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Biology deals with the study of the many varieties of living organisms (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle

Biology (from Greek βιολογία - βίος, bios, "life"; -λογία, -logia, study of) is the natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. The term biology in its modern sense appears to have been introduced independently by Karl Friedrich Burdach (1800), Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur, 1802), and Jean-Baptiste Lamarck (Hydrogéologie, 1802).

Biology is a vast subject containing many subdivisions, topics, and theories. Five unifying principles form the fundamental axioms of modern biology: cell theory, evolution, gene theory, energy, and homeostasis.

These fields are further divided based on the scale at which organisms are studied and the methods used to study them: biochemistry examines the rudimentary chemistry of life; molecular biology studies the complex interactions of systems of biological molecules; cellular biology examines the basic building block of all life, the cell; physiology examines the physical and chemical functions of the tissues, organs, and organ systems of an organism; and ecology examines how various organisms interrelate with their environment.

The classification, taxonomy, and nomenclature of biological organisms is administered by the International Code of Zoological Nomenclature, International Code of Botanical Nomenclature, and International Code of Nomenclature of Bacteria for animals, plants, and bacteria, respectively. Viruses, viroids, prions, and all other sub-viral agents that demonstrate biological characteristics are controlled by the International Code of Virus classification and nomenclature. However, several other viral classification systems do exist.


Although biology in its modern form is a relatively recent development, sciences related to and included within biology have been studied since ancient times. Natural philosophy was studied as early as the ancient civilizations of Mesopotamia, Egyptmarker, the Indian subcontinent, and Chinamarker. However, the origins of modern biology and its approach to the study of nature are most often traced back to ancient Greece. While the formal study of medicine dates back to Hippocrates, it was Aristotle who contributed most extensively to the development of biology. Especially important are his History of Animals and other works where he showed naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany that survived as the most important contribution of antiquity to botany, even into the Middle Ages.

Significant advances in the study and development of biology were promoted through the efforts of such Muslim physicians as the Afro-Arab scholar al-Jahiz (781–869) in zoology, the Kurdish biologist Al-Dinawari (828–896) in botany, , in and the Persian physician Rhazes (865–925) in anatomy and physiology. These philosophers elaborated on, expanded, and improved the Greek biological theories and systematics. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.

Biology began to quickly develop and grow with Antony van Leeuwenhoek's dramatic improvement of the microscope. It was then that scholars discovered spermatozoa, bacteria, infusoria and the sheer strangeness and diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and built the basic techniques of microscopic dissection and staining.

Advances in microscopy also had a profound impact on biological thinking itself. In the early 19th century, a number of biologists pointed to the central importance of the cell. In 1838 and 1839, Schleiden and Schwann began promoting the ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, though they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory.

Meanwhile, taxonomy and classification began to present a focal point in the study of natural history. Carolus Linnaeus published a basic taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species. Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Though he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought; his work would influence the evolutionary theories of both Lamarck and Darwin.

Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck. However, it was the British naturalist Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell, Thomas Malthus's writings on population growth, and his own morphological expertise, that created a more successful evolutionary theory based on natural selection; similar evidence led Alfred Russel Wallace to independently reach the same conclusions.

The discovery of the physical representation of heredity came along with evoluttionary principles and population genetics. In the 1940s and early 1950s, experiments pointed to DNA as the portion of chromosomes (and perhaps other nucleoproteins) that held genes. A focus on new model organisms such as viruses and bacteria, along with the discovery of the double helical structure of DNA in 1953, marked the transition to the era of molecular genetics.

Foundations of modern biology

Much of modern biology can be encompassed within five unifying principles: cell theory, evolution, genetics, homeostasis, and energy.

Cell theory

Cell theory states that the cell is the fundamental unit of life, and that all living things are composed of one or more cells or the secreted products of those cells, such as shells. Cells arise from other cells through cell division. In multicellular organisms, every cell in the organism's body derives ultimately from a single cell in a fertilized egg. The cell is also considered to be the basic unit in many pathological processes.


