals. This two-kingdom scheme was reasonable for most familiar, multicellular organisms. However, the later discovery of microscopic organisms posed a problem. Some unicellular forms were easily placed in one kingdom or the other. However, many did not fit neatly with either group. For example, Euglena (yoo GLEE nuh) has features of both kingdoms. Euglena is mobile like an animal and autotrophic like a plant. Also, at certain times it may lose its chlorophyll and become heterotrophic.
Euglena have both plant and animal like characteristics that make them hard to classify.
There are other problems with a two-kingdom system. For example, how should fungi such as mushrooms and molds be classified? They are like plants in structure and do not move, but they are heterotrophic like animals. Sponges are heterotrophic, but they do not move around and they show little response to changes in their environment.
Development of the five-kingdom system has solved some of these problems. However, the new system was not invented just to solve those problems; it is based on current knowledge of evolutionary relationships among organisms. Problems have been solved because the new system makes a great deal more "scientific sense."
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REVIEWING IDEAS
6. What are the major classification groups from broadest to most specific? Which of the groups are used in the scientific name of an organism?
Binomial nomenclature:
Match the scientific name with the common names:
__ Beta vulgaris
__ Salmo gairdneni
__ Citrus limon
__ Piper nigrum
__ Ipomoea batatus
__ Tarpor atlanticus
__ Crocodylus americanus
__ Rattus norvegicus
__ Perea flavescens
__ Daucus carota
__ Serinus canerius
__ Clupea harengus
__ Penicillium chrysogenum
black pepper perch rat canary penicillin herring beet carrot
SUMMARY
1. Classification brings order to the great variety of organisms alive today. Precise classification of organisms is necessary so that each living thing has a name recognizable to biologists everywhere. 12: 1
2. Linnaeus devised a system of classification based on structural similarities. His system of binomial nomenclature is still used today. 12:2
3. Using the system of binomial nomenclature, organisms are given a two-word Latin name. The first identifies the genus and the second, the species. 12:2
4. Modern taxonomy depends on knowledge of evolution. Classification is based on factors such as homologous structures, biochemical similarities, and genetics. 12:3
5. The classification categories, from general to specific, are kingdom, phylum, class, order, family, genus, and species. Each of these major categories may be
subdivided. 12:4
6. Comparison of classification of organisms reveals that closely related forms are classified very similarly. 12:5
7. Many systems of classification employ a five kingdom plan: prokaryotes, protists, fungi, plants, and animals. 12:6
8. The five-kingdom system of classification is based on current knowledge of evolutionary relationships and solves many of the problems associated with the older systems. 12:6
LANGUAGE OF BIOLOGY
binomial nomenclature
class
family
genus
kingdom
order
phylum
taxa
