Biology

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Posted by kaori 03/02/2009 @ 01:05

Tags : biology, life sciences, sciences

News headlines
BHS biology teacher honored - Bismarck Tribune
By Sara Kincaid Bismarck High School biology teacher Brad Lick thought he had an emergency staff meeting to go to this morning. Then he saw his family, and knew it had to be something else. "I had no idea what the meeting was about," he said....
Commencement 2009: Top Biology Student Represents the True Vision ... - Rensselaer Polytechnic Institute
During her time at the Institute, Lund has earned a bachelor's degree in biology and master's degree in management, and on May 16, 2009, she will walk across the stage and receive her doctorate in biology. Truly a student of Rensselaer, Lund has grown...
Turner Named Head of Biology Department - Virginia Military Institute
James E. Turner '65 has been named head of the Department of Biology effective June 1. Turner joined the faculty in 2001 as a professor of biology and chemistry and founding director of VMI's Undergraduate Research Initiative....
Biology students test the water - Ludington Daily News
Students in Mike Daynes' advanced biology class tested the waters Tuesday. They collected samples from Lincoln Lake and Lake Michigan and analyzed their findings. Daynes said everyone did their own tests which consisted of determining the nutrient...
Penguins' secret ocean food stash found - msnbc.com
The study, published in the latest Royal Society Biology Letters, is the first to reveal the movements and feeding habits of large numbers of penguins using relatively non-invasive geolocation sensors. In the future, the technique may be applied to...
Growing Ecological Awareness, Partner Programs, Drive Interest in ... - Fordham University
By Janet Sassi A new advanced graduate certificate in conservation biology, rolled out this academic year, aims to prepare Fordham's students for the 21st-century race to save the world's endangered species and to preserve biodiversity....
Society of In Vitro Biology meeting in Charleston June 6-10 - Charleston Regional Business
The Society of In Vitro Biology is hosting its annual meeting June 6-10 at the Embassy Suites North Charleston Convention Center. The meeting brings together international leaders and colleagues in the scientific and biotechnology sectors,...
Case Closed On Shark Mystery - Washington Post
The scientists, whose study was published in the journal Current Biology, had tagged 25 sharks off Cape Cod with tracking devices, designed to release from the animals' skin after days or weeks and transmit data about where it had been....
MHT-CET: Biology balm after physics bouncer - Times of India
Following that tough ride, the biology and maths papers marked sweet endings to two years of seemingly endless toil. When Vihang Soni walked out of his exam centre on Tuesday after appearing for the first paper-physics and chemistry-he said,...
Climate Change Driving Michigan Mammals North - Science Daily (press release)
The finding, by researchers at the University of Michigan, Michigan State University and Ohio's Miami University, appears in the June issue of the journal Global Change Biology. "When you read about changes in flora and fauna related to climatic...

History of biology

The "central dogma of molecular biology" (originally a "dogma" only in jest) was proposed by Francis Crick in 1958.[83] This is Crick's reconstruction of how he conceived of the central dogma at the time. The solid lines represent (as it seemed in 1958) known modes of information transfer, and the dashed lines represent postulated ones.

The history of biology traces the study of the living world from ancient to modern times. Although the concept of biology as a single coherent field arose in the 19th century, the biological sciences emerged from traditions of medicine and natural history reaching back to ancient Egyptian medicine and the works of Aristotle and Galen in the ancient Greco-Roman world, which were then further developed in the Middle Ages by Muslim physicians and scholars such as al-Jahiz, Avicenna, Avenzoar, Ibn al-Baitar and Ibn al-Nafis. During the European Renaissance and early modern period, biological thought was revolutionized in Europe by a renewed interest in empiricism and the discovery of many novel organisms. Prominent in this movement were Vesalius and Harvey, who used experimentation and careful observation in physiology, and naturalists such as Linnaeus and Buffon who began to classify the diversity of life and the fossil record, as well as the development and behavior of organisms. Microscopy revealed the previously unknown world of microorganisms, laying the groundwork for cell theory. The growing importance of natural theology, partly a response to the rise of mechanical philosophy, encouraged the growth of natural history (although it entrenched the argument from design).

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—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.

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 "neo-Darwinian synthesis". 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 organismal biology—the fields that deal with whole organisms and groups of organisms—and the fields related to cellular and molecular biology. 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.

The word biology is formed by combining the Greek βίος (bios), meaning "life", and the suffix '-logy', meaning "science of", "knowledge of", "study of", based on the Greek verb λεγειν, 'legein' = "to select", "to gather" (cf. the noun λόγος, 'logos' = "word"). The term biology in its modern sense appears to have been introduced independently by Karl Friedrich Burdach (in 1800), Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur, 1802) and Jean-Baptiste Lamarck (Hydrogéologie, 1802). The word itself appears in the title of Volume 3 of Michael Christoph Hanov's Philosophiae naturalis sive physicae dogmaticae: Geologia, biologia, phytologia generalis et dendrologia, published in 1766.

Before biology, there were several terms used for the study of animals and plants. Natural history referred to the descriptive aspects of biology, though it also included mineralogy and other non-biological fields; from the Middle Ages through the Renaissance, the unifying framework of natural history was the scala naturae or Great Chain of Being. Natural philosophy and natural theology encompassed the conceptual and metaphysical basis of plant and animal life, dealing with problems of why organisms exist and behave the way they do, though these subjects also included what is now geology, physics, chemistry, and astronomy. Physiology and (botanical) pharmacology were the province of medicine. Botany, zoology, and (in the case of fossils) geology replaced natural history and natural philosophy in the 18th and 19th centuries before biology was widely adopted.

The earliest humans must have had and passed on knowledge about plants and animals to increase their chances of survival. This may have included knowledge of human and animal anatomy and aspects of animal behavior (such as migration patterns). However, the first major turning point in biological knowledge came with the Neolithic Revolution about 10,000 years ago. Humans first domesticated plants for farming, then livestock animals to accompany the resulting sedentary societies.

The ancient cultures of Mesopotamia, Egypt, the Indian subcontinent, and China (among others) had sophisticated systems of philosophical, religious, and technical knowledge that encompassed the living world, and creation myths often centered on some aspect of life. However, the roots of modern biology are usually traced back to the secular tradition of ancient Greek philosophy.

The pre-Socratic philosophers asked many questions about life but produced little systematic knowledge of specifically biological interest—though the attempts of the atomists to explain life in purely physical terms would recur periodically through the history of biology. However, the medical theories of Hippocrates and his followers, especially humorism, had a lasting impact.

The philosopher Aristotle was the most influential scholar of the living world from classical antiquity. Though his early work in natural philosophy was speculative, Aristotle's later biological writings were more empirical, focusing on biological causation and the diversity of life. He made countless observations of nature, especially the habits and attributes of plants and animals in the world around him, which he devoted considerable attention to categorizing. In all, Aristotle classified 540 animal species, and dissected at least 50. He believed that intellectual purposes, formal causes, guided all natural processes.

Aristotle, and nearly all Western scholars after him until the 18th century, believed that creatures were arranged in a graded scale of perfection rising from plants on up to humans: the scala naturae or Great Chain of Being. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany—the History of Plants—which survived as the most important contribution of antiquity to botany, even into the Middle Ages. Many of Theophrastus' names survive into modern times, such as carpos for fruit, and pericarpion for seed vessel. Pliny the Elder was also known for his knowledge of plants and nature, and was the most prolific compiler of zoological descriptions.

A few scholars in the Hellenistic period under the Ptolemies—particularly Herophilus of Chalcedon and Erasistratus of Chios—amended Aristotle's physiological work, even performing experimental dissections and vivisections. Claudius Galen became the most important authority on medicine and anatomy. Though a few ancient atomists such as Lucretius challenged the teleological Aristotelian viewpoint that all aspects of life are the result of design or purpose, teleology (and after the rise of Christianity, natural theology) would remain central to biological thought essentially until the 18th and 19th centuries. Ernst W. Mayr argued that "Nothing of any real consequence happened in biology after Lucretius and Galen until the Renaissance." The ideas of the Greek traditions of natural history and medicine survived, but they were generally taken unquestioningly in medieval Europe.