A central organizing concept in biology is that life changes and develops through evolution, and that all life-forms known have a common origin. Introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809, Charles Darwin established evolution fifty years later as a viable theory by articulating its driving force: natural selection. The Complete Works of Darwin Online - Biography. Retrieved on 2006-12-15

(Alfred Russel Wallace is recognized as the co-discoverer of this concept as he helped research and experiment with the concept of evolution). Evolution is now used to explain the great variations of life found on Earth.

Darwin theorized that species and breeds developed through the processes of natural selection and artificial selection or selective breeding. Genetic drift was embraced as an additional mechanism of evolutionary development in the modern synthesis of the theory.

The evolutionary history of the species— which describes the characteristics of the various species from which it descended— together with its genealogical relationship to every other species is known as its phylogeny. Widely varied approaches to biology generate information about phylogeny. These include the comparisons of DNA sequences conducted within molecular biology or genomics, and comparisons of fossils or other records of ancient organisms in paleontology.

Biologists organize and analyze evolutionary relationships through various methods, including phylogenetics, phenetics, and cladistics. For a summary of major events in the evolution of life as currently understood by biologists, see evolutionary timeline.

Up into the 19th century, spontaneous generation, the belief that life forms could appear spontaneously under certain conditions, was widely accepted. This misconception was challenged by William Harvey, who even before the invention of the microscope was led by his studies to suggest that life came from invisible 'eggs.' In the frontispiece of his book Exercitationes de Generatione Animalium (Essays on the Generation of Animals), he expressed the basic principle of biogenesis: "Omnia ex ovo" (everything from eggs).

A group of organisms have a common descent if they share a common ancestor. All organisms on the Earth, both living and extinct, have been or are descended from a common ancestor or an ancestral gene pool. This last universal common ancestor of all organisms is believed to have appeared about 3.5 billion years ago. Biologists generally regard the collective universality of the genetic code as definitive evidence in favor of the theory of universal common descent for all bacteria, archaea, and eukaryotes (see: origin of life).

Evolution does not always give rise to progressively more complex organisms. For example, the process of dysgenics has been reportedly observed among the human population.


A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms
Genes are the primary units of inheritance in all organisms. A gene is a unit of heredity and a region of DNA that influences a particular characteristic in an organism. All organisms, from bacteria to animals, share the same basic machinery that copies and translates DNA into proteins. Cells transcribe a DNA gene into an RNA version of the gene, and a ribosome then translates the RNA into a protein. Additionally, DNA codes for the same proteins regardless of what organism it is present in. A sequence of DNA that codes for insulin in humans will also code for insulin when inserted into other organisms, such as plants.

DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell is collectively known as its genome. A chromosome is an organized structure consisting of DNA and histones. Genomic DNA is located in the cell nucleus of eukaryotes, as well as small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its genotype.


Homeostasis is the ability of an open system to regulate its internal environment to maintain a stable condition by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. All living organisms, whether unicellular or multicellular, exhibit homeostasis.

In order to maintain dynamic equilibrium, a system must detect and respond to perturbations. After the detection of a perturbation, a biological system will respond through at least one of the two forms of feedback: negative feedback and positive feedback. Negative feedback consists of reducing the output or activity of an organ or system back to its normal range of functioning. One example is the human body's release of insulin when blood sugar levels are too high. Another example is the release of glucagon when sugar levels are too low. Positive feedback mechanisms are designed to accelerate or enhance an output. One example of a positive feedback event in the human body is blood platelet accumulation, which, in turn, causes blood clotting in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin to intensify the contractions that take place during childbirth.


The survival of a living organism depends on the continuous input of energy. Chemical reactions that are responsible for its structure and function are tuned to extract energy from substances that act as its food and transform them to help form new cells and sustain them. In this process, molecules of chemical substances that constitute food play two roles; first, they contain energy that can be transformed for biological chemical reactions; and also develop molecular structures made up of biomolecules.

Nearly all of the energy needed for life processes originates from the Sun, which plants and other autotrophs convert into chemical energy (organic molecules) via photosynthesis. A few ecosystems, however, depend entirely on energy extracted from methane, sulfides, or other non-luminal energy sources by chemotrophs.

Some of the captured energy is used to produce biomass to sustain life and provide energy for its growth and development. The majority of the rest of this energy is lost as heat and waste molecules. The most common processes for converting the energy trapped in chemical substances into energy useful to sustain life are metabolism and cellular respiration.



