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Biology is the science of living systems. It is inherently interdisciplinary, requiring knowledge of the physical sciences and mathematics, although specialities may be oriented toward a group of organisms or a level of organization. BOTANY is concerned with plant life, ZOOLOGY with animal life, algology with ALGAE, MYCOLOGY with fungi, MICROBIOLOGY with microorganisms such as protozoa and bacteria, CYTOLOGY with CELLS, and so on. All biological specialties, however, are concerned with life and its characteristics. These characteristics include cellular organization, METABOLISM, response to stimuli, development and growth, and reproduction. Furthermore, the information needed to control the expression of such characteristics is contained within each organism. FUNDAMENTAL DISCIPLINES Life is divided into many levels of organization--atoms, molecules, cells, tissues, organs, organ systems, organisms, and populations. The basic disciplines of biology may study life at one or more of these levels. Taxonomy attempts to arrange organisms in natural groups based on common features. It is concerned with the identification, naming, and classification of organisms. The seven major taxonomic categories, or taxa, used in classification are kingdom, phylum, class, order, family, genus, and species. Early systems used only two kingdoms, plant and animal, whereas most modern systems use five: MONERA (BACTERIA and BLUE-GREEN ALGAE), PROTISTA (PROTOZOA and the other ALGAE), FUNGI, PLANT, and ANIMAL. The discipline of ECOLOGY is concerned with the interrelationships of organisms, both among themselves and between them and their environment. Studies of the energy flow through communities of organisms and of the environment (the ecosystem approach) are especially valuable in assessing the effects of human activities. An ecologist must be knowledgeable in other disciplines of biology. Organisms respond to stimuli from other organisms and from the environment; behaviorists are concerned with these responses. Most of them study animals--as individuals, groups, or entire species--in describing ANIMAL BEHAVIOR patterns. These patterns include ANIMAL MIGRATION, courtship and mating, social organization, TERRITORIALITY, INSTINCT, and learning. When humans are included, biology overlaps with psychology and sociology. Growth and orientation responses of plants can also be studied in the discipline of behavior, although they are traditionally considered as belonging under development and PHYSIOLOGY, respectively. Descriptive and comparative EMBRYOLOGY are the classic areas of DEVELOPMENT studies, although postembryological development, particularly the aging process, is also examined. The biochemical and biophysical mechanisms that control normal development are of particular interest when they are related to birth defects, cancer, and other abnormalities. Inheritance of physical and biochemical characteristics, and the variations that appear from generation to generation, are the general subjects of GENETICS. The emphasis may be on improving domestic plants and animals through controlled breeding, or it may be on the more fundamental questions of molecular and cellular mechanisms of HEREDITY. A branch of biology growing in importance since the 1940s, molecular biology essentially developed out of genetics and biochemistry. It seeks to explain biological events by studying the molecules within

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