The decline of the Roman Empire led to the disappearance or destruction of much knowledge, though physicians still incorporated many aspects of the Greek tradition into training and practice. In Byzantium and the Islamic world, many of the Greek works were translated into Arabic and many of the works of Aristotle were preserved.

Medieval Muslim physicians, scientists and philosophers made significant contributions to biological knowledge between the 8th and 13th centuries during what is known as the "Islamic Golden Age" or "Muslim Agricultural Revolution". In zoology, for example, the Afro-Arab scholar al-Jahiz (781-869) described early evolutionary ideas such as the struggle for existence. He also introduced the idea of a food chain, and was an early adherent of environmental determinism. The Kurdish biologist Al-Dinawari (828-896) is considered the founder of Arabic botany for his Book of Plants, in which he described at least 637 plants and discussed plant evolution from its birth to its death, describing the phases of plant growth and the production of flowers and fruit. In anatomy and physiology, the Persian physician Rhazes (865-925) carried out an early experiment to discredit the Galenic theory of humorism.

In experimental medicine, the Persian physician Avicenna (980-1037) introduced clinical trials and clinical pharmacology in The Canon of Medicine, which remained an authoritative text in European medical education up until the 17th century. The Andalusian-Arabian physician Avenzoar (1091-1161) was an early adherent of experimental dissection and autopsy, which he carried out to prove that the skin disease scabies was caused by a parasite, a discovery which upset the theory of humorism. He also introduced experimental surgery, where animal testing is used to experiment with surgical techniques prior to using them on humans. During a famine in Egypt in 1200, Abd-el-latif observed and examined a large number of skeletons, and he discovered that Galen was incorrect regarding the formation of the bones of the lower jaw and sacrum.

In the early 13th century, the Andalusian-Arabian biologist Abu al-Abbas al-Nabati developed an early scientific method for botany, introducing empirical and experimental techniques in the testing, description and identification of numerous materia medica, and separating unverified reports from those supported by actual tests and observations. His student Ibn al-Baitar (d. 1248) wrote a pharmaceutical encyclopedia describing 1,400 plants, foods, and drugs, 300 of which were his own original discoveries. A Latin translation of his work was useful to European biologists and pharmacists in the 18th and 19th centuries.

The Arabian physician Ibn al-Nafis (1213-1288) was another early adherent of experimental dissection and autopsy, who in 1242, discovered the pulmonary circulation and coronary circulation, which form the basis of the circulatory system. He also described the concept of metabolism, and discredited the incorrect Galenic and Avicennian theories on the four humours, pulsation, bones, muscles, intestines, sensory organs, bilious canals, esophagus and stomach.

During the High Middle Ages, a few European scholars such as Hildegard of Bingen, Albertus Magnus and Frederick II expanded the natural history canon. The rise of European universities, though important for the development of physics and philosophy, had little impact on biological scholarship.

The European Renaissance brought expanded interest in both empirical natural history and physiology. In 1543, Andreas Vesalius inaugurated the modern era of Western medicine with his seminal human anatomy treatise De humani corporis fabrica, which was based on dissection of corpses. Vesalius was the first in a series of anatomists who gradually replaced scholasticism with empiricism in physiology and medicine, relying on first-hand experience rather than authority and abstract reasoning. Via herbalism, medicine was also indirectly the source of renewed empiricism in the study of plants. Otto Brunfels, Hieronymus Bock and Leonhart Fuchs wrote extensively on wild plants, the beginning of a nature-based approach to the full range of plant life. Bestiaries—a genre that combines both the natural and figurative knowledge of animals—also became more sophisticated, especially with the work of William Turner, Pierre Belon, Guillaume Rondelet, Conrad Gessner, and Ulisse Aldrovandi.

Artists such as Albrecht Dürer and Leonardo da Vinci, often working with naturalists, were also interested in the bodies of animals and humans, studying physiology in detail and contributing to the growth of anatomical knowledge. The traditions of alchemy and natural magic, especially in the work of Paracelsus, also laid claim to knowledge of the living world. Alchemists subjected organic matter to chemical analysis and experimented liberally with both biological and mineral pharmacology. This was part of a larger transition in world views (the rise of the mechanical philosophy) that continued into the 17th century, as the traditional metaphor of nature as organism was replaced by the nature as machine metaphor.

Extending the work of Vesalius into experiments on still living bodies (of both humans and animals), William Harvey and other natural philosophers investigated the roles of blood, veins and arteries. Harvey's De motu cordis in 1628 was the beginning of the end for Galenic theory, and alongside Santorio Santorio's studies of metabolism, it served as an influential model of quantitative approaches to physiology.

In the early 17th century, the micro-world of biology was just beginning to open up. A few lensmakers and natural philosophers had been creating crude microscopes since the late 16th century, and Robert Hooke published the seminal Micrographia based on observations with his own compound microscope in 1665. But it was not until Antony van Leeuwenhoek's dramatic improvements in lensmaking beginning in the 1670s—ultimately producing up to 200-fold magnification with a single lens—that scholars discovered spermatozoa, bacteria, infusoria and the sheer strangeness and diversity of microscopic life. Similar investigations by Jan Swammerdam led to new interest in entomology and built the basic techniques of microscopic dissection and staining.

As the microscopic world was expanding, the macroscopic world was shrinking. Botanists such as John Ray worked to incorporate the flood of newly discovered organisms shipped from across the globe into a coherent taxonomy, and a coherent theology (natural theology). Debate over another flood, the Noachian, catalyzed the development of paleontology; in 1669 Nicholas Steno published an essay on how the remains of living organisms could be trapped in layers of sediment and mineralized to produce fossils. Although Steno's ideas about fossilization were well known and much debated among natural philosophers, an organic origin for all fossils would not be accepted by all naturalists until the end of the 18th century due to philosophical and theological debate about issues such as the age of the earth and extinction.

Systematizing, naming and classifying dominated natural history throughout much of the 17th and 18th centuries. 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. While Linnaeus conceived of species as unchanging parts of a designed hierarchy, the other great naturalist of the 18th century, 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.

The discovery and description of new species and the collection of specimens became a passion of scientific gentlemen and a lucrative enterprise for entrepreneurs; many naturalists traveled the globe in search of scientific knowledge and adventure.

Up through the nineteenth century, the scope of biology was largely divided between medicine, which investigated questions of form and function (i.e., physiology), and natural history, which was concerned with the diversity of life and interactions among different forms of life and between life and non-life. By 1900, much of these domains overlapped, while natural history (and its counterpart natural philosophy) had largely given way to more specialized scientific disciplines—cytology, bacteriology, morphology, embryology, geography, and geology.

Widespread travel by naturalists in the early- to mid-nineteenth century resulted in a wealth of new information about the diversity and distribution of living organisms. Of particular importance was the work of Alexander von Humboldt, which analyzed the relationship between organisms and their environment (i.e., the domain of natural history) using the quantitative approaches of natural philosophy (i.e., physics and chemistry). Humboldt's work laid the foundations of biogeography and inspired several generations of scientists.