Molecular biology is the study of biology at a molecular level. This field overlaps with other areas of biology, particularly with genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interrelationship of DNA, RNA, and protein synthesis and learning how these interactions are regulated.

Cell biology studies the physiological properties of cells, as well as their behaviors, interactions, and environment. This is done both on a microscopic and molecular level. Cell biology researches both single-celled organisms like bacteria and specialized cells in multicellular organisms like humans.

Understanding cell composition and how they function is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important in the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types.

Genetics is the science of genes, heredity, and the variation of organisms.Hartl D, Jones E (2005) Genes encode the information necessary for synthesizing proteins, which in turn play a large role in influencing (though, in many instances, not completely determining) the final phenotype of the organism. In modern research, genetics provides important tools in the investigation of the function of a particular gene, or the analysis of genetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.

Developmental biology studies the process by which organisms grow and develop. Originating in embryology, modern developmental biology studies the genetic control of cell growth, differentiation, and "morphogenesis," which is the process that progressively gives rise to tissues, organs, and anatomy.Model organisms for developmental biology include the round worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, the zebrafish Danio rerio, the mouse Mus musculus,, and the weed Arabidopsis thaliana. A model organism is a species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms.


Physiology studies the mechanical, physical, and biochemical processes of living organisms by attempting to understand how all of the structures function as a whole. The theme of "structure to function" is central to biology. Physiological studies have traditionally been divided into plant physiology and animal physiology, but the principles of physiology are universal, no matter what particular organism is being studied. For example, what is learned about the physiology of yeast cells can also apply to human cells. The field of animal physiology extends the tools and methods of human physiology to non-human species. Plant physiology borrows techniques from both research fields.

Anatomy is an important branch of physiology and considers how organ systems in animals, such as the nervous, immune, endocrine, respiratory, and circulatory systems, function and interact. The study of these systems is shared with medically oriented disciplines such as neurology and immunology.


Evolution is concerned with the origin and descent of species, as well as their change over time, and includes scientists from many taxonomically-oriented disciplines. For example, it generally involves scientists who have special training in particular organisms such as mammalogy, ornithology, botany, or herpetology, but use those organisms as systems to answer general questions about evolution.

Evolutionary biology is partly based on paleontology, which uses the fossil record to answer questions about the mode and tempo of evolution, and partly on the developments in areas such as population genetics and evolutionary theory. In the 1980s, developmental biology re-entered evolutionary biology from its initial exclusion from the modern synthesis through the study of evolutionary developmental biology. Related fields which are often considered part of evolutionary biology are phylogenetics, systematics, and taxonomy.


Classification is the province of the disciplines of systematics and taxonomy. Taxonomy places organisms in groups called taxa, while systematics seeks to define their relationships with each other.

This classification technique has evolved to reflect advances in cladistics and genetics, shifting the focus from physical similarities and shared characteristics to phylogenetics.

Traditionally, living things have been divided into five kingdoms:

  1. Monera
  2. Protista
  3. Fungi
  4. Plantae
  5. Animalia

However, many scientists now consider this five-kingdom system outdated. Modern alternative classification systems generally begin with the three-domain system:

  1. Archaea (originally Archaebacteria)
  2. Bacteria (originally Eubacteria)
  3. Eukarya (including protists, fungi, plants, and animals)

These domains reflect whether the cells have nuclei or not, as well as differences in the cell exteriors.

Further, each kingdom is broken down recursively until each species is separately classified. The order is:

There is also a series of intracellular parasites that are "on the edge of life" in terms of metabolic activity, meaning that many scientists do not actually classify these structures as alive, due to their lack of at least one or more of the fundamental functions by which life is defined. They are classified as:
  1. Viruses
  2. Viroids
  3. Prions
The scientific name of an organism is obtained from its genus and species. For example, humans would be listed as Homo sapiens. Homo would be the genus and sapiens is the species. Whenever writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the species in lowercase. Additionally, the entire term would be italicized or underlined.

The dominant classification system is called Linnaean taxonomy, which includes ranks and binomial nomenclature. How organisms are named is governed by international agreements such as the International Code of Botanical Nomenclature (ICBN), the International Code of Zoological Nomenclature (ICZN), and the International Code of Nomenclature of Bacteria (ICNB).

