Biology is the science of living systems. It is inherently interdisciplinary and requires knowledge of the physical and mathematical sciences, although specialties may be oriented towards a group of organisms or a level of organization. BOTANY deals with plant life, ZOOLOGY with animal life, algology with ALGAE, MYCHOLOGY with fungi, MICROBIOLOGY with microorganisms such as protozoa and bacteria, CYTOLOGY with CELLS and so on. All biological specialties, however, are about life and its characteristics. These characteristics include cellular organization, METABOLISM, response to stimuli, development, growth and reproduction. Furthermore, the information necessary to control the expression of these 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 can study life at one or more of these levels. Taxonomy attempts to organize organisms into natural groups based on common characteristics. It concerns 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, while most modern systems use five: MONERA (BACTERIA and BLUE-GREEN ALGAE), PROTIST (PROTOZOA and other ALGAE), FUNGI, PLANT and ANIMAL. The discipline of ECOLOGY deals with the interrelationships of organisms, both among themselves and between them and their environment. Studies of energy flow through communities of organisms and the environment (the ecosystem approach) are particularly useful for evaluating the effects of human activities. The anecologist must be expert in other disciplines of biology. Organisms respond to stimuli from other organisms and the environment; behaviorists are interested in these answers. Most of them study animals – individuals, groups or entire species – in describing patterns of ANIMAL BEHAVIOR. These models include ANIMAL MIGRATION, courtship and mating, social organization, TERRITORIALITY, INSTINCT, and learning. When humans are included, biology overlaps with psychology and sociology. Plant growth and orientation responses can also be studied in the discipline of behavior, although they are traditionally considered to belong to the development phase and PHYSIOLOGY respectively. Descriptive and comparative EMBRYOLOGY are the classic areas of DEVELOPMENT studies, although post-embryological development, particularly the aging process, is also examined. The biochemical and biophysical mechanisms that control normal development are of particular interest when linked to birth defects, cancer, and other anomalies. The inheritance of physical and biochemical characteristics and the variations that appear from generation to generation are the general topics of GENETICS. The emphasis could be placed on the improvement of domesticated plants and animals through controlled breeding, or it could be placed on the more fundamental questions of the molecular and cellular mechanisms of INHERITANCE. A branch of biology that has gained prominence since the 1940s, molecular biology developed essentially from genetics and biochemistry. It tries to explain biological events by studying the molecules inside them