The emerging discipline of geology also brought natural history and natural philosophy closer together; the establishment of the stratigraphic column linked the spacial distribution of organisms to their temporal distribution, a key precursor to concepts of evolution. Georges Cuvier and others made great strides in comparative anatomy and paleontology in the late 1790s and early 1800s. In a series of lectures and papers that made detailed comparisons between living mammals and fossil remains Cuvier was able to establish that the fossils were remains of species that had become extinct—rather than being remains of species still alive elsewhere in the world, as had been widely believed. Fossils discovered and described by Gideon Mantell, William Buckland, Mary Anning, and Richard Owen among others helped establish that there had been an 'age of reptiles' that had preceded even the prehistoric mammals. These discoveries captured the public imagination and focused attention on the history of life on earth. Most of these geologists held to catastrophism, but Charles Lyell's influential Principles of Geology (1830) popularised Hutton's uniformitarianism, a theory that explained the geological past and present on equal terms.

The most significant evolutionary theory before Darwin's was that of Jean-Baptiste Lamarck; based on the inheritance of acquired characteristics (an inheritance mechanism that was widely accepted until the 20th century), it described a chain of development stretching from the lowliest microbe to humans. 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, created a more successful evolutionary theory based on natural selection; similar evidence lead Alfred Russel Wallace to independently reach the same conclusions.

The 1859 publication of Darwin's theory in On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life is often considered the central event in the history of modern biology. Darwin's established credibility as a naturalist, the sober tone of the work, and most of all the sheer strength and volume of evidence presented, allowed Origin to succeed where previous evolutionary works such as the anonymous Vestiges of Creation had failed. Most scientists were convinced of evolution and common descent by the end of the 19th century. However, natural selection would not be accepted as the primary mechanism of evolution until well into the 20th century, as most contemporary theories of heredity seemed incompatible with the inheritance of random variation.

Wallace, following on earlier work by de Candolle, Humbolt and Darwin, made major contributions to zoogeography. Because of his interest in the transmutation hypothosis, he paid particular attention to the geographical distribution of closely allied species during his field work first in South America and then in the Malay archipelago. While in the archipelago he identified the Wallace line, which runs through the spice islands dividing the fauna of the archipelago between an Asian zone and a New Guinea/Australian zone. His key question, as to why the fauna of islands with such similar climates should be so different, could only be answered by considering their origin. In 1876 he wrote The geographical distribution of animals, which was the standard reference work for over half a century, and a sequel, Island Life, in 1880 that focused on island biogeography. He extended the 6 zone system developed by Philip Sclater for describing the geographical distribution of birds to animals of all kinds. His method of tabulating data on animal groups in geographic zones highlighted the discontinuities; and his appreciation of evolution allowed him to propose rational explanations, which had not been done before.

The scientific study of heredity grew rapidly in the wake of Darwin's Origin of Species with the work of Francis Galton and the biometricians. The origin of genetics is usually traced to the 1866 work of the monk Gregor Mendel, who would later be credited with the laws of inheritance. However, his work was not recognized as significant until 35 years afterward. In the meantime, a variety of theories of inheritance (based on pangenesis, orthogenesis, or other mechanisms) were debated and investigated vigorously. Embryology and ecology also became central biological fields, especially as linked to evolution and popularized in the work of Ernst Haeckel. Most of the 19th century work on heredity, however, was not in the realm of natural history, but that of experimental physiology.

Over the course of the 19th century, the scope of physiology expanded greatly, from a primarily medically-oriented field to a wide-ranging investigation of the physical and chemical processes of life—including plants, animals, and even microorganisms in addition to man. Living things as machines became a dominant metaphor in biological (and social) thinking.

Advances in microscopy also had a profound impact on biological thinking. 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.

Cell theory led biologists to re-envision individual organisms as interdependent assemblages of individual cells. Scientists in the rising field of cytology, armed with increasingly powerful microscopes and new staining methods, soon found that even single cells were far more complex than the homogeneous fluid-filled chambers described by earlier microscopists. Robert Brown had described the nucleus in 1831, and by the end of the 19th century cytologists identified many of the key cell components: chromosomes, centrosomes mitochondria, chloroplasts, and other structures made visible through staining. Between 1874 and 1884 Walther Flemming described the discrete stages of mitosis, showing that they were not artifacts of staining but occurred in living cells, and moreover, that chromosomes doubled in number just before the cell divided and a daughter cell was produced. Much of the research on cell reproduction came together in August Weismann's theory of heredity: he identified the nucleus (in particular chromosomes) as the hereditary material, proposed the distinction between somatic cells and germ cells (arguing that chromosome number must be halved for germ cells, a precursor to the concept of meiosis), and adopted Hugo de Vries's theory of pangenes. Weismannism was extremely influential, especially in the new field of experimental embryology.

By the mid 1850s the miasma theory of disease was largely superseded by the germ theory of disease, creating extensive interest in microorganisms and their interactions with other forms of life. By the 1880s, bacteriology was becoming a coherent discipline, especially through the work of Robert Koch, who introduced methods for growing pure cultures on agar gels containing specific nutrients in Petri dishes. The long-held idea that living organisms could easily originate from nonliving matter (spontaneous generation) was attacked in a series of experiments carried out by Louis Pasteur, while debates over vitalism vs. mechanism (a perennial issue since the time of Aristotle and the Greek atomists) continued apace.

In chemistry, one central issue was the distinction between organic and inorganic substances, especially in the context of organic transformations such as fermentation and putrefaction. Since Aristotle these had been considered essentially biological (vital) processes. However, Friedrich Wöhler, Justus Liebig and other pioneers of the rising field of organic chemistry—building on the work of Lavoisier—showed that the organic world could often be analyzed by physical and chemical methods. In 1828 Wöhler showed that the organic substance urea could be created by chemical means that do not involve life, providing a powerful argument against vitalism. Cell extracts ("ferments") that could effect chemical transformations were discovered, beginning with diastase in 1833, and by the end of the 19th century the concept of enzymes was well established, though equations of chemical kinetics would not be applied to enzymatic reactions until the early 20th century.

Physiologists such as Claude Bernard explored (through vivisection and other experimental methods) the chemical and physical functions of living bodies to an unprecedented degree, laying the groundwork for endocrinology (a field that developed quickly after the discovery of the first hormone, secretin, in 1902), biomechanics, and the study of nutrition and digestion. The importance and diversity of experimental physiology methods, within both medicine and biology, grew dramatically over the second half of the 19th century. The control and manipulation of life processes became a central concern, and experiment was placed at the center of biological education.

At the beginning of the 20th century, biological research was largely a professional endeavour. Most work was still done in the natural history mode, which emphasized morhphological and phylogenetic analysis over experiment-based causal explanations. However, anti-vitalist experimental physiologists and embryologists, especially in Europe, were increasingly influential. The tremendous success of experimental approaches to development, heredity, and metabolism in the 1900s and 1910s demonstrated the power of experimentation in biology. In the following decades, experimental work replaced natural history as the dominant mode of research.

In the early 20th century, naturalists were faced with increasing pressure to add rigor and preferably experimentation to their methods, as the newly prominent laboratory-based biological disciplines had done. Ecology had emerged as a combination of biogeography with the biogeochemical cycle concept pioneered by chemists; field biologists developed quantitative methods such as the quadrat and adapted laboratory instruments and cameras for the field to further set their work apart from traditional natural history. Zoologists and botanists did what they could to mitigate the unpredictability of the living world, performing laboratory experiments and studying semi-controlled natural environments such as gardens; new institutions like the Carnegie Station for Experimental Evolution and the Marine Biological Laboratory provided more controlled environments for studying organisms through their entire life cycles.

The ecological succession concept, pioneered in the 1900s and 1910s by Henry Chandler Cowles and Frederic Clements, was important in early plant ecology. Alfred Lotka's predator-prey equations, G. Evelyn Hutchinson's studies of the biogeography and biogeochemical structure of lakes and rivers (limnology) and Charles Elton's studies of animal food chains were pioneers among the succession of quantitative methods that colonized the developing ecological specialties. Ecology became an independent discipline in the 1940s and 1950s after Eugene P. Odum synthesized many of the concepts of ecosystem ecology, placing relationships between groups of organisms (especially material and energy relationships) at the center of the field.