A merging draft, BioCode, was published in 1997 in an attempt to standardize naming in these three areas, but it has yet to be formally adopted. The BioCode draft has received little attention since 1997; its originally planned implementation date of January 1, 2000, has passed unnoticed. However, a 2004 paper concerning the cyanobacteria does advocate a future adoption of a BioCode and interim steps consisting of reducing the differences between the codes. The International Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.


Ecology studies the distribution and abundance of living organisms, and the interactions between organisms and their environment. The habitat of an organism can be described as the local abiotic factors such as climate and ecology, plus the other organisms and biotic factors that share its environment.

Ecological systems are studied at several different levels, from individuals and populations to ecosystems and the biosphere. The term population biology is often used interchangeably with population ecology, although population biology is more frequently used when studying diseases, viruses, and microbes, while population ecology is more commonly when studying plants and animals. As can be surmised, ecology is a science that draws on several disciplines.

Ethology studies animal behavior (particularly that of social animals such as primates and canids), and is sometimes considered a branch of zoology. Ethologists have been particularly concerned with the evolution of behavior and the understanding of behavior in terms of the theory of natural selection. In one sense, the first modern ethologist was Charles Darwin, whose book, The Expression of the Emotions in Man and Animals, influenced many ethologists.

Biogeography studies the spatial distribution of organisms on the Earth, focusing on topics like plate tectonics, climate change, dispersal and migration, and cladistics.

Every living thing interacts with other organisms and its environment. One reason that biological systems can be difficult to study is that so many different interactions with other organisms and the environment are possible, even on the smallest of scales. A microscopic bacterium responding to a local sugar gradient is responding to its environment as much as a lion is responding to its environment when it searches for food in the African savanna.

For any given species, behaviors can be co-operative, aggressive, parasitic or symbiotic. Matters become more complex when two or more different species interact in an ecosystem. Studies of this type are within the province of ecology.

Branches of Biology

These are the main branches of biology:

  • Agriculture - study of producing crops from the land, with an emphasis on practical applications

  • Anatomy - the study of form and function, in plants, animals, and other organisms, or specifically in humans

  • Biochemistry - the study of the chemical reactions required for life to exist and function, usually a focus on the cellular level

  • Bioengineering - the study of biology through the means of engineering with an emphasis on applied knowledge and especially related to biotechnology.

  • Bioinformatics - also classified as a branch of information technology (IT) it is the study, collection, and storage of genomic and other biological data

  • Biomechanics - often considered a branch of medicine, the study of the mechanics of living beings, with an emphasis on applied use through artificial limbs, etc.

  • Biomimetics - science of adapting designs from nature to solve modern problems.

  • Biophysics - the study of biological processes through physics, by applying the theories and methods traditionally used in the physical sciences

  • Biotechnology - a new and sometimes controversial branch of biology that studies the manipulation of living matter, including genetic modification

  • Botany - the study of plants

  • Cell biology - the study of the cell as a complete unit, and the molecular and chemical interactions that occur within a living cell.

  • Conservation Biology - the study of the preservation, protection, or restoration of the natural environment, natural ecosystems, vegetation, and wildlife

  • Cryobiology - the study of the effects of lower than normally preferred temperatures on living beings.

  • Ecology - the study of the interactions of living organisms with one another and with the non-living elements of their environment.

  • Environmental Biology - the study of the natural world, as a whole or in a particular area, especially as affected by human activity

  • Epidemiology - a major component of public health research, it is the study of factors affecting the health and illness of populations

  • Ethology - the study of animal behavior.

  • Genetics - the study of genes and heredity.

  • Histology - the study of cells and tissues, a microscopic branch of anatomy.

  • Marine Biology - the study of ocean ecosystems, plants, animals, and other living beings.

  • Microbiology - the study of microscopic organisms (microorganisms) and their interactions with other living things

  • Molecular Biology - the study of biology and biological functions at the molecular level, some cross over with biochemistry

  • Neurobiology - the study of the nervous system, including anatomy, physiology, even pathology

  • Oceanography - the study of the ocean, including ocean life, environment, geography, weather, and other aspects influencing the ocean.

