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<p>Biology as a unified science was first developed in the nineteenth century, as scientists discovered that all living things shared certain fundamental characteristics and were best studied as a whole. Over a million papers are published annually in a wide array of biology and medicine journals,<sup class="reference" id="_ref-0">[1]</sup> and biology is a standard subject of instruction at schools and universities around the world.</p>

<p>As such a vast field, biology is divided into a number of disciplines. The old divisions by type of organism remains with subjects such as botany encompassing the study of plants, zoology with the study of animals, and microbiology as the study of microorganisms. The field may also be divided based on the scale at which it is studied: biochemistry examines the fundamental chemistry of life; cellular biology examines the basic building block of all life, the cell; Physiology examines the mechanical and physical functions of an organism; and ecology examines how various organisms interrelate. Applied fields of biology such as medicine are more complex and involve many specialized sub-disciplines.</p>

<h2><span class="mw-headline">Foundations of modern biology</span></h2>

<p>Biology is a branch of science that characterizes and investigates living organisms utilizing the scientific method. There are four broad unifying principles of biology:</p>

<li>Homeostasis. The physiological processes that allow an organism to maintain its internal environment notwithstanding its external environment. </li>

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<h3><span class="mw-headline">Cell Theory</span></h3>

<p>Cell theory states that all living things are composed of one or more cells as well as the secreted products of those cells, for example, plasma, extracellular matrix, and bone. These cells arise from other cells through cell division, and that in multicellular organisms, every cell in the organism's body has been produced from the single cell in a fertilized egg.</p>

<h3><span class="mw-headline">Evolution</span></h3>

<p>A central organizing concept in biology is that all life has a common origin and has changed and developed through the process of the theory of evolution (see Common descent). This has led to the striking similarity of units and processes discussed in the previous section. Charles Darwin established evolution as a viable theory by articulating its driving force, natural selection (Alfred Russel Wallace is recognized as the co-discoverer of this concept). Darwin theorized that species and breeds developed through the processes of natural selection as well as by artificial selection or selective breeding.Genetic drift was embraced as an additional mechanism of evolutionary development in the modern synthesis of the theory.</p>

<p>Up into the 19th century, it was commonly believed that life forms could appear spontaneously under certain conditions (see abiogenesis). This misconception was challenged by William Harvey's diction that &quot;all life [is] from [an] egg&quot; (from the Latin &quot;Omne vivum ex ovo&quot;), a foundational concept of modern biology. It simply means that there is an unbroken continuity of life from its initial origin to the present time.</p>

<p>A group of organisms shares a common descent if they share a common ancestor. All organisms on the Earth have been and 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 universality of the genetic code as definitive evidence in favor of the theory of universal common descent (UCD) for all bacteria, archaea, and eukaryotes (see: origin of life).</p>

<h3><span class="mw-headline">Gene theory</span></h3>

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Schematic representation of DNA, the primary genetic material.</div>

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<p>While organisms may vary immensely in appearance, habitat, and behaviour it is a central principle of biology that all life shares certain universal fundamentals. A key feature is reproduction or replication. The entity being replicated, the replicator, in the past was considered to be the organism during the time of Darwin, but since the 1970s increasingly reduced to the scale of molecules.<sup class="reference" id="_ref-1">[2]</sup> All known life has a carbon-based biochemistry, carbon is the fundamental building block of the molecules that make up all known living things. Similarly water is the basic solvent for all known living organisms. While all these things are true of all organisms observed on Earth, in theory alternative forms of life could exist and some scientists do look at alternative biochemistry.</p>

<p>All terrestrial organisms use DNA and RNA-based genetic mechanisms to hold genetic information. Another universal principle is that all observed organisms with the exception of viruses are made of cells. Similarly, all organisms share common developmental processes.</p>

<h3><span class="mw-headline">Homeostasis</span></h3>

<p>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. Homeostasis manifests itself at the cellular level through the maintenance of a stable internal acidity (pH); at the organismic level, warm-blooded animals maintain a constant internal body temperature; and at the level of the ecosystem, as when atmospheric carbon dioxide levels rise and plants are theoretically able to grow healthier and remove more of the gas from the atmosphere. Tissues and organs can also maintain homeostasis.</p>

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<h2><span class="mw-headline">Scope</span></h2>

<h3><span class="mw-headline">Common descent</span></h3>

<p>Up into the 19th century, it was believed that life forms were being continuously created under certain conditions (see spontaneous generation). This misconception was challenged by William Harvey's diction that &quot;all life [is] from [an] egg&quot; (from the Latin &quot;Omne vivum ex ovo&quot;), a foundational concept of modern biology. It simply means that there is an unbroken continuity of life from its initial origin to the present time.</p>