In the 1960s, as evolutionary theorists explored the possibility of multiple units of selection, ecologists turned to evolutionary approaches. In population ecology, debate over group selection was brief but vigorous; by 1970, most biologists agreed that natural selection was rarely effective above the level of individual organisms. The evolution of ecosystems, however, became a lasting research focus. Ecology expanded rapidly with the rise of the environmental movement; the International Biological Program attempted to apply the methods of big science (which had been so successful in the physical sciences) to ecosystem ecology and pressing environmental issues, while smaller-scale independent efforts such as island biogeography and the Hubbard Brook Experimental Forest helped redefine the scope of an increasingly diverse discipline.

1900 marked the so-called rediscovery of Mendel: Hugo de Vries, Carl Correns, and Erich von Tschermak independently arrived at Mendel's laws (which were not actually present in Mendel's work). Soon after, cytologists (cell biologists) proposed that chromosomes were the hereditary material. Between 1910 and 1915, Thomas Hunt Morgan and the "Drosophilists" in his fly lab forged these two ideas—both controversial—into the "Mendelian-chromosome theory" of heredity. They quantified the phenomenon of genetic linkage and postulated that genes reside on chromosomes like beads on string; they hypothesized crossing over to explain linkage and constructed genetic maps of the fruit fly Drosophila melanogaster, which became a widely used model organism.

Hugo de Vries tried to link the new genetics with evolution; building on his work with heredity and hybridization, he proposed a theory of mutationism, which was widely accepted in the early 20th century. Lamarckism also had many adherents. Darwinism was seen as incompatible with the continuously variable traits studied by biometricians, which seemed only partially heritable. In the 1920s and 1930s—following the acceptance of the Mendelian-chromosome theory— the emergence of the discipline of population genetics, with the work of R.A. Fisher, J.B.S. Haldane and Sewall Wright, unified the idea of evolution by natural selection with Mendelian genetics, producing the modern synthesis. The inheritance of acquired characters was rejected, while mutationism gave way as genetic theories matured.

In the second half of the century the ideas of population genetics began to be applied in the new discipline of the genetics of behavior, sociobiology, and, especially in humans, evolutionary psychology. In the 1960s W.D. Hamilton and others developed game theory approaches to explain altruism from an evolutionary perspective through kin selection. The possible origin of higher organisms through endosymbiosis, and contrasting approaches to molecular evolution in the gene-centered view (which held selection as the predominant cause of evolution) and the neutral theory (which made genetic drift a key factor) spawned perennial debates over the proper balance of adaptationism and contingency in evolutionary theory.

In the 1970s Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibrium which holds that stasis is the most prominent feature of the fossil record, and that most evolutionary changes occur rapidly over relatively short periods of time. In 1980 Luis Alvarez and Walter Alvarez proposed the hypothesis that an impact event was responsible for the Cretaceous-Tertiary extinction event. Also in the early 1980s, statistical analysis of the fossil record of marine organisms published by Jack Sepkoski and David M. Raup lead to a better appreciation of the importance of mass extinction events to the history of life on earth.

By the end of the 19th century all of the major pathways of drug metabolism had been discovered, along with the outlines of protein and fatty acid metabolism and urea synthesis. In the early decades of the twentieth century, the minor components of foods in human nutrition, the vitamins, began to be isolated and synthesized. Improved laboratory techniques such as chromatography and electrophoresis led to rapid advances in physiological chemistry, which—as biochemistry—began to achieve independence from its medical origins. In the 1920s and 1930s, biochemists—led by Hans Krebs and Carl and Gerty Cori—began to work out many of the central metabolic pathways of life: the citric acid cycle, glycogenesis and glycolysis, and the synthesis of steroids and porphyrins. Between the 1930s and 1950s, Fritz Lipmann and others established the role of ATP as the universal carrier of energy in the cell, and mitochondria as the powerhouse of the cell. Such traditionally biochemical work continued to be very actively pursued throughout the 20th century and into the 21st.

Following the rise of classical genetics, many biologists—including a new wave of physical scientists in biology—pursued the question of the gene and its physical nature. Warren Weaver—head of the science division of the Rockefeller Foundation—issued grants to promote research that applied the methods of physics and chemistry to basic biological problems, coining the term molecular biology for this approach in 1938; many of the significant biological breakthroughs of the 1930s and 1940s were funded by the Rockefeller Foundation.

Like biochemistry, the overlapping disciplines of bacteriology and virology (later combined as microbiology), situated between science and medicine, developed rapidly in the early 20th century. Félix d'Herelle's isolation of bacteriophage during World War I initiated a long line of research focused on phage viruses and the bacteria they infect.

The development of standard, genetically uniform organisms that could produce repeatable experimental results was essential for the development of molecular genetics. After early work with Drosophila and maize, the adoption of simpler model systems like the bread mold Neurospora crassa made it possible to connect genetics to biochemistry, most importantly with Beadle and Tatum's "one gene, one enzyme" hypothesis in 1941. Genetics experiments on even simpler systems like tobacco mosaic virus and bacteriophage, aided by the new technologies of electron microscopy and ultracentrifugation, forced scientists to re-evaluate the literal meaning of life; virus heredity and reproducing nucleoprotein cell structures outside the nucleus ("plasmagenes") complicated the accepted Mendelian-chromosome theory.

Oswald Avery showed in 1943 that DNA was likely the genetic material of the chromosome, not its protein; the issue was settled decisively with the 1952 Hershey-Chase experiment—one of many contribution from the so-called phage group centered around physicist-turned-biologist Max Delbrück. In 1953 James D. Watson and Francis Crick, building on the work of Maurice Wilkins and Rosalind Franklin, suggested that the structure of DNA was a double helix. In their famous paper "Molecular structure of Nucleic Acids", Watson and Crick noted coyly, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." After the 1958 Meselson-Stahl experiment confirmed the semiconservative replication of DNA, it was clear to most biologists that nucleic acid sequence must somehow determine amino acid sequence in proteins; physicist George Gamow proposed that a fixed genetic code connected proteins and DNA. Between 1953 and 1961, there were few known biological sequences—either DNA or protein—but an abundance of proposed code systems, a situation made even more complicated by expanding knowledge of the intermediate role of RNA. To actually decipher the code, it took an extensive series of experiments in biochemistry and bacterial genetics, between 1961 and 1966—most importantly the work of Nirenberg and Khorana.

In addition to the Division of Biology at Caltech, the Laboratory of Molecular Biology (and its precursors) at Cambridge, and a handful of other institutions, the Pasteur Institute became a major center for molecular biology research in the late 1950s. Scientists at Cambridge, led by Max Perutz and John Kendrew, focused on the rapidly developing field of structural biology, combining X-ray crystallography with molecular modelling and the new computational possibilities of digital computing (benefiting both directly and indirectly from the military funding of science). A number of biochemists led by Fred Sanger later joined the Cambridge lab, bringing together the study of macromolecular structure and function. At the Pasteur Institute, François Jacob and Jacques Monod followed the 1959 PaJaMo experiment with a series of publications regarding the lac operon that established the concept of gene regulation and identified what came to be known as messenger RNA. By the mid-1960s, the intellectual core of molecular biology—a model for the molecular basis of metabolism and reproduction— was largely complete.

The late 1950s to the early 1970s was a period of intense research and institutional expansion for molecular biology, which had only recently become a somewhat coherent discipline. In what organismic biologist E. O. Wilson called "The Molecular Wars", the methods and practitioners of molecular biology spread rapidly, often coming to dominate departments and even entire disciplines. Molecularization was particularly important in genetics, immunology, embryology, and neurobiology, while the idea that life is controlled by a "genetic program"—a metaphor Jacob and Monod introduced from the emerging fields of cybernetics and computer science—became an influential perspective throughout biology. Immunology in particular became linked with molecular biology, with innovation flowing both ways: the clonal selection theory developed by Niels Jerne and Frank Macfarlane Burnet in the mid 1950s helped shed light on the general mechanisms of protein synthesis.