  • Population ecology - the study of populations of organisms, including how they increase and go extinct

  • Paleontology - the study of fossils and sometimes geographic evidence of prehistoric life

  • Pharmacology - the study and practical application of preparation, use, and effects of drugs and synthetic medicines.

  • Physiology - the study of the functioning of living organisms and the organs and parts of living organisms

  • Phytopathology - the study of plant diseases (also called Plant Pathology)

  • Virology - the study of viruses and some other virus-like agents

  • Zoology - the study of animals, including classification, physiology, development, and behavior (See also Entomology, Ethology, Herpetology, Ichthyology, Mammology, and Ornithology)

See also

Notes and References

  1. Based on definition from Aquarena Wetlands Project glossary of terms.
  2. Junker Geschichte der Biologie, p8.
  3. Coleman, Biology in the Nineteenth Century, pp 1–2.
  4. Life Science, Weber State Museum of Natural Science
  5. ICTV Virus Taxonomy 2009
  6. "80.001 Popsiviroidae - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2009-10-28.
  7. "90. Prions - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2009-10-28.
  8. "81. Satellites - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2009-10-28.
  9. Magner, A History of the Life Sciences
  10. Mehmet Bayrakdar, "Al-Jahiz And the Rise of Biological Evolutionism", The Islamic Quarterly, Third Quarter, 1983, London.
  11. Magner, A History of the Life Sciences, pp 133–144
  12. Sapp, Genesis, chapter 7; Coleman, Biology in the Nineteenth Century, chapters 2
  13. Mayr, The Growth of Biological Thought, chapter 4
  14. Mayr, The Growth of Biological Thought, chapter 7
  15. Mayr, The Growth of Biological Thought, chapter 10: "Darwin's evidence for evolution and common descent"; and chapter 11: "The causation of evolution: natural selection"; Larson, Evolution, chapter 3
  16. As Darwinian scholar Joseph Carroll of the University of Missouri–St. Louis puts it in his introduction to a modern reprint of Darwin's work: "The Origin of Species has special claims on our attention. It is one of the two or three most significant works of all time—one of those works that fundamentally and permanently alter our vision of the world....It is argued with a singularly rigorous consistency but it is also eloquent, imaginatively evocative, and rhetorically compelling."
  17. Shermer p. 149.
  18. Darwin, Charles (1859). On the Origin of Species, 1st, John Murray
  19. Phylogeny on
  20. Thinking About Life, Paul S. Agutter and Denys N. Wheatley
  21. From SemBiosys, A New Kind Of Insulin INSIDE WALL STREET By Gene G. Marcial(AUGUST 13, 2007)
  23. Genotype definition - Medical Dictionary definitions
  24. Kelvin Rodolfo, Explanation of Homeostasis on Retrieved Oct. 16, 2009.
  25. Marieb, Elaine N. & Hoehn, Katja (2007). Human Anatomy & Physiology (Seventh ed.). San Francisco, CA: Pearson Benjamin Cummings.
  26. Bartsch/Colvard, The Living Environment. (2009) New York State Prentice Hall Regents Review. Retrieved Oct. 16, 2009.
  27. Katrina Edwards. Microbiology of a Sediment Pond and the Underlying Young, Cold, Hydrologically Active Ridge Flank. Woods Hole Oceanographic Institution.
  28. Molecular Biology - Definition from
  29. "Anatomy of the Human Body". 20th edition. 1918. Henry Gray.
  30. John H. Gillespie Population Genetics: A Concise Guide, Johns Hopkins Press, 1998. ISBN 0-8018-5755-4.
  31. Vassiliki Betta Smocovitis Unifiying Biology: the evolutionary synthesis and evolutionary biology ISBN 0-691-03343-9.
  32. Michener, Charles D., John O. Corliss, Richard S. Cowan, Peter H. Raven, Curtis W. Sabrosky, Donald S. Squires, and G. W. Wharton (1970). Systematics In Support of Biological Research. Division of Biology and Agriculture, National Research Council. Washington, D.C. 25 pp.
  33. Rybicki EP (1990) "The classification of organisms at the edge of life, or problems with virus systematics." S Aft J Sci 86:182–186
  34. Wiley, 1981
  35. Branches of Biology on
  36. Biology on

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