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European robin, Erithacus rubecula, Turdus migratorius, classifying organisms, units of heredity

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Classification, in biology, identification, naming, and grouping of organisms into a formal system based on similarities such as internal and external anatomy, physiological functions, genetic makeup, or evolutionary history. With an estimated 10 million to 13 million species on Earth, the diversity of life is immense. Determining an underlying order in the complex web of life is a difficult undertaking that encompasses advanced scientific methods as well as fundamental philosophical issues about how to view the living world. Among the scientists who work on classification problems are systematists, biologists who study the diversity of organisms and their evolutionary relationship. In a related field known as taxonomy, scientists identify new organisms and determine how to place them into an existing classification scheme.

Classification determines methods for organizing the diversity of life on Earth. It is a dynamic process that reflects the very nature of organisms, which are subject to modification and change over many, many generations in the process of evolution. Since life first appeared on Earth 3.5 billion years ago, many new types of organisms have evolved. Many of these organisms have become extinct, while some have developed into the present fauna and flora of the world. Extinction and diversification continue nonstop, and scientists are frequently encountering fluctuations that may affect the way an organism is classified.

In addition to ordering organisms, scientists give a new species a scientific name, typically a two-word name in Latin, to distinguish it from similar organisms. This naming process creates a standard way for scientists around the world to communicate about the same organism. This standard minimizes confusion, particularly when common names are applied to organisms. For instance, the bird Europeans commonly call a robin is a different species of bird from the robin Americans recognize. The confusion ends when the birds are referred to by their scientific names: the European robin is Erithacus rubecula, while the American robin is Turdus migratorius.

When classifying organisms, scientists study a wide range of features, including those visible to the naked eye, those detectable only under a microscope, and those that can be determined only by chemical tests. Scientists compare the external shapes and sizes of organisms as well as the anatomy and function of internal organs and organ systems, such as the digestive or reproductive systems. Biochemists study and compare the molecular interactions within an organism that enable it to grow, make and store energy, and reproduce. The early stages of an organismís development, or embryology, as well as an organismís behavior, or ethology, are also useful in grouping organisms. Even the role an organism plays in its habitat can help place it in a particular group. Scientists use the fossil record to learn how certain animals have changed and evolved through Earthís history, which may provide clues for classification.

More recently, scientists have employed the techniques of molecular biology to compare the units of heredity, or genes, among organisms. Scientists study the fundamental units of deoxyribonucleic acid (DNA), the molecule that makes up genes, and organisms that share a similar DNA structure may be more closely related. Called molecular systematics, this approach is a powerful analytical tool. Used in combination with the other features studied in classification, molecular systematics can provide valuable insight into classification problems. For many organisms, molecular systematics studies have supported traditional classification; however, in some cases, the evidence from genetics studies has indicated that organisms should be reclassified. Skunks, for example, traditionally have been classified with badgers, ferrets, and minks in the family Mustelidae. But recent studies of molecular traits indicate that skunks differ significantly from these animals and may warrant classification in their own family.

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