Biotechnology in the general sense has been an important part of biology since the late 19th century. With the industrialization of brewing and agriculture, chemists and biologists became aware of the great potential of human-controlled biological processes. In particular, fermentation proved a great boon to chemical industries. By the early 1970s, a wide range of biotechnologies were being developed, from drugs like penicillin and steroids to foods like Chlorella and single-cell protein to gasohol—as well as a wide range of hybrid high-yield crops and agricultural technologies, the basis for the Green Revolution.

Biotechnology in the modern sense of genetic engineering began in the 1970s, with the invention of recombinant DNA techniques. Restriction enzymes were discovered and characterized in the late 1960s, following on the heels of the isolation, then duplication, then synthesis of viral genes. Beginning with the lab of Paul Berg in 1972 (aided by EcoRI from Herbert Boyer's lab, building on work with ligase by Arthur Kornberg's lab), molecular biologists put these pieces together to produce the first transgenic organisms. Soon after, others began using plasmid vectors and adding genes for antibiotic resistance, greatly increasing the reach of the recombinant techniques.

Wary of the potential dangers (particularly the possibility of a prolific bacteria with a viral cancer-causing gene), the scientific community as well as a wide range of scientific outsiders reacted to these developments with both enthusiasm and fearful restraint. Prominent molecular biologists led by Berg suggested a temporary moratorium on recombinant DNA research until the dangers could be assessed and policies could be created. This moratorium was largely respected, until the participants in the 1975 Asilomar Conference on Recombinant DNA created policy recommendations and concluded that the technology could be used safely.

Following Asilomar, new genetic engineering techniques and applications developed rapidly. DNA sequencing methods improved greatly (pioneered by Fred Sanger and Walter Gilbert), as did oligonucleotide synthesis and transfection techniques. Researchers learned to control the expression of transgenes, and were soon racing—in both academic and industrial contexts—to create organisms capable of expressing human genes for the production of human hormones. However, this was a more daunting task than molecular biologists had expected; developments between 1977 and 1980 showed that, due to the phenomena of split genes and splicing, higher organisms had a much more complex system of gene expression than the bacteria models of earlier studies. The first such race, for synthesizing human insulin, was won by Genentech. This marked the beginning of the biotech boom (and with it, the era of gene patents), with an unprecedented level of overlap between biology, industry, and law.

By the 1980s, protein sequencing had already transformed methods of scientific classification of organisms (especially cladistics) but biologists soon began to use RNA and DNA sequences as characters; this expanded the significance of molecular evolution within evolutionary biology, as the results of molecular systematics could be compared with traditional evolutionary trees based on morphology. Following the pioneering ideas of Lynn Margulis on endosymbiotic theory, which holds that some of the organelles of eukaryotic cells originated from free living prokaryotic organisms through symbiotic relationships, even the overall division of the tree of life was revised. Into the 1990s, the five domains (Plants, Animals, Fungi, Protists, and Monerans) became three (the Archaea, the Bacteria, and the Eukarya) based on Carl Woese's pioneering molecular systematics work with 16S rRNA sequencing.

The development and popularization of the polymerase chain reaction (PCR) in mid 1980s (by Kary Mullis and others at Cetus Corp.) marked another watershed in the history of modern biotechnology, greatly increasing the ease and speed of genetic analysis. Coupled with the use of expressed sequence tags, PCR led to the discovery of many more genes than could be found through traditional biochemical or genetic methods and opened the possibility of sequencing entire genomes.

The unity of much of the morphogenesis of organisms from fertilized egg to adult began to be unraveled after the discovery of the homeobox genes, first in fruit flies, then in other insects and animals, including humans. These developments led to advances in the field of evolutionary developmental biology towards understanding how the various body plans of the animal phyla have evolved and how they are related to one another.

The Human Genome Project—the largest, most costly single biological study ever undertaken—began in 1988 under the leadership of James D. Watson, after preliminary work with genetically simpler model organisms such as E. coli, S. cerevisiae and C. elegans. Shotgun sequencing and gene discovery methods pioneered by Craig Venter—and fueled by the financial promise of gene patents with Celera Genomics— led to a public-private sequencing competition that ended in compromise with the first draft of the human DNA sequence announced in 2000.

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Biology

Escherichia coli

Biology (from Greek βιολογία - βίος, bios, "life"; -λογία, -logia, study of) is a branch of the natural sciences concerned with the study of living organisms and their interaction with each other and their environment. The term was first used by the French naturalist Jean-Baptiste Lamarck. The science of biology examines the structure, function, growth, origin, evolution, distribution and classification of living things. Five unifying principles form the foundation of modern biology: cell theory, evolution, gene theory, energy, and homeostasis.

Biology as a separate science was developed in the nineteenth century, as scientists discovered that organisms shared fundamental characteristics. It is now a standard subject of instruction at schools and universities around the world, and over a million papers are published annually in a wide array of biology and medicine journals.

Traditionally, the specialized disciplines of biology are grouped by the type of organism being studied: botany, the study of plants; zoology, the study of animals; and microbiology, the study of microorganisms. These fields are further divided based on the scale at which organisms are studied and the methods used to study them: biochemistry examines the fundamental 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 and organ systems of an organism; and ecology examines how various organisms interrelate with their environment.

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 (Alfred Russel Wallace is recognized as the co-discoverer of this concept as he helped research and experiment with the concept of evolution). 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 called 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 believed. This misconception was challenged by William Harvey's diction that "all life from egg" (from the Latin "Omne vivum ex ovo"), a foundational concept of modern biology. It means that there is an unbroken continuity of life from its initial origin to the present time.

A group of organisms share 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 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 observed among the human population.

Biological form and the corresponding human protein.

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 exists at the cellular level, for example cells maintain a stable internal acidity (pH); and at the level of the organism, for example warm-blooded animals maintain a constant internal body temperature. Homeostasis is a term that is also used in association with ecosystems, for example, the roots of plants help prevent soil from eroding, which helps to maintain the ecosystem. Tissues and organs can also maintain homeostasis.

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 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 in the presence of water and minerals. A few ecosystems, however, depend entirely on energy extracted from methane, sulfides, or other inorganic molecules by chemosynthetic microorganisms. Some of the captured energy is used to produce biomass to sustain life and provide energy for its growth and development. A part of this energy is lost as heat and waste molecules. The common processes for converting energy in chemical substances into energy useful to sustain life are metabolism and 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. 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 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 Brachydanio rerio, the mouse Mus musculus, and the weed Arabidopsis thaliana.

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 also borrows techniques from both 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 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 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.

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 "all life from egg" (from the Latin "Omne vivum ex ovo"), 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.

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).

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.

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.

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

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 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 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 environment of an organism includes both its habitat, which can be described as the sum of local abiotic factors such as climate and ecology, as well as the other organisms that share its habitat. 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 the term with biology is more frequently used when studying diseases, viruses, and microbes, and the term with ecology is used when studying plants and animals. As can be surmised, ecology is a science that draws on several disciplines.

Ethology studies animal behavior (particularly 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 savannah. 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 the province of ecology.

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Molecular biology

Schematic relationship between biochemistry, genetics, and molecular biology

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

Much of the work in molecular biology is quantitative, and recently much work has been done at the interface of molecular biology and computer science in bioinformatics and computational biology. As of the early 2000s, the study of gene structure and function, molecular genetics, has been amongst the most prominent sub-field of molecular biology.

Increasingly many other fields of biology focus on molecules, either directly studying their interactions in their own right such as in cell biology and developmental biology, or indirectly, where the techniques of molecular biology are used to infer historical attributes of populations or species, as in fields in evolutionary biology such as population genetics and phylogenetics. There is also a long tradition of studying biomolecules "from the ground up" in biophysics.

