Bases of Modern Classification

Though similarity of structure still remains the basis for grouping organisms, biologists also use other kinds of evidence in classification. Modern taxonomists study chromosome structure, reproductive potential, biochemical similarities, and embryology to determine the relationships among organisms.

Comparing one organism’s chromosome makeup, or karyotipe, with that of another organism helps the biologists determine relationships. Taxonomists know that similarity of karyotypes usually points out a close taxonomic relationship. Taxonomists also study reproductive potential. Botanists, for example, perform many experimental crosses with related plants to see which will produce fertile offspring. Those that do are said to belong to the same species. An organism’s biochemical makeup also provides evidence of its relationships. Similar sequence in the amino acids of proteins from two organisms may indicate that they are closely related.

Scientists also study the embryological development of homologous structures to determine relationships. For example, a taxonomists might note that the wing of a bat and a flipper of a whale originate from the same embryonic tissues and initially developed in a similar manner. The taxonomists might then suggest that the two animals had a common ancestor.

Categories of Classification

Taxonomists do not use only genus and species in classifying organisms. Other classification levels also exist. When taken in order from the largest, most generalized group ton the smallest, most specific group, these levels are kingdom, phylum, class, order, family, genus, and species. Each organism has its place in this taxonomic system. For example, a tiger is a member of kingdom Animalia, phylum Chordata, class mammalia, order Carnivora, family Felidae, genus Panthera, species tigris. The scientific name of the tigers, therefore, is Panthera tigris.

Members of the same species that differ in some important way- such as flower size or ear shape- are said to be members of different varieties, or subdivisions of a species. The variety of a species is often listed as a part of a scientific name. It is written after a species name. When two varieties are separated geographically from each other, many taxonomists prefer to call them subspecies.

The Five Kingdoms

When Linnaeus developed a new system of classification, he retained the idea of two kingdoms. All autotrophs, or organisms that produce their own food, were placed into the plant kingdom. All heterotrophs, which are organisms dependent on others for food, were placed into the animal kingdom. As scientists learned more, they realized that the two-kingdom system was unadequate.

Life scientists have agreed to use a classification system that has five categories. All living things can be placed into one of these five categories - or kingdoms. Monerans are simple, single cells. Some monerans can make their own food. Some cannot. The bacteria that live on your skin are monerans. The protist kingdom includes organisms that are complex, single cells. Like the monerans, some can make their own food and some cannot. Many of the organisms found in a drop of pond water are protests. Fungi are many-celled organisms that absorb food from dead or living things. The mushrooms you eat on a pizza are fungi. Green plants are many- celled organisms that make their own food. Roses and pine trees are plants. The animal kingdom also includes many- celled organisms. Unlike plants, however, animals do not make their own food. You and your friends-like mice, birds, and worms are members of the animal kingdom.