<p>A group of organisms shares a common descent if they share a common ancestor. All organisms on the Earth have been and 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 universality of the genetic code as definitive evidence in favor of the theory of universal common descent (UCD) for all bacteria, archaea, and eukaryotes (see: origin of life).</p>

<h3><span class="mw-headline">Structure of life</span></h3>

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Schematic of typical animal cell depicting the various organelles and structures.</div>

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<p>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.</p>

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

<p>Genetics is the science of genes, heredity, and the variation of organisms. 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.</p>

<p>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 &quot;morphogenesis,&quot; which is the process that gives rise to tissues, organs, and anatomy. Model organisms for developmental biology include the round worm <em>Caenorhabditis elegans</em>, the fruit fly <em>Drosophila melanogaster</em>, the zebrafish <em>Brachydanio rerio</em>, the mouse <em>Mus musculus</em>, and the weed <em>Arabidopsis thaliana</em>.</p>

<h3><span class="mw-headline">Physiology of organisms</span></h3>

<p>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 &quot;structure to function&quot; 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 also borrows techniques from both fields.</p>

<p>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.</p>

<h3><span class="mw-headline">Diversity and evolution of organisms</span></h3>

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<p><em>Main articles:</em> <strong>Evolutionary biology</strong>, Biodiversity, Botany, Zoology 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, or herpetology, but use those organisms as systems to answer general questions about evolution. Evolutionary biology is mainly based on paleontology, which uses the fossil record to answer questions about the mode and tempo of evolution, as well as the developments in areas such as population genetics and evolutionary theory. In the 1990s, 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.</p>

<p>The two major traditional taxonomically-oriented disciplines are botany and zoology. Botany is the scientific study of plants. Botany covers a wide range of scientific disciplines that study the growth, reproduction, metabolism, development, diseases, and evolution of plant life. Zoology involves the study of animals, including the study of their physiology within the fields of anatomy and embryology. The common genetic and developmental mechanisms of animals and plants is studied in molecular biology, molecular genetics, and developmental biology. The ecology of animals is covered under behavioral ecology and other fields.</p>

<h3><span class="mw-headline">Taxonomy</span></h3>

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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 fourth 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 Virus cInternational Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.</p>

<h3><span class="mw-headline">Interactions of organisms</span></h3>

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<h2><span class="mw-headline">History</span></h2>

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<p>Over the 18th and 19th centuries, biological sciences such as botany and zoology became increasingly professional scientific disciplines. Lavoisier and other physical scientists began to connect the animate and inanimate worlds through physics and chemistry. Explorer-naturalists such as Alexander von Humboldt investigated the interaction between organisms and their environment, and the ways this relationship depends on geography&mdash;laying the foundations for biogeography, ecology and ethology. Naturalists began to reject essentialism and consider the importance of extinction and the mutability of species. Cell theory provided a new perspective on the fundamental basis of life. These developments, as well as the results from embryology and paleontology, were synthesized in Charles Darwin's theory of evolution by natural selection. The end of the 19th century saw the fall of spontaneous generation and the rise of the germ theory of disease, though the mechanism of inheritance remained a mystery.<sup class="reference" id="_ref-4">[5]</sup></p>

<p>In the early 20th century, the rediscovery of Mendel's work led to the rapid development of genetics by Thomas Hunt Morgan and his students, and by the 1930s the combination of population genetics and natural selection in the &quot;neo-Darwinian synthesis&quot;. New disciplines developed rapidly, especially after Watson and Crick proposed the structure of DNA. Following the establishment of the Central Dogma and the cracking of the genetic code, biology was largely split between <em>organismal biology</em>&mdash;the fields that deal with whole organisms and groups of organisms&mdash;and the fields related to <em>cellular and molecular biology</em>. By the late 20th century, new fields like genomics and proteomics were reversing this trend, with organismal biologists using molecular techniques, and molecular and cell biologists investigating the interplay between genes and the environment, as well as the genetics of natural populations of organisms.<sup class="reference" id="_ref-5">[6]</sup></p>

<h2><span class="mw-headline">See also</span></h2>

<dl><dd><em>Main lists: List of biology topics, List of basic biology topics and List of biologists</em></dd></dl>

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<h2><span class="mw-headline">References</span></h2>

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<h2><span class="mw-headline">Further reading</span></h2>

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<div style="MARGIN-LEFT: 10px"><a class="extiw" title="v:School:Biology" href="http://en.wikiversity.org/wiki/School:Biology">The School of Biology</a></div>

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<li><a class="external text" title="http://www.biologynews.net/" rel="nofollow" href="http://www.biologynews.net/">Biology News Net</a>. </li>

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<h3><span class="mw-headline">Journal links</span></h3>

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