Since the late 1950s and early 1960s, molecular biologists have learned to characterize, isolate, and manipulate the molecular components of cells and organisms. These components include DNA, the repository of genetic information; RNA, a close relative of DNA whose functions range from serving as a temporary working copy of DNA to actual structural and enzymatic functions as well as a functional and structural part of the translational apparatus; and proteins, the major structural and enzymatic type of molecule in cells.

One of the most basic techniques of molecular biology to study protein function is expression cloning. In this technique, DNA coding for a protein of interest is cloned (using PCR and/or restriction enzymes) into a plasmid (known as an expression vector). This plasmid may have special promoter elements to drive production of the protein of interest, and may also have antibiotic resistance markers to help follow the plasmid.

This plasmid can be inserted into either bacterial or animal cells. Introducing DNA into bacterial cells can be done by transformation (via uptake of naked DNA), conjugation (via cell-cell contact) or by transduction (via viral vector). Introducing DNA into eukaryotic cells, such as animal cells, by physical or chemical means is called transfection. Several different transfection techniques are available, such as calcium phosphate transfection,electroporation, microinjection and liposome transfection. DNA can also be introduced into eukaryotic cells using viruses or bacteria as carriers, the latter is sometimes called bactofection and in particular uses Agrobacterium tumefaciens. The plasmid may be integrated into the genome, resulting in a stable transfection, or may remain independent of the genome, called transient transfection.

In either case, DNA coding for a protein of interest is now inside a cell, and the protein can now be expressed. A variety of systems, such as inducible promoters and specific cell-signaling factors, are available to help express the protein of interest at high levels. Large quantities of a protein can then be extracted from the bacterial or eukaryotic cell. The protein can be tested for enzymatic activity under a variety of situations, the protein may be crystallized so its tertiary structure can be studied, or, in the pharmaceutical industry, the activity of new drugs against the protein can be studied.

The polymerase chain reaction is an extremely versatile technique for copying DNA. In brief, PCR allows a single DNA sequence to be copied (millions of times), or altered in predetermined ways. For example, PCR can be used to introduce restriction enzyme sites, or to mutate (change) particular bases of DNA, the latter is a method referred to as "Quick change". PCR can also be used to determine whether a particular DNA fragment is found in a cDNA library. PCR has many variations, like reverse transcription PCR (RT-PCR) for amplification of RNA, and, more recently, real-time PCR (QPCR) which allow for quantitative measurement of DNA or RNA molecules.

Gel electrophoresis is one of the principal tools of molecular biology. The basic principle is that DNA, RNA, and proteins can all be separated by means of an electric field. In agarose gel electrophoresis, DNA and RNA can be separated on the basis of size by running the DNA through an agarose gel. Proteins can be separated on the basis of size by using an SDS-PAGE gel, or on the basis of size and their electric charge by using what is known as a 2D gel electrophoresis.

Named after its inventor, biologist Edwin Southern, the Southern blot is a method for probing for the presence of a specific DNA sequence within a DNA sample. DNA samples before or after restriction enzyme digestion are separated by gel electrophoresis and then transferred to a membrane by blotting via capillary action. The membrane is then exposed to a labeled DNA probe that has a complement base sequence to the sequence on the DNA of interest. Most original protocols used radioactive labels, however non-radioactive alternatives are now available. Southern blotting is less commonly used in laboratory science due to the capacity of other techniques, such as PCR, to detect specific DNA sequences from DNA samples. These blots are still used for some applications, however, such as measuring transgene copy number in transgenic mice, or in the engineering of gene knockout embryonic stem cell lines.

The northern blot is used to study the expression patterns a specific type of RNA molecule as relative comparison among of a set of different samples of RNA. It is essentially a combination of denaturing RNA gel electrophoresis, and a blot. In this process RNA is separated based on size and is then transferred to a membrane that is then probed with a labeled complement of a sequence of interest. The results may be visualized through a variety of ways depending on the label used; however, most result in the revelation of bands representing the sizes of the RNA detected in sample. The intensity of these bands is related to the amount of the target RNA in the samples analyzed. The procedure is commonly used to study when and how much gene expression is occurring by measuring how much of that RNA is present in different samples. It is one of the most basic tools for determining at what time, and under what conditions, certain genes are expressed in living tissues.

Antibodies to most proteins can be created by injecting small amounts of the protein into an animal such as a mouse, rabbit, sheep, or donkey (polyclonal antibodies)or produced in cell culture (monoclonal antibodies). These antibodies can be used for a variety of analytical and preparative techniques.

In western blotting, proteins are first separated by size, in a thin gel sandwiched between two glass plates in a technique known as SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis). The proteins in the gel are then transferred to a PVDF, nitrocellulose, nylon or other support membrane. This membrane can then be probed with solutions of antibodies. Antibodies that specifically bind to the protein of interest can then be visualized by a variety of techniques, including colored products, chemiluminescence, or autoradiography. Often, the antibodies are labeled with an enzymes. When a chemiluminescent substrate is exposed to the enzyme it allows detection. Using western blotting techniques allows not only detection but also quantitative analysis.

Analogous methods to western blotting can be used to directly stain specific proteins in live cells or tissue sections. However, these immunostaining methods, such as FISH, are used more often in cell biology research.

The terms "western" and "northern" are molecular biology jokes that play on the term southern blot. The first blots were with DNA, and since they were done by Ed Southern, they came to be known as Southerns. Patricia Thomas, inventor of the RNA blot, which became known as a "northern", actually didn't use the term. . To carry the joke further, one can find references in the literature to "southwesterns" (protein-DNA interactions), "northwesterns" (protein-RNA interactions) and "farwesterns" (protein-protein interactions).

A DNA array is a collection of spots attached to a solid support such as a microscope slide where each spot contains one or more single-stranded DNA oligonucleotide fragment. Arrays make it possible to put down a large quantity of very small (100 micrometre diameter) spots on a single slide. Each spot has a DNA fragment molecule that is complementary to a single DNA sequence (similar to Southern blotting). A variation of this technique allows the gene expression of an organism at a particular stage in development to be qualified (expression profiling). In this technique the RNA in a tissue is isolated and converted to labeled cDNA. This cDNA is then hybridized to the fragments on the array and visualization of the hybridization can be done. Since multiple arrays can be made with the exact same position of fragments they are particularly useful for comparing the gene expression of two different tissues, such as a healthy and cancerous tissue. Also, one can measure what genes are expressed and how that expression changes with time or with other factors. For instance, the common baker's yeast, Saccharomyces cerevisiae, contains about 7000 genes; with a microarray, one can measure qualitatively how each gene is expressed, and how that expression changes, for example, with a change in temperature. There are many different ways to fabricate microarrays; the most common are silicon chips, microscope slides with spots of ~ 100 micrometre diameter, custom arrays, and arrays with larger spots on porous membranes (macroarrays). There can be anywhere from 100 spots to more than 10,000 on a given array.

Arrays can also be made with molecules other than DNA. For example, an antibody array can be used to determine what proteins or bacteria are present in a blood sample.

Allele specific oligonucleotide (ASO) is a technique that allows detection of single base mutations without the need for PCR or gel electrophoresis. Short (20-25 nucleotides in length), labeled probes are exposed to the non-fragmented target DNA. Hybridization occurs with high specificity due to the short length of the probes and even a single base change will hinder hybridization. The target DNA is then washed and the labeled probes that didn't hybridize are removed. The target DNA is then analyzed for the presence of the probe via radioactivity or fluorescence. In this experiment, as in most molecular biology techniques, a control must be used to ensure successful experimentation. The Illumina Methylation Assay is an example of a method that takes advantage of the ASO technique to measure one base pair differences in sequence.

