There are about two million known species of living organisms. One of the
activities of biologist is to classify organisms. A classification system brings
logic and order to the study of living things.
The science of classifying organisms is called taxonomy. A very important reason for classifying organisms is that it provides a logical means for naming organisms. The common names applied to organisms are often misleading. For example, a silver fish is not a fish. A groundhog is not a hog. A sheepshead is not a sheep. A sea horse is not a horse. A polecat is not a cat. Ringworm is not a worm. Scientific names are based upon taxonomy and are more meaningful.
Aristotle devised one of the first systems of classifying organisms, classifying them as being either plants or animals. The first significant change to Aristotle's system was accomplished by 18th century biologist Carolus Linnaeus. Linnaeus classified all organisms on the basis of structure.
Linnaeus introduced a two-term naming system called binomial nomenclature, in which each organism is given a two-word Latin name. The first word, a noun, is the genus to which the organism belongs. The second word, an adjective, represents the species. The genus name is capitalized, the species name is not. The scientific name for man is Homo sapiens. Homo, our genus is capitalized; sapiens, our species is not capitalized. Homo--the noun, means man. Sapiens, the adjective, means wise or thinking.
The genus is a broader category than species. A single genus may contain many species. All cats belong to the genus Felis. However, there are many species of cats. The cougar is Felis concolor. A house cat is Felis domesticus. Oak trees belong to the genus Quercus. A red oak is Quercus rubra, and a white oak is Quercus alba. Notice that when printed both names are italicized. When they are handwritten the names are underlined.
Biologist classify organisms according to their evolutionary beginnings. The word "genus" means "beginning." The various cats, for example, are thought to have evolved from a common ancestor. Therefore, they are grouped together in the genus Felis. Although Linnaeus did not realize it, by classifying organisms according to structure he was also classifying them according to evolutionary relationships. For this reason, much of his classification system remains valid today.
Genus and species are not the only classification groups. Before organisms are placed in genus and species, they first must be classified as to: kingdom, phylum, class, order and family. Then they are classed as to genus and species, which becomes their scientific name. Each group from kingdom to species become more specific as each step narrows the number of organisms of the previous group.
The categories used by biologist to classify organisms are called taxa (taxon is
singular). Kingdom is the broadest of all taxa. Today there are five kingdoms:
animals, plants, protists, fungi, and monerans.
1. Animals have these characteristics: they are multicellular, heterotrophic (cannot make their own food), organismic (capable of carrying on life processes), and ingested food.
2. Plants have these characteristics: they are autotrophic (make their own food), with chlorophyll which is in a chloroplast, have cell walls made of cellulose, and are usually multicellular.
3. Protists have these characteristics: They are simple eukaryotic organisms (have cells with a true nucleus enclosed in a membrane), such as simple algae, protozoa, and slime molds.
4. Fungi are plant-like heterotrophic organisms which reproduce by spores.
5. Monerans are prokaryotic (have no distinct nucleus) usually unicellular organisms such as bacteria.
Kingdoms are divided into phyla. A phylum is the primary division of a kingdom
by shared characteristics. Examples of phyla are chordates, arthropods and
mollusks.
1. Chordates--all chordates, at some stage in life have a stiff dorsal rod of cartilage called a notochord. Chordates include all the vertebrates (animals with a backbone). We are chordates. Our notochord turns into the vertebral canal which houses our spinal cord.
2. Arthropods--have an exoskeleton and jointed legs. Insects are the largest class of arthropods.
3. Mollusks--soft-bodied, mostly marine animals, usually enclosed within a hard outer shell of calcium carbonate. Examples are oysters, clams and octopuses (octopi).
Phyla are sometime subdivided into subphyla. Subphyla are divided into classes;
classes into orders; orders into families; families into genus; and genus into
species.
The complete classification of human beings is as follows:
1. Kingdom--animal
2. phylum--chordate
3. subphylum--vertebrate
4. Class--mammal
5. Order--primates
6. family--hominid
7. genus--Homo
8. species--sapiens