As new procedures and technology become available, the older technology is rapidly abandoned. A good example is methods for determining the size of DNA molecules. Prior to gel electrophoresis (agarose or polyacrylamide) DNA was sized with rate sedimentation in sucrose gradients, a slow and labor intensive technology requiring expensive instrumentation; prior to sucrose gradients, viscometry was used.

Aside from their historical interest, it is worth knowing about older technology as it may be useful to solve a particular problem.

While molecular biology was established in the 1930s, the term was first coined by Warren Weaver in 1938. Warren was the director of Natural Sciences for the Rockefeller Foundation at the time and believed that biology was about to undergo a period of significant change given recent advances in fields such as X-ray crystallography. He therefore channeled significant amounts of (Rockefeller Institute) money into biological fields.

Clinical research and medical therapies arising from molecular biology are covered under gene therapy and ??.

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Biology and sexual orientation

LGBT pride flag

Biology and sexual orientation is research into possible biological influences on the development of human sexual orientation. No simple cause for sexual orientation has been conclusively demonstrated, and there is no scientific consensus as to whether the contributing factors are primarily biological or environmental. Many think both play complex roles. The American Academy of Pediatrics and the American Psychological Association have both stated that sexual orientation probably has multiple causes. Research has identified several biological factors which may be related to the development of a heterosexual, homosexual or bisexual orientation. These include genes, prenatal hormones, and brain structure. Conclusive proof of a biological cause of sexual orientation would have significant political and cultural implications.

Researchers have traditionally used twin studies to try to isolate genetic influences from environmental or other influences. One common type of twin study compares identical twins (known as monozygotic or "MZ twins") who both have a particular trait to non-identical or fraternal twins (known as dizygotic or "DZ twins") with that same trait. Since identical twins have the same genetic makeup (genotype) while non-identical twins share only 50% of their genes, a difference between these types of twins provides evidence of a genetic component. For example, if a high percentage of identical twins both have red hair (while a low percentage of non-identical twins both have red hair), that suggests that red hair has a genetic basis. On the other hand, if identical twins share a characteristic just as often as fraternal twins (such as love of music), that suggests that there is not a genetic basis for that trait.

A number of twin studies have attempted this kind of isolation. As Bearman and Bruckner (2002) describe it, early studies concentrated on small, select samples, which showed very high genetic influences; however, they were also criticized for non-representative selection of their subjects. Later studies, performed on increasingly representative samples, showed much lesser concordance among MZ twins, although still significantly larger than among DZ twins.

For example, a recent meta-study by Hershberger (2001) compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Two additional examples: Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ) twins were concordant for homosexuality. Also, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins. Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. MZ twin concordance for homosexuality was found to be 30%.

A recent study of all adult twins in Sweden (more than 7,600 twins) found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and societal attitudes had a weaker significant effect. Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twin may influence the results (see below).

Twin studies have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies. Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins (who are genetically identical, and shared the same fetal environment), sexual orientation cannot be purely biologically caused.

Another issue is the recent finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Gringas and Chen (2001) describe a number of mechanisms which can lead to differences between monozygotic twins, the most relevant here being chorionicity and amniocity. Dichorionic twins potentially have different hormonal environments and receive maternal blood from separate placenta. Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated". If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.

Earlier chromosome studies of homosexuality in males have not been replicated, or have had doubt cast on these early suggestions. For example, in 1993, Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly (but inaccurately) dubbed as the 'gay gene' in the media, causing significant controversy.

However, a later analysis by Hu et al. revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28. Sanders et al. (1998) replicated the study, finding 66% Xq28 marker sharing in 54 pairs of gay brothers. On the other hand, two other studies (Bailey et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men. Also, a study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.

Additionally, Mustanski et al. (2005) performed a full-genome scan (instead of just an X chromosome scan) on individuals and families previously reported on in Hamer et al. (1993) and Hu et al. (1995), as well as additional new subjects. With the larger sample set and complete genome scan, the study found much weaker link for Xq28 than reported by Hamer et al. However, they did find other markers with significant likelihood scores at 8p12, 7q36 and 10q26, the latter two having approximately equivalent maternal and paternal contributions.

A recent study suggests linkage between a mother's genetic make-up and homosexuality of her sons. Women have two X chromosomes, one of which is "switched off". The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active. In some cases though, it appears that this switching off can occur in a non-random fashion. Bocklandt et al. (2006) reported that, in mothers of homosexual men, the number of women with extreme skewing of X chromosome inactivation is significantly higher than in mothers without gay sons. Thirteen percent of mothers with one gay son, and 23% of mothers with two gay sons showed extreme skewing, compared to 4% percent of mothers without gay sons. One problem in building consensus using this type of study stems from their heavy reliance on participants truthfully reporting their sexual preference.

Blanchard and Klassen (1997) reported that each older brother increases the odds of being gay by 33%. This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation." To explain this finding, it has been proposed that male fetuses provoke a maternal immune reaction that becomes stronger with each successive male fetus. Male fetuses produce HY antigens which are "almost certainly involved in the sexual differentiation of vertebrates." It is this antigen which maternal H-Y antibodies are proposed to both react to and 'remember'. Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinisation.

Bocklandt, Horvath, Vilain and Hamer (2006) reported that some mothers of gay babies have extreme skewing of X chromosome inactivation. Using a sample of 97 mothers of homosexual men and 103 mothers of heterosexual men, the pattern of X inactivation was ascertained from blood assays. 4% of the mothers of straight men showed extreme skewing compared to 13% of the mothers of gay men. Mothers of two or more gay babies had extreme skewing of X inactivation of 23%. This extreme skewing may influence male sexual orientation through the fraternal birth order effect.

An alternate theory was proposed by Italian researchers in 2004 supported by a study of about 4,600 people who were the relatives of 98 homosexual and 100 heterosexual men. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring. The connections discovered, however, would explain only 20% of the cases studied, indicating that this might not be the sole genetic factor determining sexual orientation.

Recent research conducted in Sweden has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal. The research showed that when both heterosexual women (lesbians were included in the study, but the results regarding them were "somewhat confused") and gay men are exposed to a testosterone derivative found in men's sweat, a region in the hypothalamus is activated. Heterosexual men, on the other hand, have a similar response to an estrogen-like compound found in women's urine. The conclusion, that sexual attraction, whether same-sex or opposite-sex oriented, operates similarly on a biological level, does not mean that there is necessarily a biological cause for homosexuality. Researchers have suggested that this possibility could be further explored by studying young subjects to see if similar responses in the hypothalamus are found and then correlating this data with adult sexual orientation.

A number of sections of the brain have been reported to be sexually dimorphic; that is, they vary between men and women. There have also been reports of variations in brain structure corresponding to sexual orientation. In 1990, Swaab and Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men. In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure.

Early work of this type was also done by Simon LeVay. LeVay studied four groups of neurons in the hypothalamus, called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that this part of the brain played a role in the regulation of sexual behaviour in animals, and because INAH2 and INAH3 had previously been reported to differ in size between men and women.

William Byne and colleagues attempted to replicate the differences reported in INAH 1-4 size using a different sample of brains from 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). They found a significant difference in INAH3 size between heterosexual men and women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men and larger than that of the heterosexual women, though neither difference quite reached statistical significance.

Byne and colleagues also weighed and counted numbers of neurons in INAH3, tests not carried out by LeVay. The results for INAH3 weight were similar to those for INAH3 size; that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either. The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation.

The early fixation hypothesis includes research into prenatal development and the environmental factors that control masculinization of the brain. Studies have concluded that there is empirical evidence to support this hypothesis, including the observed differences in brain structure and cognitive processing between homosexual and heterosexual men. One explanation for these differences is the idea that differential exposure to hormone levels in the womb during fetal development may block or exaggerate masculinization of the brain in homosexual men. The concentrations of these chemicals is thought to be influenced by fetal and maternal immune systems, maternal consumption of certain drugs, maternal stress, and direct injection. This hypothesis is also connected to the fraternal birth order research.

This type of theory holds that the formation of gender identity occurs in the first few years of life after birth. It argues that individuals can be predisposed to homosexual orientation by biological factors but are triggered in some cases by upbringing. Part of adopting a gender identity involves establishing the gender(s) of sexual attraction. This process is analogous to the "imprinting" process observed in animals. A baby duckling may be genetically programmed to "imprint" on a mother, but what entity it actually imprints upon depends on what objects it sees immediately after hatching. Most importantly, once this process has occurred, it cannot be reversed, any more than the duckling can hatch twice.

A sort of reverse sexual imprinting has been observed in heterosexual humans; see the section on the "Westermarck effect" in Behavioral imprinting.

Several different triggers for imprinting upon a particular sexual orientation have been proposed.

One hypothesis is that something about what young children see in the gender-roles behavior of adults, or some differences (possibly unconscious) in the way adults treat young children, somehow influences or determines a child's eventual sexual orientation.

Daryl Bem, a social psychologist at Cornell University, has theorized that the influence of biological factors on sexual orientation may be mediated by experiences in childhood. A child's temperament predisposes the child to prefer certain activities over others. Because of their temperament, which is influenced by biological variables such as genetic factors, some children will be attracted to activities that are commonly enjoyed by other children of the same gender. Others will prefer activities that are typical of another gender. This will make a gender-conforming child feel different from opposite-gender children, while gender-nonconforming children will feel different from children of their own gender. According to Bem, this feeling of difference will evoke physiological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this physiological arousal will later be transformed into sexual arousal: children will become sexually attracted to the gender which they see as different ("exotic"). This theory is known as Exotic Becomes Erotic (EBE) theory.

The theory is based in part on the frequent finding that a majority of gay men and lesbians report being gender-nonconforming during their childhood years. A meta-analysis of 48 studies showed childhood gender nonconformity to be the strongest predictor of a homosexual orientation for both men and women. Fourteen studies published since Bailey & Zucker's 1995 also show the same results. In one study by the Kinsey Institute of approximately 1000 gay men and lesbians (and a control group of 500 heterosexual men and women), 63% of both gay men and lesbians reported that they were gender nonconforming in childhood (i.e., did not like activities typical of their sex), compared with only 10-15% of heterosexual men and women. There are also six "prospective" studies--that is longitudinal studies that begin with gender-nonconforming boys at about age 7 and follow them up into adolescence and adulthood. These also show that a majority (63%) of the gender nonconforming boys become gay or bisexual as adults. There are very few prospective studies of gender nonconforming girls. In a group of eighteen behaviorally masculine girls (mean age of assessment: 9 years), all reported a homosexual sexual orientation at adolescence, and eight had requested sex reassignment.

William Reiner, a psychiatrist and urologist with the University of Oklahoma has evaluated more than a hundred cases of children born with sexual differentiation disorders. In the 1960s and 70s, it was common in developed countries for doctors to castrate boys born with a micropenis and have them raised as girls. However, this practice has come under attack, because even though these boys were raised as girls, they nearly all report as adults that they are sexually attracted to women. This suggests that their sexual orientation was determined at birth. The only cases Reiner found where children born with a X and Y chromosome are attracted to males as adults were those where testosterone receptors were absent, which prevented the male sex hormones from masculinizing the fetus.

The pathogenic hypothesis of homosexuality, also called the 'gay germ' hypothesis, suggests that homosexuality might be caused by an infectious agent. The speculative hypothesis was suggested by Gregory Cochran and Paul Ewald as part of a larger project advocating a number of pathogenic theories of disease. They argue that because of the supposedly reduced number of offspring produced by gay and lesbian people, evolution would strongly select against it. They also draw an analogy to diseases that alter brain structure and behavior, such as narcolepsy, which are suspected of being triggered by viral infection. Cochran also argues that the prevalence of homosexuality in urban areas suggests that an infectious disease causes homosexuality. They conclude that it is a "feasible hypothesis... no more and no less." After being unable to publish this account in a peer-reviewed journal, the idea appeared in the popular press. An American Philosophical Association newsletter the following year stated "there is ultimately very little to be said in favor of these contentions", and criticised the press attention gained, given a lack of peer reviewed publication of the theory, and questioned the general ethics of communication of theories about homosexuality by researchers to the public. In an article in Out Magazine, brain researcher William Byne stated "Cochran and Ewald are guilty of pathologizing homosexuality", while in the same article psychology professor J. Michael Bailey posited that a 'germ theory' did not necessarily mean homosexuality was a disease, but recognised the political ammunition such a belief could give to homophobes.

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they would appear to be maladaptive in an evolutionary context following a simple Darwinian model of Natural Selection—on the assumption that homosexuality would reduce this frequency.

Those who believe that homosexuality is purely genetic argue that maladaptive traits will only be removed from a population if the trait is under simple, direct selection, if it derives from a heritable component of a genotype and if the intensity of selection is greater than other evolutionary forces like genetic drift, or inclusive fitness.

Some scholars have suggested that homosexuality is adaptive in a non-obvious way. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present may also confer resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage).

The so-called "gay uncle" theory posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (food, supervision, defense, shelter, etc.) to the offspring of their closest relatives. This hypothesis is an extension of the theory of kin selection. Kin selection was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J.B.S. Haldane in 1932 and later elaborated by many others including John Maynard Smith and West Eberhard. This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.

The primary criticism of this theory has to do with the fact that children share on average 25% of their genes with their uncles and aunts, but on average 50% with their parents. This means that to be adaptive, a "gay uncle" would need to somehow assist an extra two nieces or nephews, on average, to reach adulthood for every one of their own offspring they give up. Critics of the theory find this trade-off to be unlikely to produce a net reproductive gain.

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved to not drive them to complete rejection of heterosexuality.

Whether genetic or other physiological determinants as the basis of sexual orientation is a highly politicised issue. The Advocate, a U.S. gay and lesbian newsmagazine, reported in 1996 that 61% of its readers believed that "it would mostly help gay and lesbian rights if homosexuality were found to be biologically determined". A cross-national study in the United States, the Philippines, and Sweden found that those who believed that "homosexuals are born that way" held significantly more positive attitudes toward homosexuality than those who believed that "homosexuals choose to be that way" and/or "learn to be that way".

Some advocates for the rights of sexual minorities also resist the idea that sexuality is biologically determined, or fixed at birth. They point out that sexual orientation can shift over the course of one's life. Many resist any attempts to pathologise or medicalise 'deviant' sexuality, and choose to fight for acceptance in a moral or social realm. Others fear that discoveries about medical causes of sexuality may be used by doctors and parents to eradicate non-heterosexual orientations.

There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors. Many think that nature and nurture both play complex roles; most people experience little or no sense of choice about their sexual orientation.

No one knows what causes heterosexuality, homosexuality, or bisexuality.... there is a renewed interest in searching for biological etiologies for homosexuality. However, to date there are no replicated scientific studies supporting any specific biological etiology for homosexuality.

Despite some degree of logical plausibility, there is ultimately very little to be said in favor of these contentions. In its focus on the reduced reproductive rates of homosexual men and women, the account ignores other mechanisms by which genetic traits endure across generations. More importantly, the account is offered without any evidence whatsoever about which microbe might work how to generate homosexual interests. A peer-reviewed science journal turned this account away, but it nevertheless found its way into the pages of the public press.... the ease with which theories of homosexuality seep into public discourse raises important ethical questions about the way in which researchers ought to communicate their various theories to the public. Given that an unfounded theory of homosexuality can do more damage than good, researchers should raise the bar in regard to the views they propound about its origin.

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Source : Wikipedia